abstract stringlengths 3 192k | title stringlengths 4 857 |
|---|---|
the amount and configuration of land on a synchronously rotating, habitable-zone rocky planet can have a substantial impact on its climate. in a previous study, we showed using the 3d circulation model exoplasim that surface temperature can vary by up to 20 k, and atmospheric humidity can vary by several orders of magnitude, due to the effect of land cover on the amount of sea ice and liquid ocean on the planet's dayside. we now investigate how land-related climate differences might present in observations. in this work, we use a radiative transfer model to generate synthetic transit spectra for our simulation outputs with varying land cover. we also include a new set of simulations with varying dayside land in a more favourable parameter space for transit observations; these simulations also span a range of surface pressures. although some land-related climate differences are visible in our synthetic spectra, we find that it will be challenging to constrain an exoplanet's surface conditions from observations. i will describe the influence of land cover on the climate regimes present in our expanded parameter space of simulations and discuss how these climate differences could correspond to uncertainties in transit spectra. | influence of dayside land on climates and transit spectra of synchronously rotating rocky planets |
introduction: planets that retain a primordial, h-he dominated atmosphere can have hydrogen act as a greenhouse gas: the collision induced absorption (cia) of infra-red light increases with pressure. this mechanism could replace regular greenhouse gasses in exoplanets to allow warm enough temperatures for a liquid water layer. prior work[1, 2] demonstrated that either stellar insolation or intrinsic heat could be a sufficient energy source. in the case of the latter, a 'habitable zone' for such planets could extend up to infinity[3, 4]. in this work we aim to study the long-term potential for habitability of planets with hydrogen-dominated atmospheres. we simulate planets with varying properties to investigate how these influence the likeliness of habitable conditions. importantly, we include for the first time long-term temporal evolution of both the planet and the host-star, to estimate how long liquid water could remain.methods: we model the thermal structure of a silicate-iron planet with a h-he-dominated atmosphere and vary the core mass, initial envelope mass, and semimajor-axis. an evolution model for the host-star's luminosity is included, as well as a model for the evolution of the planet's intrinsic heat and radius. the intrinsic heat model includes a radiogenic component based on earth's abundance of radioactive materials as well as cooling and contraction of both the silicate-iron core and the gaseous envelope. we furthermore include a thermal xuv-driven atmosphere loss model. by comparing the pressure and temperature at the bottom of the atmosphere with the water phase diagram, we determine when liquid water can exist.results: we find that terrestrial and super-earth planets with masses of about 1 - 10 earth masses can maintain liquid water conditions for more than 9 billion years at radial distances larger than 2 au. the required surface pressures are typically between ~100 bar (as on venus) and 1 kbar (as in the oceanic trenches on earth). hydrodynamic escape can reduce this duration significantly for planets with a smaller envelope than 10-5 earth mass within 10 au, while planets with an envelope larger than 10-3 earth mass remain mostly unaffected by this mechanism. planets that receive a negligible amount of stellar radiation can maintain the conditions as long as the internal heat source is sufficient, which is at maximum 80 billion years in our simulation.conclusions: under the assumptions of our model we show that there is a wide range of conditions under which liquid water can exist and remain on the order of 10 billion years. this raises the question whether most potentially habitable planets are very different from earth. our model is simplified and future work should investigate the formation and retention of a liquid water ocean in more detail. the pathway and the conditions towards planets with the right initial conditions should be studied in the future as well. this will enable us to better predict the occurence rate of such a planet and the chances of observing them at the current age of the universe.references: [1] stevenson d. j. (1999) nature 400:32. [2] pierrehumbert r. & gaidos e. (2011). the astrophysical journal letters 734:l13. [3] seager s. (2013) science, 340:577-581. [4] madhusudhan, n. et al. (2021) the astrophysical journal, 918:1. | potential long-term habitable conditions on planets with primordial h-he atmospheres. |
climate modeling has shown that tidal locking, particularly of terrestrial exoplanets orbiting m-dwarfs, can lead unique atmospheric dynamics and surface conditions. however, chaotic libration and rotation induced by planetary interactions in compact planetary systems may destabilize attendant exoplanets away from synchronized states. here, we use a three-dimensional n-body integrator and an intermediately-complex general circulation model to simulate the evolving climates of trappist-1 e and f under the influence of different orbital and spin evolution pathways. planet f perturbed by mean motion resonance effects are colder and dryer compared to their synchronized counterparts due to the zonal drift of the substellar point away from the eyeball open water belt. the differences between perturbed and synchronized planet e are minor due to higher instellation and warmer climates. this is the first study to incorporate the outcomes of direct gravitational n-body simulations into 3d climate modeling and our results show that planets at the outer edge of the habitable zones (ohz) of multiplanet systems are vulnerable to rapid global glaciation. future missions to characterize habitable exoplanets should avoid ohz planets in compact | combining 3d n-body simulation and climate modeling to decipher the climates of exoplanets in compact multi-planet systems |
energetic particles, such as stellar energetic particles and galactic cosmic rays, are an important part of space weather for exoplanets orbiting cool stars and the young earth. energetic particles bombard exoplanetary atmospheres, leading to unique chemical effects that may be detectable with jwst. the flux of energetic particles reaching an exoplanet depends on the stellar wind properties which vary with stellar age, as the star spins down. this means it is important to constrain the stellar wind properties of other stars. i will present our results which modelled the energetic particle flux reaching earth at different ages, such as when life is thought to have begun (approximately 3.8gyr ago). i will discuss how, at this time, our model shows that stellar energetic particles dominated over galactic cosmic rays up to gev energies. at these energies, energetic particles can cause particle showers in the planet atmosphere that can reach the surface of the planet. at the same time, to connect with upcoming observations we need to consider exoplanets orbiting stars with well-constrained stellar winds. for instance, the stellar mass-loss rate is important in determining the size of the astrosphere. thus, i will also discuss our recent results for the galactic cosmic ray fluxes reaching the habitable zone and exoplanets of a number of nearby stars. finally, i will discuss our ongoing efforts to connect these energetic particle fluxes closely to upcoming jwst observations. | the interaction of energetic particles with the winds of cool stars |
planned and future direct imaging missions, such as roman and the large space-based infrared/optical/ultraviolet (ir/o/uv) telescope prioritized in astro2020 will have the instrumentation capable of imaging the reflected light from exoplanets. while current direct imaging surveys probe the thermal emission of young, giant, self-luminous planets, future missions will push toward the detection and characterization of earth-like planets around sun-like stars. however, determining whether a planet is in the habitable zone of its star may be difficult in multi-planet systems, which are now known to occur frequently. previous work has shown that planets in multi-planet systems can be "confused" in direct images taken over multiple epochs due to lack of prior knowledge about the planet's orbital parameters or planetary characteristics. in this work, we consider a system to be confused when it has multiple possible orbit combinations for the planets that describe the data equally well and it's not obvious which detection belongs to which planet. being able to differentiate planets in a multi-planet system is necessary in order to determine orbital parameters, characterize atmospheres, and estimate habitability. in this work, we address the confusion problem with a "deconfusion" algorithm, which leverages astrometric information to fit orbits to planets and help differentiate between multiple planets in an image. the deconfusion algorithm accepts unlabeled planet detections with astrometric information and generates a list of orbit matches. the matches are ranked based on their consistency with the presented data and the number of matched observations. we introduce the inclusion of photometric considerations in the deconfusion algorithm to help rank and differentiate between possible orbits. we are investigating the utility of orbital phase information for each planet in simulated planetary systems to reduce the rate of confusion in a multi-planet system. here we present estimated confusion rates for simulated systems with both astrometric and photometric information. the results of our investigation are used to determine the ability of photometric information to mitigate confusion for future direct imaging missions. | leveraging photometry for deconfusion of directly imaged multi-planet systems |
in order to recognize a habitable exoplanet from future observed spectra, we present new model reflected spectra and geometric albedo for modern and prebiotic (∼3.9 ga) earth-like exoplanets orbiting within the habitable zone of stars of spectral types f, g, k and m. we compute this for various atmospheric and surface compositions of the planets. molecules that are potential biosignatures and act as greenhouse agents are incorporated in our model atmosphere. various combinations of solid and liquid materials such as ocean, coast, land consisting of trees, grass, sand or rocks determine the surface albedo of the planet. geometric albedo and model reflected spectra for a set of nine potential habitable planets, including proxima centauri b, trappist-1d, kepler-1649c and teegarden's star-b, are also presented. we employ the opacity data derived by using the open-source package exo-transmit and adopt different atmospheric temperature-pressure profiles depending on the properties of the terrestrial exoplanets. the model-reflected spectra are constructed by numerically solving the multiple scattering radiative transfer equations. we verified our model reflected spectra for a few specific cases by comparing with those published by other researchers. we demonstrate that prebiotic earth-like exoplanets and present earth-like exoplanets with increased amount of greenhouse gases in their atmospheres scatter more starlight in the optical. we also present the transmission spectra for modern and prebiotic earth-like exoplanets with cloudy and cloudless atmospheres. | new models of reflection spectra for terrestrial exoplanets: present and prebiotic earth orbiting around stars of different spectral types |
the astro2020 decadal survey recommended a "future large ir/o/uv telescope optimized for observing habitable exoplanets and general astrophysics" that would "search for biosignatures from a robust number of about 25 habitable zone [exo]planets," now dubbed the habitable worlds observatory (hwo). the search for biosignatures requires high quality spectra over a broad bandwidth and sufficient signal-to-noise. the combination of wavelength, spectral resolution, bandwidth, and signal-to-noise-ratio impacts the number of exo-earths that can be spectrally characterized. previous work (morgan et al. 2022) evaluated the number of earth-size, habitable zone exoplanets (denoted here as yield) that could be spectrally characterized over the wavelength range of 500 nm to 1000 nm for a 6-m diameter exoplanet direct imaging mission for coronagraph-only and hybrid coronagraph-starshade architectures for three prior knowledge cases: the nominal case of a blind-search survey, the upper-bound case of perfect prior knowledge, which is useful to determine if target depletion occurs, and the partial prior knowledge case of a hypothetical extreme precision radial velocity survey. in this paper, we extend previous exoplanet yields to include wavelengths out to 1.8 microns. because the iwa for coronagraphs is proportional to wavelength, the achievable spectral coverage will be different for every planet detected. we present the spectral coverage achieved across individual target stars, as well as the ensemble target set, for a coronagraph-only architecture and a hybrid coronagraph + starshade architecture. we use the three prior knowledge cases. the coronagraph spectral characterization is simulated in the near infrared for each of the 10% sub-bands individually, as if it were the only spectral characterization performed during the mission, and then as a broadband spectral characterization performed in sequence over the sub-bands. the starshade achieves the broadband spectral equivalent simultaneously. we also examine the capabilities of the 60 m starshade point design and investigate the benefits of refueling. | exo-earth yield of a 6m space telescope in the near-infrared |
the launch of the james webb space telescope (jwst) in december 2021 opens up the possibility of studying the composition of exoplanetary atmospheres in habitable zones, such as trappist-1e, in the near future. with the help of numerical models of the exoplanetary atmospheres, the observations and the processes behind them can be better understood and interpreted (herbst et al., 2022). we investigate the influence of stellar energetic particles (seps) and galactic cosmic rays (gcrs) on the atmospheric chemistry of exoplanets around a very active m-star using the ion chemistry model exotic. in collaboration with the university of kiel and dlr berlin, we perform model experiments with different n2 or co2 dominated atmospheres, depending on the initial co2 partial pressure, as well as humid and dry conditions (wunderlich et al., 2020), taking into account the ionization rates for such events. a further specification regarding the scenarios results from the distinction between dead and alive atmospheres, whose atmospheric composition is characterized by a lower or higher oxygen fraction in the initial conditions. within exotic we can calculate the impact of the ionization events on these atmospheres both as a single and as a series of events with different strengths. preliminary results show a significant impact of sep events on the chemical composition of the atmosphere, including biosignatures such as o3 . the strength and structure of these impacts depend on the composition of the starting atmosphere, in particular on the availability of oxygen as well as nitrogen and water vapour. | investigation of the influence of stellar particle events and galactic cosmic rays on the atmosphere of trappist-1e |
atmospheric escape shapes the fate of exoplanets, with statistical evidence for atmospheric mass loss imprinted across the mass-radius-insolation distribution. the habitable zone planet finder (hpf) spectrograph exospheres program aims to expand the search across a diverse range of planet and host star properties via the 10833 å helium triplet, a sensitive probe of active atmospheric escape. we present a detection of up to 10% helium excess absorption depth during transit spectroscopy of the highly irradiated, low-gravity, inflated hot saturn hat-p-67 b. the 13.8 hours of on-sky integration time over 39 nights sample the entire planet orbit, uncovering excess helium absorption preceding the transit by up to 130 planetary radii in a large leading tail. this configuration can be understood as the escaping material overflowing its small roche lobe, and advecting most of the gas into the stellar—and not planetary — rest frame, consistent with the doppler velocity structure seen in the helium line profiles. the prominent leading tail serves as direct evidence for dayside mass loss with a strong day-/night- side asymmetry. we see some transit-to-transit variability in the line profile, consistent with the interplay of stellar and planetary winds. we employ 1d parker wind models to estimate the mass loss rate, finding values on the order of 2 x 1013 g/s, with large uncertainties owing to the unknown xuv flux of the f host star. the large mass loss in hat-p-67 b represents a valuable example of an inflated hot saturn, a class of planets recently identified to be rare as their atmospheres are predicted to evaporate quickly. we contrast two physical mechanisms for runaway evaporation: ohmic dissipation and xuv irradiation, slightly favoring the latter. | a large and variable leading tail of helium in hat-p-67b, a sub-saturn undergoing runaway inflation |
toi-700 d is the first earth-sized planet in the habitable zone (hz) discovered by the transiting exoplanet survey satellite. here, we assess whether a venus-like exoplanet at the toi-700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics simulations. we investigate the effects of x-ray and euv (xuv) radiation from the host star, the interplanetary magnetic field (imf) orientation, and the planetary intrinsic magnetic field. in unmagnetized cases, major ion loss is caused by o+ escape through a ring-shaped region by the mass loading process after the ionization of the extended oxygen corona. as the imf parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions (o2+ co2+) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. in magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp-origin escape from the lower ionosphere. these results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where xuv is 30 times stronger than at the current earth. however, the dipole intrinsic magnetic field of 1,000 nt at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong xuv conditions. | study of atmospheric ion escape from exoplanet toi-700 d: venus analogs |
the existence of an earth-like habitat (defined as a rocky exoplanet within the habitable zone of complex life that hosts an n2-o2-dominated atmosphere with minor amounts of co2) is depending on a certain set of known (and, potentially, unknown) astrophysical and geophysical requirements that have to be met to allow for its evolution and environmental stability. a few of these requirements are already quantifiable to a certain extent by our current scientific knowledge while others are still under debate. one crucial factor that has to be taken into account when estimating the prevalence of earth-like habitats within the galaxy is a planet's host star. its radiation and plasma environment may affect the stability of an earth-like atmosphere to such an extent that it can even render its stable existence unlikely around highly active stars. a star's metallicity and location within the galactic disk may pose further restrictions on the prevalence of earth-like habitats within the milky way. taking these factors into account, we will, based on current quantifiable scientific knowledge, derive that only a certain fraction of stars within the galaxy will in principle be able to host planets with earth-like atmospheres. interestingly, k dwarfs with a stellar mass around 0.8 msun may constitute a particularly interesting environment for the existence of earth-like habitats. m stars, on the other hand, exhibit several different problems; planets suitable for life as we know it may therefore be a rare occasion around the smallest, but most abundant, stars within the galaxy. | the host star as a crucial factor for the prevalence of earth-like habitats |
the recently discovered super-earth lp 890-9 c is an intriguing target for atmospheric studies as it transits a nearby, low-activity late-type m-dwarf star at the inner edge of the habitable zone. its position at the runaway greenhouse limit makes it a natural laboratory to study the climate evolution of hot rocky planets. we present the first 3d-global climate model exo-venus model for a modern venus-like atmosphere (92 bar surface pressure, realistic composition, and h2so4 radiatively-active clouds), applied to the tidally-locked lp 890-9 c to inform observations by jwst and future instruments. if lp 890-9 c has developed into a modern exo-venus, then the modelled temperatures suggest that h2so4 clouds are possible even in the substellar region. like on modern venus, clouds on lp 890-9 c would create a flat spectrum. the strongest co2 bands in transmission predicted by our model for lp 890-9 c are about 10 ppm, challenging detection, given jwst estimated noise floor. estimated phase curve amplitudes are 0.9 and 2.4 ppm for continuum and co2 bands, respectively. while pointing out the challenge to characterise modern exo-venus analogues, these results provide new insights for jwst proposals and highlight the influence of clouds in the spectrum of hot rocky exoplanet spectra. | 3d global climate model of an exo-venus: a modern venus-like atmosphere for the nearby super-earth lp 890-9 c |
a key objective of the upcoming habitable worlds observatory (hwo) is to search for habitable and inhabited planets. the astro2020 decadal survey recommended that the mission should be capable of searching for biosignatures in the atmospheres of roughly 25 habitable planets, and the nasa exoplanet exploration program (exep) recently released a list of 164 potential target stars. while some of those targets may be excellent targets for hwo, the exep report cautioned that other targets may be less viable due to stellar companions, high exozodi levels, or other complicating factors. assessing the viability of each potential target star early in the mission design process is important because the number of accessible targets depends on the mission design and the science output of hwo scales with the number of viable targets. if a high fraction of potential targets are not rejected until after major architecture choices are finalized, then hwo may not achieve the goal of surveying a sufficient number of habitable zones. in order to maximize the likelihood that hwo will detect a large number of habitable, and perhaps inhabited, planets, we have begun a comprehensive investigation of potential target stars. first, we are inspecting the astronomical literature and public data archives to assess current knowledge of the 164 targets listed in the exep mission star list including limits on the presence of planets in those systems. second, we are supplementing the exep mission star list by adding additional stars so that hwo mission design trade studies can explore a wider region of parameter space. third, for potential targets with known stellar or substellar companions, we are conducting dynamical simulations to determine which systems can host habitable planets over long timescales. fourth, for each viable target, we are developing an observational plan to maximize our precursor knowledge of planetary system architecture prior to hwo observations. by assessing the suitability of potential targets early in the hwo mission design process, we will help ensure that hwo is able to observe a large sample of small, temperate planets and place meaningful constraints on the frequency of life in the universe. | maximizing precursor knowledge of potential targets for the habitable worlds observatory |
when a planet has a strong dipole magnetic field and rapid rotation, superposition of the stellar wind induced and corotation electric fields results in the tear-drop-shaped region of the closed drift, where planetary ionized atmosphere can fill the magnetic flux tubes along the field lines. the region is characterized with cold dense planetary plasma and called as the plasmasphere. since the plasmasphere formation requires the existence of both the thick atmosphere and global intrinsic magnetic field, the observation of plasmasphere can provide possible evidence and clues of the exoplanetary atmosphere and intrinsic magnetic field. in this study, we estimate the size and ion density of terrestrial exoplanetary plasmasphere and assess its detectability by transit observations based on the knowledge of the solar system planets. m-dwarf stars have the habitable zone (hz) close to the host star, and because of the proximity, tidal locking is possible for most planets in hz of k and m dwarf stars. even if an exoplanet is tidally locked and its rotation is slow, rapid rotation of the atmosphere (superrotation) can provide an alternative source of upper atmospheric rotation, one of necessary condition for the plasmasphere formation. here we consider the cases when the upper atmosphere also exhibit the supperroation and rocky exoplanets possess the venus-like co2-rich atmosphere. as stellar radiation inputs, we used x-ray and ultraviolet (xuv) radiation of the proxima centauri (pc). the same intrinsic dipole magnetic field strength as that of earth is assumed and two cases of planetary mass of the imaginary exoplanets, i.e., venusian mass or 2 times of earth mass, are considered. the results show that the main ion species in the ionosphere and plasmasphere is c+. the plasmasphere size estimation show that earth/venus-like magnetized exoplanet can have a plasmasphere with a size of 4-6 times of the planetary radius. the size of the plasmasphere depends on the superrotation speed of the thermosphere, ionospheric conductance, stellar wind dynamic pressure, and imf cone angle. estimation also indicates that the fuv absorption by plasmaspheric c+ ions can cause a few to several percent of the transit depth depending on the planetary size and plasmaspheric c+ density, which might be observable by space telescopes. | plasmasphere formation at terrestrial rocky exoplanets around m-dwarf stars and its detectability: possible evidence for the intrinsic magnetic field and thick atmosphere |
bow shocks are ubiquitous astrophysical phenomena resulting from the supersonic passage of an object through a gas. recently, pre-transit absorption in uv metal transitions of the hot jupiter exoplanets hd 189733b and wasp12-b have been interpreted as being caused by material compressed in a planetary bow shock. here we present a robust detection of a time-resolved pre-transit, as well as in-transit, absorption signature around the hot jupiter exoplanet hd 189733b using high spectral resolution observations of several hydrogen balmer lines. the line shape of the pre-transit feature and the shape of the time series absorption provide the strongest constraints on the morphology and physical characteristics of extended structures around an exoplanet. the in-transit measurements confirm the exospheric hα detection of jensen et al. (2012) although the absorption depth measured here is ~50% lower. the pre-transit absorption feature occurs 125 minutes before the predicted optical transit, a projected linear distance to the stellar disk of 7.2 rp. the absorption strength observed in the balmer lines indicates an optically thick, but physically small, geometry. we model this signal as the early ingress of a planetary bow shock. if the bow shock is mediated by a planetary magnetosphere, the large standoff distance derived from the model suggests a large planetary magnetic field strength. better knowledge of exoplanet magnetic field strengths is crucial to understanding the role these fields play in star-planet interactions and protecting planets in the habitable zone from dangerous stellar flares. | optical hydrogen absorption consistent with a bow shock leading the hot jupiter hd 189733b |
for temperate exoplanets orbiting m dwarf hosts, the proximity of the habitable zone to the star necessitates careful consideration of tidal effects. spin synchronization of the planetary orbital period and rotation period is a common assumption for habitable zone planets across the entire m spectral type. this predicted tidal locking has important implications for conditions on the surface. in this manuscript, we investigate the plausibility of capture into cassini state 2 for a known sample of 280 multiplanet systems orbiting m dwarf hosts. this resonance of the spin precession and orbital precession frequencies is capable of exciting planets into stable nonzero rotational obliquities. a large axial tilt resulting from this predicted resonance capture can preclude synchronous rotation, inducing some version of "day" and "night." considering each planetary pair and estimating the spin and orbital precession frequencies, we report that 75% of detected planets orbiting m dwarfs may be plausibly excited to a high obliquity over long timescales. this is consistent with similar findings for planets orbiting close-in to fgk dwarfs. however, it is only for m dwarf planets that the parameter space relevant for capture into cassini state 2 overlaps with the stellar habitable zone. this effect is strongest for host stars with effective temperatures $t_{eff}<3000$ k, where more than half of planets with $t_{eq}<400$ k could possess non-zero obliquity due to residence in cassini state 2. this overlap renders the potential capture into cassini states extremely relevant to understanding the galaxy's most common temperate planets. | plausibility of capture into high-obliquity states for exoplanets in the m dwarf habitable zone |
exoplanet imaging has thus far enabled studies of wide-orbit ($>$10 au) giant planet ($>$2 jupiter masses) formation and giant planet atmospheres, with future 30 meter-class extremely large telescopes (elts) needed to image and characterize terrestrial exoplanets. however, current state-of-the-art exoplanet imaging technologies placed on elts would still miss the contrast required for imaging earth-mass habitable-zone exoplanets around low-mass stars by ~100x due to speckle noise--scattered starlight in the science image due to a combination of aberrations from the atmosphere after an adaptive optics (ao) correction and internal to the telescope and instrument. we have been developing a focal plane wavefront sensing technology called the fast atmospheric self-coherent camera technique (fast) to address both of these issues; in this work we present the first results of simultaneous first and second stage ao wavefront sensing and control with a shack hartmann wavefront sensor (shwfs) and fast, respectively, using two common path deformable mirrors. we demonstrate this "multi-wfs single conjugate ao" real-time control at up to 200 hz loop speeds on the santa cruz extreme ao laboratory (seal) testbed, showing a promising potential for both fast and similar high-speed diffraction-limited second-stage wavefront sensing technologies to be deployed on current and future observatories, helping to remove speckle noise as the main limitation to elt habitable exoplanet imaging. | first laboratory demonstration of real-time multi-wavefront sensor single conjugate adaptive optics |
m dwarfs show the highest rocky planet occurrence among all spectral types, in some instances within the habitable zone. because some of them are very active stars, they are often subject to frequent and powerful flaring, which can be a double-edged sword in regard of exoplanet habitability. on one hand, the increased flux during flare events can trigger the chemical reactions that are necessary to build the basis of prebiotic chemistry. on the other hand, sufficiently strong flares may erode exoplanets' atmospheres and reduce their uv protection. recent observations of flares have shown that the flaring flux can be x100 times stronger in uv than in the optical. uv is also preferable to constrain more accurately both the prebiotic abiogenesis and the atmospheric erosion. for these reasons, we are developing a cubesat payload concept to complement current flare surveys operating in the optical. this cubesat will observe a high number of flaring m dwarfs, following an all-sky scanning law coverage, both in the uv and the optical to better understand the different effective temperatures as wavelengths and flaring status go. this will complement the bright optical flares data acquired from the current ground-based, high-cadence, wide fov surveys. another scientific planned goal is to conduct few-minute after-the-flare follow-up optical ground-based time-resolved spectroscopy, that will be triggered by the detection of uv flares in space on board of the proposed cubesat. finally, the study of m dwarfs stellar activity in the uv band will provide useful data for larger forthcoming missions that will survey exoplanets, such as plato, ariel, habex and luvoir. | observing m dwarfs uv and optical flares from a cubesat and their implications for exoplanets habitability |
the far-side array for radio science investigations of the dark ages and exoplanets (farside) is a proposed mission concept to the lunar far side that seeks to deploy and operate an array of 128 dual-polarization, dipole antennas over a region of 100 square kilometers. the resulting interferometric radio telescope would provide unprecedented radio images of distant star systems, allowing for the investigation of faint radio signatures of coronal mass ejections and energetic particle events and could also lead to the detection of magnetospheres around exoplanets within their parent star's habitable zone. simultaneously, farside would also measure the "dark ages" of the early universe at a global 21-cm signal across a range of red shifts (z approximately 50-100). each discrete antenna node in the array is connected to a central hub (located at the lander) via a communication and power tether. nodes are driven by cold=operable electronics that continuously monitor an extremely wide-band of frequencies (200 khz to 40 mhz), which surpass the capabilities of earth-based telescopes by two orders of magnitude. achieving this ground-breaking capability requires a robust deployment strategy on the lunar surface, which is feasible with existing, high trl technologies (demonstrated or under active development) and is capable of delivery to the surface on next-generation commercial landers, such as blue origin's blue moon lander. this paper presents an antenna packaging, placement, and surface deployment trade study that leverages recent advances in tethered mobile robots under development at nasa's jet propulsion laboratory, which are used to deploy a flat, antenna-embedded, tape tether with optical communication and power transmission capabilities. | how to deploy a 10-km interferometric radio telescope on the moon with just four tethered robots |
the proxima centauri planetary system (proxima hereafter), the nearest neighbor to our own solar system at 1.3 pc, consists of three confirmed exoplanets. the second, proxima b, is a likely super-earth residing within the circumstellar habitable zone. we analyze the stability of the two inner planets—proxima d and b—using analytic calculations of hill stability and n-body simulations with rebound. we find that for 100% of possible orbital configurations allowed by observational constraints, the ordering of the planets is preserved, but stability is not guaranteed. using the megno technique, we find that about 25% of 10,000 tested orbital configurations are unstable or chaotic. for stable configurations, we present simulations of the full (up to 7 gyr) orbital history of all three worlds using the vplanet software package that couple an equilibrium tidal model with a semi-analytic model of orbital evolution, thus accounting both for tidal forcing and planet-planet gravitational perturbations. the inner planets have been detected only via radial velocity spectroscopy, leaving their orbital elements and masses unconstrained. we therefore adopt a monte carlo approach, initializing each of 50 simulations with a unique orbital architecture. we present the range of changes in proxima b's semi-major axis and eccentricity permitted by this model, as well as the distribution of possible tidal surface fluxes today. we find that the planet's final eccentricity after 7 gyr of evolution can be non-zero, depending on the initial conditions. these results highlight the important role of orbital dynamics in the history of this potentially habitable world. | orbital dynamics of the proxima centauri system |
the dozens of rocky exoplanets discovered in the circumstellar habitable zone (chz) currently represent the most suitable places to host life as we know it outside the solar system. however, the presumed presence of liquid water on the chz planets does not guarantee suitable environments for the emergence of life. according to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (uv) flux. on the other hand, high uv flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life. these arguments raise questions about the actual habitability of chz planets around stars other than solar-type ones, with different uv to bolometric luminosity ratios. by combining the 'principle of mediocrity' and recent experimental studies, we define uv boundary conditions (uv-habitable zone, uhz) within which life can possibly emerge and evolve. we investigate whether exoplanets discovered in chzs do indeed experience such conditions. by analysing swift-uv/optical telescope data, we measure the near ultraviolet (nuv) luminosities of 17 stars harbouring 23 planets in their chz. we derive an empirical relation between nuv luminosity and stellar effective temperature. we find that 18 of the chz exoplanets actually orbit outside the uhz, i.e. the nuv luminosity of their m-dwarf hosts is decisively too low to trigger abiogenesis - through cyanosulfidic chemistry - on them. only stars with effective temperature ≳3900 k illuminate their chz planets with enough nuv radiation to trigger abiogenesis. alternatively, colder stars would require a high-energy flaring activity. | the ultraviolet habitable zone of exoplanets |
without aerobic life, the simultaneous presence of n2 and o2 in the earth's atmosphere, as on any other planet, would be chemically incompatible over geologic timescales. the existence of an n2-o2-dominated atmosphere on an exoplanet would, hence, not only constitute a strong biosignature of aerobic life. it would also have to meet certain astro- and geophysical conditions to originate, evolve and to sustain.our definition of eta-earth (ηearth), therefore, builds on the concept of a so-called earth-like habitat (eh), i.e., a planet within the complex habitable zone for life, at which n2 and o2 are simultaneously present as the dominant species while co2 only comprises a minor constituent in its atmosphere. by our present scientific knowledge, certain criteria must be fulfilled to allow the existence of such an earth-like atmosphere. these can be subsumed within a new probabilistic formula for estimating a maximum number of ehs which we will present within this talk. some of these criteria, such as the initial mass function, the bolometric luminosity and xuv flux evolution of a star, or the distribution of rocky exoplanets within the habitable zones of different stellar spectral types, are already rather well studied and can be tested through further observations. other important criteria, like the prevalence of working carbon-silicate and nitrogen cycles, or the origin of life are by now poorly, or entirely un-constrained. further factors, like the presence of a large moon or the importance of an intrinsic magnetic field, are not only poorly constrained but its importance for the evolution and stability of an earth-like habitat are even debated. while our new formula for estimating the maximum number of ehs can in principle incorporate all these factors as well as unknowns, we by now must restrict ourselves to the ones that are either well understood or can at least be tested soon. based on our current knowledge, this approach only allows us to probabilistically estimate a maximum number of exoplanets on which an earth-like habitat can in principle evolve. the real number of ehs might, therefore, be significantly lower than our current best estimate but additional criteria should be verifiable in near future by upcoming ground- and space-based instrumentation such as plato, the e-elt, or by the kinds of the proposed space-based observatory luvoir.by considering all the factors that are presently scientifically quantifiable to at least some extent, we will present our current best estimate for the maximum number of ehs that might exist within the galaxy. if we redefine ηearth, the mean number per star of rocky planets within the habitable zone, to only account for the mean number of ehs per star, ηeh, we end up with a number much smaller than current best estimates for ηearth. it is, therefore, scientifically not justified to presume the astrobiological copernican assumption that all potential habitats inside a habitable zone for complex life will evolve similar to earth. | eta earth revisited: how many earth-like habitats might there be in the milky way? |
we are living in an extremely exciting era for exoplanet science: astronomers have detected nearly 5000 confirmed planets and we have begun to understand the links between stellar properties, planet compositions, system architectures, and formation histories. recognizing the compelling nature of the quest for other worlds, the astro2020 decadal survey selected "worlds and suns in context" as the first theme in their scientific vision for the next decade. in my talk, i will review progress along the pathways to habitable worlds and discuss the bright future of exoplanet science. i will begin by reviewing our understanding of planetary demographics and compositions. next, i will take the audience on a tour of three intriguing multi-planet systems: k2-136, toi-1246, and hip 41378. k2-136 is a member of the hyades open cluster and harbors three transiting sub-neptunes, and toi-1246 is a k dwarf hosting four transiting sub-neptunes. i will present mass measurements for the middle planet in the k2-136 system (k2-136 c) and all four transiting planets in the toi-1246 system, thereby adding k2-136 c to the sample of young planets with precise mass measurements and toi-1246 to the small sample of systems for which masses and radii have been measured for at least four planets. hip 41378 is a bright sun-like star hosting at least five transiting planets. i will present our mass measurements for all five planets and our hst/wfc3 transmission spectrum of the outermost planet hip 41378 f, a long-period, saturn-sized world with an extremely low bulk density. finally, i will conclude by looking towards the future and discussing the transformational science that will be driven by the recent launch of jwst and the upcoming launch of roman; the dawn of new ground-based facilities; and the development of a large ir/o/uv space telescope capable of searching for potentially earth-like planets in the habitable zones of over a hundred sun-like stars. | a trio of multiplanet systems and pathways to habitable worlds |
one of the most exciting scientific challenges is to detect earth-like planets in the habitable zones of other stars in the galaxy and search for evidence of life. during the past 20 years the detection of exoplanets, orbiting stars beyond our own, has moved from science fiction to science fact. from the first handful of gas giants, found through radial velocity studies, detection techniques have increased in sensitivity, finding smaller planets and diverse multi-planet systems. through enhanced ground-based spectroscopic observations, transit detection techniques and the enormous productivity of the kepler space mission, the number of confirmed planets has increased to more than 2000. several space missions, including tess (nasa), now operational, and plato (esa), will extend the parameter space for exoplanet discovery towards the regime of rocky earth-like planets and take the census of such bodies in the neighbourhood of the solar system. the ability to observe and characterise dozens of potentially rocky earth-like planets now lies within the realm of possibility due to rapid advances in key space and imaging technologies and active studies of potential missions have been underway for a number of years. the latest of these is the large uv optical ir space telescope (luvoir), one of four flagship mission studies commissioned by nasa in support of the 2020 us decadal survey. luvoir, if selected, will be of interest to a wide scientific community and will be the only telescope capable of searching for and characterizing a sufficient number of exo-earths to provide a meaningful answer to the question "are we alone?". this contribution is a white paper that has been submitted in response to the esa voyage 2050 call. | the search for living worlds and the connection to our cosmic origins |
the "radius valley" is a feature in the short-period, small exoplanet population in kepler and k2 data showing an abundance of super-earths (1-2 r⊕) and mini-neptunes (2-3.5 r⊕), with a relatively scarce population of intermediate-sized planets between the two. by employing updated stellar properties and implementing refined measures of completeness and reliability, we discover that the occurrence of super-earths over that of mini-neptunes has a period and stellar mass dependence. this result is explored in the context of atmospheric loss mechanisms. we use these dependencies to extrapolate the occurrence of earths and super-earths in the habitable zone of fgk stars. finally, we discuss our results in the context of upcoming missions' search for long-period small planets and their dependence on stellar mass. | the population of earths and super-earths into the habitable zone |
m-dwarfs are common stellar hosts of habitable-zone exoplanets. near-uv (nuv) radiation can severely impact the atmospheric and surface conditions of such planets, making the characterization of nuv flaring activity a key aspect in determining habitability. we use archival data from the galaxy evolution explorer (galex) and xmm-newton telescopes to study the flaring activity of m-dwarfs in the nuv. the galex observations form the most extensive data set of m-dwarfs in the nuv to date, with the exploitation of this data possible due to the new gphoton2 pipeline. we run a dedicated algorithm to detect flares in the pipeline-produced lightcurves and find some of the most energetic flares observed to date within the nuv bandpass, with energies of ~1034 erg. using galex data, we constrain flare frequency distributions for stars from m0-m6 in the nuv up to 105 s in equivalent duration and 1034 erg in energy, orders of magnitude above any previous study in the uv. we estimate the combined effect of nuv luminosities and flare rates of stars later than m2 to be sufficient for abiogenesis on habitable-zone exoplanets orbiting them. as a counterpoint, we speculate the high frequencies of energetic uv flares and associated coronal mass ejections would inhibit the formation of an ozone layer, possibly preventing the genesis of complex earth-like life-forms due to sterilizing levels of surface uv radiation. we also provide a framework for future observations of m-dwarfs with ultrasat, a wide field-of-view nuv telescope to be launched in 2026. | a census of near-uv m-dwarf flares using archival galex data and the gphoton2 pipeline |
results from nasa’s kepler mission and other recent exoplanet surveys have demonstrated that potentially habitable exoplanets are relatively common, especially in the case of low-mass stellar hosts. the next key question that must be addressed for such planets is whether or not these worlds are actually habitable, implying they could sustain surface liquid water. only through investigations of the potential habitability of exoplanets and through searches for biosignatures from these planets will we be able to understand if the emergence of life is a common phenomenon in our galaxy. emission spectroscopy for transiting exoplanets (sometimes called secondary eclipse spectroscopy) is a powerful technique that future missions will use to study the atmospheres and surfaces of worlds orbiting in the habitable zones of nearby, low-mass stars. emission observations that span the mid-infrared wavelength range for potentially habitable exoplanets provide opportunities to detect key habitability and life signatures, and also allow observers to probe atmospheric and surface temperatures. this presentation will outline the case for using emission spectroscopy to understand if an exoplanet can sustain surface liquid water, which is believed to be a critical precursor to the origin of life. | characterizing exoplanet habitability with emission spectroscopy |
neid is a nasa & us nsf funded ultra-stable, optical spectrometer designed to achieve radial velocity (rv) precision on the order of 10cm/s. achieving this level of measurement precision requires extreme thermo-mechanical stability within the instrument which we achieve by maintaining a vacuum on the order of microtorr as well as sub-millikelvin temperature stability. in this poster, we will outline neid's environmental control system (ecs) and temperature monitoring and control (tmc) system, which were both inherited and improved upon from that of the habitable-zone planet finder (hpf) infrared spectrograph. we have achieved our target stability by demonstrating < 0.4mk rms temperature variability over the course of a 30 day stability run in the lab. we expect our stability to improve at the observatory as the wiyn instrument room is more stable than our instrument development lab at penn state. neid will be commissioned in fall 2019 at kitt peak national observatory on the 3.5m wiyn telescope. it will serve the exoplanet community as a vital resource for the detection and confirmation of low-mass exoplanets. | design and performance of neid ultra-stable environmental control system |
venus is the most earth-like planet in the solar system in terms of its size, mass, and bulk composition, yet the surface conditions of earth and its sister world could not be more different. venus has the hottest terrestrial surface in the solar system, yet it may once have hosted oceans in its deep past as indicated by its elevated d/h ratio. recent 3-d modeling efforts (way et al. 2016, 2020) indicate that cloud feedbacks driven by venus' slow rotation could enable habitable conditions for billions of years. possibly, our solar system hosted two worlds with habitable surface environments for most of its history, and if planets receiving venus-like levels of insolation can be habitable, this has important implications for the distribution of possible habitable environments elsewhere in the galaxy. stellar tidal forces are likely to induce slow rotation for planets orbiting within and interior to the habitable zones of many low mass m dwarf stars (which represent 75% of all stars), and the same processes studied to enable habitable conditions on early venus have also been applied to these planets (e.g. yang et al. 2013; kopparapu et al. 2016; fujii et al. 2017). if hot, slowly rotating exoplanets are observed to indeed be habitable, this insight may shed light on the processes that operated on venus in the past. indeed, planets receiving venus-like insolation levels are likely to represent the most observable class of terrestrial exoplanets for the upcoming james webb space telescope (jwst; kane et al. 2014). however, transit transmission observations that will be employed by jwst are particularly challenging for venus-like planets, because they cannot probe below a global cloud layer. recently, it has been shown that a planet with a thick atmosphere and a high-altitude cloud deck could appear spectrally similar to a planet with a thin, clear-sky atmosphere (lustig-yaeger et al. 2019). the challenges inherent to studying exo-venus planets will require robust computer models validated and constrained using data from the venusian environment, but there are still many unknowns about this mysterious planet's past and present. no spacecraft has entered the venus atmosphere since the 1980s, and the u.s. has not had a venus mission since magellan (launched 1989). in recognition of the vital role venus can play in shaping our understanding of planetary habitability as a dynamic process that evolves over time, and given that venus-like exoplanets are likely to be observed in the near future, it is crucial to better understand this "exoplanet in our backyard." | venus: the exoplanet in our backyard |
we study the habitability of the two 1.5±0.4 earth-mass planets, recently detected by the carmenes collaboration, around the ultra-cool nearby m dwarf teegarden's star. with orbital periods of 4.9 and 11.4 days, both planets are likely to be within the habitable zone and tidally locked. they are among the most earth-like exoplanets yet discovered. we find (wandel and tal-or 2019, apjl) that one or both planets are likely to support liquid water on at least part of their surface for a wide range of possible atmospheres, characterized by their atmospheric heating factor (i.a. the greenhouse effect) and global circulation. at least one of the tg planets may be habitable for atmospheric heating in the range 0.3-15 that of earth. as demonstrated by detailed numerical calculations of similar planets, teegarden's star present calmness and old age favor the retaining or reproduction of a sufficiently massive atmosphere, within the habitability range. the demonstrated habitability of the teegarden's star planets for a wide range of atmospheres, combined with their small distance (3.85 pc) and highest similarity to earth (esi=0.94) makes them most attractive targets for bio-signature searches. | habitability of the teegarden's star planets |
the habitable zone gallery 2.0 provides new and improved visualization and data analysis tools to the exoplanet habitability community and beyond. modules include interactive habitable zone plotting and downloadable 3d animations. | the habitable zone gallery 2.0: the online exoplanet system visualization suite |
recent studies indicate that habitable zone planets around sun-like stars can be tidally-locked (barnes et al. 2017). it is plausible that earth-like planets around g-dwarfs, which are the primary exoplanet targets for future nasa concept missions like the habitable exoplanet imaging mission (habex) & the large ultra-violet optical infra-red (luvoir) surveyor, have slower rotational periods than the earth. we present results from 3-d climate model (rocke-3d) simulations of an earth-like planet around a sun-like star, varying the rotational period between 1-day to 365-days. from these simulations, we generate reflectance spectra as a function of (monthly) orbital phase over one year for each case, and compare it with the standard earth spectrum. the reduced rotation period broadens the hadley circulation, eliminates the characteristic westerly jet streams, and enhances clouds and precipitation around the planet (yang et al. 2014). in some cases, the characteristic upwards rayleigh scattering slope seen in the standard 1-day rotation earth spectra at short visible wavelengths reduces significantly with orbital phase. this could be due to high-altitude clouds reaching optical depth unity for downwelling stellar radiation before rayleigh scattering reaches an optical depth of unity. this gives a less pronounced rayleigh peak that appears to have the same grey opacity as the continuum out to longer wavelengths. our results show that the rotation period has a significant effect on the reflectance spectra of earth-like planets around solar-type stars, and direct imaging spectral characterization missions should consider the likely possibility that present-day earth-like planets may not be pale-blue dots. | the impact of planetary rotation rate on the reflectance spectrum of terrestrial exoplanets around sun-like stars |
saltus (single aperture large telescope for universe studies) is a probe mission concept that employs a radiatively cooled, 20 meter inflatable aperture and cryogenic detectors to better understand our cosmic origins and the possibility of life elsewhere. the science objectives of saltus are to: 1) trace astrochemical signatures of planet formation, habitable zones, and life. 2) trace galaxy evolution and heavy element production over cosmic time. 3) perform a census and probe the structure of supermassive black holes. to achieve these objectives saltus will perform spectroscopic studies towards 100's of exoplanets, protoplanetary disks, galaxies, and solar system objects over a wavelength range from ~30 to 300 microns at high and moderate spectral resolution. the focal plane will include both coherent and incoherent detectors cooled by a closed-cycle refrigeration system. the telescope will employ a sunshield and be radiatively cooled to ~45k. saltus will have ~10x the collecting area of jwst and ~30x that of herschel. the 20 meter aperture of saltus will be used as a space node for millimeter-wave vlbi observations of massive black holes in the milky way and beyond. the mission is designed to provide a lifetime of > 5 years. more than 50% of its on-orbit time will be dedicated to the saltus guest observer (go) program. with its large aperture and suite of instruments, saltus will provide a quantum leap in our understanding of the universe. | single aperture large telescope for universe studies (saltus) |
in this thesis, we investigated the extreme precision radial velocity (eprv) problem by solving both the instrumental systematics and the stellar activity contaminations with the purpose of de- tecting earth-like planets orbiting within to the habitable zones of sun-like stars. we developed several pieces of codes dedicated to large datasets processing as the historical harps and harps- n archive databases. those tools contain: 1) a python code to normalise the stellar spectra in a fast and efficient way, 2) a methodology to remove instrumental systematics and telluric line contami- nation using the spectral time-series and the stationarity of the features into the earth rest-frame, 3) an analysis of the correlation between stellar activity signals and atomic lines properties 4) a new methodology to extract activity proxy and doppler shift in template matching canvas and 5) a new methodology to extract proxies in the time-domain. all those tools are part of a single reduc- tion cascade which begins from the 1d order-merged spectra down to the corrected rvs. dozens of new signals were detected from the harps and harps-n databases beginning to populate the regime of periods longer than 60 days and mass smaller than 10 m⊙. on six years of harps-n solar observations, residual rvs of 80 cm/s root mean square were obtained with no residual periodic signals larger than 25 cm/s, which would correspond to a planet of 3 m⊙ at the location of earth. the results obtained within this phd demonstrates that with current observing rv facilities, it is possible to find planets as low as 2 m⊙ on a period of 150 days, which would correspond to the habitable zone of a k dwarfs, if enough data are available and the stars show an activity level not more than the sun. | improving the rv precision to detect the earth's twins around the sun's siblings |
in this work, we consider the potential of cometary impacts to deliver complex organic molecules and the prebiotic building blocks required for life to rocky exoplanets. numerical experiments have demonstrated that for these molecules to survive, impacts at very low velocities are required. this work shows that for comets scattered from beyond the snow-line into the habitable zone, the minimum impact velocity is always lower for planets orbiting solar-type stars than m-dwarfs. using both an analytical model and numerical n-body simulations, we show that the lowest velocity impacts occur onto planets in tightly packed planetary systems around high-mass (i.e. solar-mass) stars, enabling the intact delivery of complex organic molecules. impacts onto planets around low-mass stars are found to be very sensitive to the planetary architecture, with the survival of complex prebiotic molecules potentially impossible in loosely packed systems. rocky planets around m-dwarfs also suffer significantly more high velocity impacts, potentially posing unique challenges for life on these planets. in the scenario that cometary delivery is important for the origins of life, this study predicts the presence of biosignatures will be correlated with (i) decreasing planetary mass (i.e. escape velocity), (ii) increasing stellar-mass and (iii) decreasing planetary separation (i.e. exoplanets in tightly-packed systems). | can comets deliver prebiotic molecules to rocky exoplanets? |
the temperate rocky exoplanets that will be best characterized with future observations orbit m-dwarf stars. due to their close-in orbits, these planets are expected to be tidally locked to their host stars, with large day-night temperature and moisture contrasts and climates that are significantly different from that of earth. hurricanes can cause emergent variability in cloud patterns and atmospheric moisture content, and if present may affect future observations of terrestrial exoplanets. in this work, we explore whether hurricanes can form on tidally locked terrestrial exoplanets. we use earth-based metrics for the favorability of hurricane genesis (i.e., formation) to study whether the environmental conditions in three-dimensional aquaplanet simulations of terrestrial exoplanets allow for hurricane genesis. we study a wide range of planetary parameters, including rotation period, incident stellar flux, planetary radius, surface gravity, surface pressure, and host star type. we find that hurricanes can form over a wide range of rotation period, surface gravity, surface pressure, and incident stellar flux. notably, we find that for an incident stellar flux similar to that of earth, hurricane genesis is most favorable on planets with intermediate rotation periods of 8-10 days. this is because the effect of decreasing absolute vorticity due to decreased planetary spin with increasing rotation period causes hurricane genesis to be less favorable at longer rotation periods. additionally, increasing mid-level ventilation and wind shear with decreasing rotation period cause hurricane genesis to be less favorable at shorter rotation periods. as a result, we find that hurricane genesis can be favorable on planets within the habitable zones of late-type m dwarf stars. such planets orbiting small red dwarf stars are some of the most accessible to telescopic observations, which may be able to detect variability in cloud cover or stratospheric water mixing ratio due to hurricanes. | favorability of hurricane genesis on tidally locked exoplanets orbiting m-dwarf stars |
to date, nearly all multi-wavelength modeling of long-duration γ-ray bursts has ignored synchrotron radiation from the significant population of electrons expected to pass the shock without acceleration into a power-law distribution. we investigate the effect of including the contribution of thermal, non-accelerated electrons to synchrotron absorption and emission in the standard afterglow model, and show that these thermal electrons provide an additional source of opacity to synchrotron self-absorption, and yield an additional emission component at higher energies. the extra opacity results in an increase in the synchrotron self-absorption frequency by factors of 10-100 for fiducial parameters. the nature of the additional emission depends on the details of the thermal population, but is generally observed to yield a spectral peak in the optical brighter than radiation from the nonthermal population by similar factors a few seconds after the burst, remaining detectable at millimeter and radio frequencies several days later. | thermal electrons in gamma-ray burst afterglows |
grb 190829a at z = 0.0785 is the fourth closest long grb ever detected by the neil gehrels swift observatory, and the third confirmed case with a very high-energy component. we present our multiwavelength analysis of this rare event, focusing on its early stages of evolution, and including data from swift, the master global network of optical telescopes, alma, and atca. we report sensitive limits on the linear polarization of the optical emission, disfavouring models of off-axis jets to explain the delayed afterglow peak. the study of the multiwavelength light curves and broad-band spectra supports a model with at least two emission components: a bright reverse shock emission, visible at early times in the optical and x-rays and, later, in the radio band; and a forward shock component dominating at later times and lower radio frequencies. a combined study of the prompt and afterglow properties shows many similarities with cosmological long grbs, suggesting that grb 190829a is an example of classical grbs in the nearby universe. | the early afterglow of grb 190829a |
the nearest grb 170817a provided an opportunity to probe the angular structure of the jet of this short gamma-ray burst (sgrb), by using its off-axis observed afterglow emission. it is investigated that whether the afterglow-constrained jet structures can be consistent with the luminosity of the prompt emission of grb 170817a. furthermore, by assuming that all sgrbs including grb 170817a have the same explosive mechanism and jet structure, we apply the different jet structures into the calculation of the flux and redshfit distributions of the sgrb population, in comparison with the observational distributions of the swift and fermi sources. as a result, it is found that the single-gaussian structure can be basically ruled out, whereas the power-law and two-gaussian models can in principle survive. | joint constraint on the jet structure from the short grb population and grb 170817a |
motivated by the detection of very-high-energy (vhe) gamma rays deep in the afterglow emission of a gamma-ray burst (grb), we revisit predictions of the maximum energy to which electrons can be accelerated at a relativistic blast wave. acceleration at the weakly magnetized forward shock of a blast wave can be limited by either the rapid damping of turbulence generated behind the shock, the effect of a large-scale ambient magnetic field, or radiation losses. within the confines of a standard, single-zone, synchrotron self-compton (ssc) model, we show that observations of grb 190829a rule out a rapid damping of the downstream turbulence. furthermore, simultaneous fits to the x-ray and tev gamma-ray emission of this object are not possible unless the limit on acceleration imposed by the ambient magnetic field is comparable to or weaker than that imposed by radiation losses. this requires the dominant length scale of the turbulence behind the shock to be larger than that implied by particle-in-cell simulations. however, even then, klein-nishina effects prevent production of the hard vhe gamma-ray spectrum suggested by observations. thus, tev observations of grb afterglows, though still very sparse, are already in tension with the ssc emission scenario. | the implications of tev-detected grb afterglows for acceleration at relativistic shocks |
we report the first detection of radio polarization of a gamma-ray burst (grb) afterglow with the first intensive combined use of telescopes in the millimeter and submillimeter ranges for grb 171205a. the linear polarization degree in the millimeter band at the subpercent level (0.27% ± 0.04%) is lower than those observed in late-time optical afterglows (weighted average of ∼1%). the faraday depolarization by nonaccelerated, cool electrons in the shocked region is one of the possible mechanisms for the low value. this scenario requires a total energy that is larger by a factor of ∼10 than ordinary estimates without considering nonaccelerated electrons. the polarization position angle varies by at least 20° across the millimeter band, which is not inconsistent with this scenario. this result indicates that polarimetry in the millimeter and submillimeter ranges is a unique tool for investigating grb energetics, and coincident observations with multiple frequencies or bands would provide more accurate measurements of the nonaccelerated electron fraction. | first detection of radio linear polarization in a gamma-ray burst afterglow |
well-sampled optical light curves of 50 gamma-ray bursts (grbs) with plateau features are compiled from the literature. by empirical fitting, we obtained the parameters of the optical plateaus, such as the decay slopes (α 1 and α 2), the break times (t b), and the corresponding optical fluxes (f b) at the break times. the break time of optical plateaus ranges from tens of seconds to 106 s, with a typical value of about 104 s. we have calculated the break luminosity, and it mainly ranges from 1044 erg s-1 to 1047 erg s-1, which is generally two or three orders of magnitude less than the corresponding break luminosity of the x-ray afterglow plateaus. we reanalyzed the optical plateaus and also found that a significantly tighter correlation exists when we added the isotropic equivalent energy of grbs eγ,iso into the l b,z-t b,z relation. the best-fit correlation is obtained to be {l}{{b},{{z}}}\propto {t}{{b},{{z}}}-0.9 {e}γ ,{iso}0.4. we next explored the possible correlations among l b,z, t b,z and e p,i, and found there is also a tight correlation between them, which takes the form of {l}{{b},{{z}}}\propto {t}{{b},{{z}}}-0.9 {e}{{p},{{i}}}0.5. we argue that these two tight l b,z-t b,z-eγ,iso and l b,z-t b,z-e p,i correlations are more physical, and it may be directly related to the radiation physics of grbs. the tight correlations can possibly be used as standard candles. | the three-parameter correlations about the optical plateaus of gamma-ray bursts |
we present multi-wavelength follow-up campaigns by the astrosat czti and growth collaborations in search of an electromagnetic counterpart to the gravitational wave event gw 170104. at the time of the gw 170104 trigger, the astrosat czti field of view covered 50.3% of the sky localization. we do not detect any hard x-ray (>100 kev) signal at this time, and place an upper limit of ≈ 4.5× {10}-7 {erg} {cm}}-2 {{{s}}}-1, for a 1 s timescale. separately, the atlas survey reported a rapidly fading optical source dubbed atlas17aeu in the error circle of gw 170104. our panchromatic investigation of atlas17aeu shows that it is the afterglow of an unrelated long, soft grb 170105a, with only a fortuitous spatial coincidence with gw 170104. we then discuss the properties of this transient in the context of standard long grb afterglow models. | a tale of two transients: gw 170104 and grb 170105a |
the afterglow emission from gamma-ray bursts (grbs) is believed to originate from a relativistic blast wave driven into the circumburst medium. although the afterglow emission from radio up to x-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (he; ≳gev) remains uncertain. the recent detection of sub-tev emission from grb 190114c by the major atmospheric gamma imaging cherenkov telescopes (magic) raises further debate on what powers the very high energy (vhe; ≳300 gev) emission. here, we explore the inverse compton scenario as a candidate for the he and vhe emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-compton or ssc) and isotropic photon fields external to the blast wave (external compton). for each case, we compute the multiwavelength afterglow spectra and light curves. we find that ssc will dominate particle cooling and the gev emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. additionally, considering the extragalactic background light attenuation, we discuss the detectability of vhe afterglows by existing and future gamma-ray instruments for a wide range of model parameters. studying grb 190114c, we find that its afterglow emission in the fermi-large area telescope (lat) band is synchrotron dominated. the late-time fermi-lat measurement (i.e. t ∼ 104 s), and the magic observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$ ), making the inverse compton dominant in the sub-tev energies. | inverse compton signatures of gamma-ray burst afterglows |
an important constraint for galaxy evolution models is how much gas resides in galaxies, in particular, at the peak of star formation z = 1-3. we attempt a novel approach by letting long-duration gamma ray bursts (lgrbs) x-ray their host galaxies and deliver column densities to us. this requires a good understanding of the obscurer and biases introduced by incomplete follow-up observations. we analyse the x-ray afterglow of all 844 swift lgrbs to date for their column density nh. to derive the population properties, we propagate all uncertainties in a consistent bayesian methodology. the nh distribution covers the 1020-23 cm-2 range and shows no evolutionary effect. higher obscurations, e.g. compton-thick columns, could have been detected but are not observed. the nh distribution is consistent with sources randomly populating a ellipsoidal gas cloud of major axis {n^{major}h }=10^{23}cm^{-2} with 0.22 dex intrinsic scatter between objects. the unbiased shoals survey of afterglows and hosts allows us to constrain the relation between spitzer-derived stellar masses and x-ray derived column densities nh. we find a well-constrained power-law relation of nh = 1021.7 cm-2 × (m⋆/109.5 m⊙)1/3, with 0.5 dex intrinsic scatter between objects. the milky way and the magellanic clouds also follow this relation. from the geometry of the obscurer, its stellar mass dependence and comparison with local galaxies, we conclude that lgrbs are primarily obscured by galaxy-scale gas. ray tracing of simulated illustris galaxies reveals a relation of the same normalization, but a steeper stellar-mass dependence and mild redshift evolution. our new approach provides valuable insight into the gas residing in high-redshift galaxies. | galaxy gas as obscurer - i. grbs x-ray galaxies and find an nh3∝ m_{star} relation |
at 2022cmc is a luminous optical transient (νlν ≳ 1045 erg s-1) accompanied by decaying non-thermal x-rays (peak duration tx ≲ days and isotropic energy ex,iso ≳ 1053 erg) and a long-lived radio/mm synchrotron afterglow, which has been interpreted as a jetted tidal disruption event (tde). both an equipartition analysis and a detailed afterglow model reveal the radio/mm emitting plasma to be expanding mildly relativistically (lorentz factor $\gamma \gtrsim \, \mathrm{ few}$ ) with an opening angle θj ≃ 0.1 and roughly fixed energy ej,iso ≳ few × 1053 erg into an external medium of density profile n ∝ r-k with k ≃ 1.5-2, broadly similar to that of the first jetted tde candidate swift j1644+57 and consistent with bondi accretion at a rate of ~$10^{-3}\,\dot{m}_{\rm edd}$ on to a 106 m⊙ black hole before the outburst. the rapidly decaying optical emission over the first days is consistent with fast-cooling synchrotron radiation from the same forward shock as the radio/mm emission, while the bluer slowly decaying phase to follow likely represents a separate thermal emission component. emission from the reverse shock may have peaked during the first days, but its non-detection in the optical band places an upper bound γj ≲ 100 on the lorentz factor of the unshocked jet. although a tde origin for at 2022cmc is indeed supported by some observations, the vast difference between the short-lived jet activity phase tx ≲ days and the months-long thermal optical emission also challenges this scenario. a stellar core-collapse event giving birth to a magnetar or black hole engine of peak duration ~1 d offers an alternative model also consistent with the circumburst environment, if interpreted as a massive star wind. | synchrotron afterglow model for at 2022cmc: jetted tidal disruption event or engine-powered supernova? |
we present a spectropolarimetric analysis of grb 171010a using data from astrosat, fermi, and swift to provide insights into the physical mechanisms of the prompt radiation and jet geometry. the prompt emission from grb 171010a was very bright (fluence >10-4 erg cm-2) and had a complex structure that was composed of the superimposition of several pulses. the energy spectra deviate from the typical band function to show a low-energy peak ∼15 kev, which we interpret as a power law with two breaks, with a synchrotron origin. alternatively, the prompt spectra can also be interpreted as comptonized emission, or a blackbody combined with a band function. a time-resolved analysis confirms the presence of the low-energy component, while the peak energy is found to be confined in the range of 100-200 kev. the afterglow emission detected by fermi-lat is typical of an external shock model, and we constrain the initial lorentz factor using the peak time of the emission. swift-xrt measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to <6°. the detection of a large number of compton-scattered events by astrosat-czti provides an opportunity to study the hard x-ray polarization of the prompt emission. we find that the burst has a high time-variable polarization, and the emission has a higher polarization at energies above the peak energy. we discuss all observations in the context of grb models and polarization arising as a result of physical or geometric effects: synchrotron emission from multiple shocks with ordered or random magnetic fields, a jet dominated by poynting flux that undergoes abrupt magnetic dissipation, subphotospheric dissipation, a jet consisting of fragmented fireballs, and the comptonization model. | astrosat-czti detection of variable prompt emission polarization in grb 171010a |
based on the early-year observations from neil gehrels swift observatory, liang et al. performed a systematic analysis for the shallow decay component of gamma-ray bursts (grbs) x-ray afterglow, in order to explore its physical origin. here we revisit the analysis with an updated sample (with swift/xrt grbs between 2004 february and 2017 july). we find that with a larger sample, (1) the distributions of the characteristic properties of the shallow decay phase (e.g., tb , s x, γx,1, and α x,1) still accord with normal or lognormal distribution; (2) γx,1 and γ γstill show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and x-ray phases still exist; (3) for most grbs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; (4) assuming that the central engine has a power-law luminosity release history as l≤ft(t\right)={l}0{≤ft(\tfrac{t}{{t}0}\right)}-q, we find that the value q is mainly distributed between -0.5 and 0.5, with an average value of 0.16 ± 0.12 (5) the tentative correlation between {e}iso,{{x}}} and {t}b{\prime } disappears, so that the global three-parameter correlation ({e}iso,{{x}}}-{e}p{\prime }-{t}b{\prime }) becomes less significant; (6) the anticorrelation between l x and {t}b{\prime } and the three-parameter correlation ({e}iso,γ }-{l}{{x}}-{t}b) indeed exist with a high confidence level. overall, our results are generally consistent with liang et al., confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine. | the shallow decay segment of grb x-ray afterglow revisited |
we consider here a proton-synchrotron model to explain the magic observation of grb 190114c afterglow in the energy band of 0.2-1 tev, while the x-ray spectra are explained by electron-synchrotron emission. given the uncertainty of the particle acceleration process, we consider several variations of the model, and show that they all match the data very well. we find that the values of the uncertain model parameters are reasonable: isotropic explosion energy ~1054.5 erg, ambient density ~10-100 cm-3, and the fraction of electrons/protons accelerated to a high-energy power law is of a few percent. all these values are directly derived from the observed teraelectronvolt and x-ray fluxes. the model also requires that protons be accelerated to observed energies as high as a few 1020 ev. further, assuming that the jet break takes place after 106 s gives the beaming-corrected energy of the burst to be ≈1053 erg, which is one to two orders of magnitude higher than usually inferred, because of the small fraction of electrons accelerated. our modeling is consistent with both late time data at all bands, from optical to x-rays, and with numerical models of particle acceleration. our results thus demonstrate the relevance of proton-synchrotron emission to the high-energy observations of gamma-ray bursts during their afterglow phase. | proton synchrotron origin of the very-high-energy emission of grb 190114c |
gamma-ray bursts (grbs) are fascinating extragalactic objects. they represent a fantastic opportunity to investigate unique properties not exhibited in other sources. multiwavelength afterglow observations from some short- and long-duration grbs reveal an atypical long-lasting emission that evolves differently from the canonical afterglow light curves favoring the off-axis emission. we present an analytical synchrotron afterglow scenario and the hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified surrounding environment. the analytical synchrotron afterglow model is shown during the coasting, deceleration (off- and on-axis emission), and post-jet break decay phases, and the hydrodynamical evolution is computed by numerical simulations showing the time evolution of the doppler factor, the half-opening angle, the bulk lorentz factor, and the deceleration radius. we show that numerical simulations are in good agreement with those derived with our analytical approach. we apply the current synchrotron model and successfully describe the delayed nonthermal emission observed in a sample of long and short grbs with evidence of off-axis emission. furthermore, we provide constraints on the possible afterglow emission by requiring the multiwavelength upper limits derived for the closest swift-detected grbs and promising gravitational-wave events. | modeling gamma-ray burst afterglow observations with an off-axis jet emission |
gamma-ray bursts (grbs) are detectable in the γ-ray band if their jets are oriented toward the observer. however, for each grb with a typical θjet, there should be ~2/θ2jet bursts whose emission cone is oriented elsewhere in space. these off-axis bursts can eventually be detected when, due to the deceleration of their relativistic jets, the beaming angle becomes comparable to the viewing angle. orphan afterglows (oas) should outnumber the current population of bursts detected in the γ-ray band even if they have not been conclusively observed so far at any frequency. we compute the expected flux of the population of orphan afterglows in the mm, optical, and x-ray bands through a population synthesis code of grbs and the standard afterglow emission model. we estimate the detection rate of oas with ongoing and forthcoming surveys. the average duration of oas as transients above a given limiting flux is derived and described with analytical expressions: in general oas should appear as daily transients in optical surveys and as monthly/yearly transients in the mm/radio band. we find that ~2 oa yr-1 could already be detected by gaia and up to 20 oa yr-1 could be observed by the ztf survey. a larger number of 50 oa yr-1 should be detected by lsst in the optical band. for the x-ray band, ~26 oa yr-1 could be detected by the erosita. for the large population of oa detectable by lsst, the x-ray and optical follow up of the light curve (for the brightest cases) and/or the extensive follow up of their emission in the mm and radio band could be the key to disentangling their grb nature from other extragalactic transients of comparable flux density. | unveiling the population of orphan γ-ray bursts |
we present the discovery of the fading radio transient first j153350.8+272729. the source had a maximum observed 5 ghz radio luminosity of 8 × 1039 erg s-1 in 1986, but by 2019 had faded by a factor of nearly 400. it is located at the center of a galaxy (sdss j153350.89+272729) at 147 mpc, which shows weak type ii seyfert activity. we show that a tidal disruption event (tde) is the preferred scenario for first j153350.8+272729, although it could plausibly be interpreted as the afterglow of a long-duration γ-ray burst. this is only the second tde candidate to be first discovered at radio wavelengths. its luminosity fills a gap between the radio afterglows of subrelativistic tdes in the local universe, and relativistic tdes at high redshifts. the unusual properties of first j153350.8+272729 (ongoing nuclear activity in the host galaxy, high radio luminosity) motivate more extensive tde searches in untargeted radio surveys. | first j153350.8+272729: the radio afterglow of a decades-old tidal disruption event |
gamma-ray bursts (grbs) are the most powerful explosions in the universe. how efficiently the jet converts its energy to radiation is a long-standing problem, which is poorly constrained. the standard model invokes a relativistic fireball with a bright photosphere emission component. a definitive diagnosis of grb radiation components and the measurement of grb radiative efficiency require prompt emission and afterglow data, with high resolution and wide band coverage in time and energy. here, we present a comprehensive temporal and spectral analysis of the tev-emitting bright grb 190114c. its fluence is one of the highest for all the grbs that have been detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolutions, down to a timescale of about 0.1 s. we observe that each of the initial pulses has a thermal component contributing ~20% of the total energy and that the corresponding temperature and inferred lorentz factor of the photosphere evolve following broken power-law shapes. from the observation of the nonthermal spectra and the light curve, the onset of the afterglow corresponding to the deceleration of the fireball is considered to start at ~6 s. by incorporating the thermal and nonthermal observations, as well as the photosphere and synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters, measuring a ~16% radiative efficiency for this grb. with the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data. | a cosmological fireball with 16% gamma-ray radiative efficiency |
observations of tev emission from early gamma-ray burst (grb) afterglows revealed the long sought for inverse compton (ic) upscattering of the lower energy synchrotron. however, it turned out that the long hoped for ability to easily interpret the synchrotron self-compton (ssc) spectra didn't materialize. the tev emission is in the klein-nishina (kn) regime and the simple thomson regime ssc spectrum is modified, complicating the scene. we describe here a methodology to obtain an analytic approximation to an observed spectrum and infer the conditions at the emitting region. the methodology is general and can be used in any such source. as a test case we apply it to the observations of grb 190114c. we find that the procedure of fitting the model parameters using the analytic ssc spectrum suffers from some generic problems. however, at the same time, it conveniently gives a useful insight into the conditions that shape the spectrum. once we introduce a correction to the standard kn approximation, the best-fitting solution is consistent with the one found in detailed numerical simulations. as in the numerical analysis, we find a family of solutions that provide a good approximation to the data and satisfy roughly b ∝ γ-3 between the magnetic field and the bulk lorentz factor, and we provide a tentative explanation why such a family arises. | analytic modelling of synchrotron self-compton spectra: application to grb 190114c |
we present an implementation of a self-consistent way of modelling synchrotron self-compton (ssc) effects in gamma-ray burst afterglows, with and without approximated klein-nishina suppressed scattering for the afterglow modelling code boxfit, which is currently based on pure synchrotron emission. we discuss the changes in spectral shape and evolution due to ssc effects, and comment on how these changes affect physical parameters derived from broad-band modelling. we show that ssc effects can have a profound impact on the shape of the x-ray light curve using simulations including these effects. this leads to data that cannot be simultaneously fit well in both the x-ray and radio bands when considering synchrotron-only fits, and an inability to recover the correct physical parameters, with some fitted parameters deviating orders of magnitude from the simulated input parameters. this may have a significant impact on the physical parameter distributions based on previous broad-band modelling efforts. | modelling synchrotron self-compton and klein-nishina effects in gamma-ray burst afterglows |
a newly born magnetar is thought to be central engine of some long gamma-ray bursts (grbs). we investigate the evolution of the electromagnetic (em) emission from the magnetic dipole (md) radiation wind injected by spin-down of a newly born magnetar via both quadrupole gravitational-wave (gw) and md radiations. we show that the em luminosity evolves as lem ∝ (1 + t/τc)α, and α is -1 and -2 in the gw and md radiation dominated scenarios, respectively. transition from the gw to md radiation dominated epoch may show up as a smooth break with slope changing from -1 to -2. if the magnetar collapses to a black hole before τc, the md radiation should be shut down, then the em light curve should be a plateau followed by a sharp drop. the expected generic light curve in this paradigm is consistent with the canonical x-ray light curve of swift long grbs. the x-ray emission of several long grbs is identified and interpreted as magnetar spin-down via gw or md, as well as constrains the physical parameters of magnetar. the combination of md emission and grb afterglows may make the diversity of the observed x-ray light curves. this may interpret the observed chromatic behaviours of the x-ray and optical afterglow light curves and the extremely low detection rate of a jet-like break in the x-ray afterglow light curves of long grbs. | electromagnetic emission from newly born magnetar spin-down by gravitational-wave and magnetic dipole radiations |
within the framework of the external shock model of gamma-ray burst (grb) afterglows, we perform a morphological analysis of the early-optical light curves to directly constrain model parameters. we define four morphological types, i.e., the reverse shock-dominated cases with/without the emergence of the forward shock peak (type i/type ii), and the forward shock-dominated cases without/with νm crossing the band (type iii/iv). we systematically investigate all of the swift grbs that have optical detection earlier than 500 s and find 3/63 type i bursts (4.8%), 12/63 type ii bursts (19.0%), 30/63 type iii bursts (47.6%), 8/63 type iv bursts (12.7%), and 10/63 type iii/iv bursts (15.9%). we perform monte carlo simulations to constrain model parameters in order to reproduce the observations. we find that the favored value of the magnetic equipartition parameter in the forward shock ({ɛ }b{{f}}) ranges from 10-6 to 10-2, and the reverse-to-forward ratio of ɛb ({{r}}b) is about 100. the preferred electron equipartition parameter {ɛ }{{e}}{{r},{{f}}} value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. this could mitigate the so-called “efficiency problem” for the internal shock model, if ɛe during the prompt emission phase (in the internal shocks) is large (say, ∼0.1). the preferred {{r}}b value is in agreement with the results in previous works that indicate a moderately magnetized baryonic jet for grbs. | a morphological analysis of gamma-ray burst early-optical afterglows |
distinct x-ray plateau and flare phases have been observed in the afterglows of gamma-ray bursts (grbs), and most of them should be related to central engine activities. in this paper, we collect 174 grbs with x-ray plateau phases and 106 grbs with x-ray flares. there are 51 grbs that overlap in the two selected samples. we analyze the distributions of the proportions of the plateau energy e plateau and the flare energy ${e}_{\mathrm{flare}}$ relative to the isotropic prompt emission energy eγ,iso. the results indicate that they well meet the gaussian distributions and the medians of the logarithmic ratios are ~-0.96 and -1.39 in the two cases. moreover, strong positive correlations between e plateau (or ${e}_{\mathrm{flare}}$ ) and eγ,iso with slopes of ~0.95 (or ~0.80) are presented. for the overlapping sample, the slope is ~0.80. we argue that most of x-ray plateaus and flares might have the same physical origin but appear with different features because of the different circumstances and radiation mechanisms. we also test the applicabilities of two models, i.e., black holes surrounded by fractured hyperaccretion disks and millisecond magnetars, on the origins of x-ray plateaus and flares. | statistical analyses of the energies of x-ray plateaus and flares in gamma-ray bursts |
in recent years, dynamical relativistic jet simulation techniques have progressed to a point where it is becoming possible to fully numerically resolve gamma-ray burst (grb) blast-wave evolution across scales. however, the modelling of emission is currently lagging behind and limits our efforts to fully interpret the physics of grbs. in this work we combine recent developments in moving-mesh relativistic dynamics with a local treatment of non-thermal emission in a new code: gamma. the code involves an arbitrary lagrangian-eulerian approach only in the dominant direction of fluid motion that avoids mesh entanglement and associated computational costs. shock detection, particle injection, and local calculation of their evolution including radiative cooling are done at runtime. even though gamma has been designed with grb physics applications in mind, it is modular such that new solvers and geometries can be implemented easily with a wide range of potential applications. in this paper, we demonstrate the validity of our approach and compute accurate broad-band grb afterglow radiation from early to late times. our results show that the spectral cooling break shifts by a factor of ~40 compared to existing methods. its temporal behaviour also significantly changes from the previously calculated temporary steep increase after the jet break. instead, we find that the cooling break does not shift with time between the relativistic and newtonian asymptotes when computed from our local algorithm. gamma is publicly available at: https://github.com/eliotayache/gamma. | gamma: a new method for modelling relativistic hydrodynamics and non-thermal emission on a moving mesh |
we extend previous work on gamma-ray burst afterglows involving hot thermal electrons at the base of a shock-accelerated tail. using a physically motivated electron distribution based on first-principles simulations, we compute the broadband emission from radio to tev gamma rays. for the first time, we present the effects of a thermal distribution of electrons on synchrotron self-compton emission. the presence of thermal electrons causes temporal and spectral structure across the entire observable afterglow, which is substantively different from models that assume a pure power-law distribution for the electrons. we show that early-time tev emission is enhanced by more than an order of magnitude for our fiducial parameters, with a time-varying spectral index that does not occur for a pure power law of electrons. we further show that the x-ray closure relations take a very different, also time-dependent, form when thermal electrons are present; the shape traced out by the x-ray afterglows is a qualitative match to observations of the traditional decay phase. | a semianalytic afterglow with thermal electrons and synchrotron self-compton emission |
context. the origin of the prompt emission of gamma-ray bursts is highly debated. proposed scenarios involve various dissipation processes (shocks, magnetic reconnection, and inelastic collisions) above or below the photosphere of an ultra-relativistic outflow.aims: we search for observational features that could help to favour one scenario over the others by constraining the dissipation radius, the magnetization of the outflow, or by indicating the presence of shocks. bursts showing peculiar behaviours can emphasize the role of a specific physical ingredient, which becomes more apparent under certain circumstances.methods: we study grb 070110, which exhibited several remarkable features during its early afterglow; i.e. a very flat plateau terminated by an extremely steep drop and immediately followed by a bump. we modelled the plateau as the photospheric emission from a long-lasting outflow of moderate lorentz factor (γ 20), which lags behind an ultra-relativistic (γ > 100) ejecta that is responsible for the prompt emission. we computed the dissipation of energy in the forward and reverse shocks resulting from the deceleration of this ejecta by the external medium (uniform or stellar wind).results: we find that photospheric emission from the long-lasting outflow can account for the plateau properties (luminosity and spectrum) assuming that some dissipation takes place in the flow. the geometrical timescale at the photospheric radius is so short that the observed decline at the end of the plateau likely corresponds to the actual shutdown of the activity in the central engine. the bump that follows results from the power dissipated in the reverse shock, which develops when the material making the plateau catches up with the initially fast shell in front, after the fast shell has decelerated.conclusions: the proposed interpretation suggests that the prompt phase results from dissipation above the photosphere while the plateau has a photospheric origin. if the bump is produced by the reverse shock, it implies an upper limit (σ ≲ 0.1) on the magnetization of the low γ material making the plateau. a plateau that is terminated by a drop as steep as in grb 070110 was not observed in any other long burst. it could mean that persistent outflows are very uncommon or that the plateau luminosity or the energy of the emitted photons are generally much lower because the outflow remains mostly adiabatic or has a lorentz factor below 10. | what can we learn from "internal plateaus"? the peculiar afterglow of grb 070110 |
the nature of the gamma-ray burst (grb) central engine still remains an enigma. entities widely believed to be capable of powering the extreme jets are magnetars and black holes. the maximum rotational energy that is available in a millisecond magnetar to form a jet is ∼1052 erg. we identify eight long grbs whose jet-opening angle-corrected energetics of the prompt emission episode are >1052 erg with high confidence level and, therefore, their central engines are expected to be black holes. the majority of these grbs present significant emission in the sub-gev energy range. the x-ray afterglow light curves of these bursts do not show any shallow decay behavior such as a plateau; however, a few cases exhibit flares and multiple breaks instead of a single power-law decay. for a minimum mass of the black hole (∼2 m⊙), we find the efficiency of producing a jet from its rotational energy to range between 2% and 270%. highly energetic jets requiring high efficiencies implies that either the mass of these black holes are much larger or there are, in addition, other sources of energy that power the jet. by considering the blandford-znajek mechanism of jet formation, we estimate the masses of these black holes to range between ∼2 and 60 m⊙. some of the lighter black holes formed in these catastrophic events are likely candidates to lie in the mass-gap region (2-5 m⊙). | identifying black hole central engines in gamma-ray bursts |
the spin-down energy of millisecond magnetars has been invoked to explain x-ray afterglow observations of a significant fraction of short and long gamma-ray bursts. here, we extend models previously introduced in the literature, incorporating radiative losses with the spin-down of a magnetar central engine through an arbitrary braking index. combining this with a model for the tail of the prompt emission, we show that our model can better explain the data than millisecond-magnetar models without radiative losses or those that invoke spin-down solely through vacuum dipole radiation. we find that our model predicts a subset of x-ray flares seen in some gamma-ray bursts. we can further explain the diversity of x-ray plateaus by altering the radiative efficiency and measure the braking index of newly born millisecond magnetars. we measure the braking index of grb061121 as $n=4.85^{+0.11}_{-0.15}$ suggesting the millisecond-magnetar born in this gamma-ray burst spins down predominantly through gravitational-wave emission. | interpreting the x-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars |
recently, lhaaso reported the detection of the brightest-of-all-time grb 221009a, revealing the early onset of a tev afterglow. we analyze the spectral evolution of the x-ray/gamma-ray emission of grb 221009a measured by the fermi gamma-ray burst monitor (gbm) during the dips of two prompt emission pulses (i.e., intervals t 0 + [300-328] s and t 0 + [338-378] s, where t 0 is the gbm trigger time). we find that the spectra at the dips transit from the band function to a power-law function, indicating a transition from the prompt emission to the afterglow. after ~t 0 + 660 s, the spectrum is well described by a power-law function, and the afterglow becomes dominant. remarkably, the underlying afterglow emission at the dips smoothly connect with the afterglow after ~t 0 + 660 s. the entire afterglow emission measured by gbm can be fitted by a power-law function f ~ t -0.95±0.05, where t is the time since the first main pulse at t* = t 0 + 226 s, consistent with the tev afterglow decay measured by lhaaso. the start time of this power-law decay indicates that the afterglow peak of grb 221009a should be earlier than t 0 + 300 s. we also test the possible presence of a jet break in the early afterglow light curve, finding that both the jet break model and single power-law decay model are consistent with the gbm data. the two models cannot be distinguished with the gbm data alone because the inferred jet break time is quite close to the end of the gbm observations. | grb 221009a: revealing a hidden afterglow during the prompt emission phase with fermi-gbm observations |
we present here a survey of high-ionization absorption lines in the afterglow spectra of long-duration gamma-ray bursts (grbs) obtained with the vlt/x-shooter spectrograph. our main goal is to investigate the circumburst medium in the natal regions of grbs. our primary focus is on the n vλλ 1238, 1242 line transitions, but we also discuss other high-ionization lines such as o vi, c iv, and si iv. we find no correlation between the column density of n v and the neutral gas properties such as metallicity, h i column density, and dust depletion; however, the relative velocity of n v, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of h i. this may be explained if the n v gas is part of an h ii region hosting the grb, where the region's expansion is confined by dense, neutral gas in the grb's host galaxy. we find tentative evidence (at 2σ significance) that the x-ray derived column density, nh, x, may be correlated with the column density of n v, which would indicate that both measurements are sensitive to the column density of the gas located in the vicinity of the grb. we investigate the scenario where n v (and also o vi) is produced by recombination after the corresponding atoms have been stripped entirely of their electrons by the initial prompt emission, in contrast to previous models where highly ionized gas is produced by photoionization from the grb afterglow. | highly ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts |
the prompt emission of low-luminosity gamma-ray bursts (llgrbs) indicates that these events originate from a relativistic shock breakout. in this case, we can estimate, based on the properties of the prompt emission, the energy distribution of the ejecta. we develop a general formalism to estimate the afterglow produced by synchrotron emission from the forward shock resulting from the interaction of this ejecta with the circumburst matter. we assess whether this emission can produce the observed radio and x-ray afterglows of the available sample of four llgrbs. all four radio afterglows can be explained within this model, providing further support for shock breakouts being the origin of llgrbs. we find that in one of the llgrbs (grb 031203), the predicted x-ray emission, using the same parameters that fit the radio, can explain the observed one. in another one (grb 980425), the observed x-rays can be explained if we allow for a slight modification of the simplest model. for the last two cases (grbs 060218 and 100316d), we find that, as is the case for previous attempts to model these afterglows, the simplest model that fits the radio emission underpredicts the observed x-ray afterglows. using general arguments, we show that the most natural location of the x-ray source is, like the radio source, within the ejecta-external medium interaction layer but that emission is due to a different population of electrons or to a different emission process. | the afterglow of a relativistic shock breakout and low-luminosity grbs |
the extinction profiles in gamma-ray burst (grb) afterglow spectral energy distributions (seds) are usually described by the small magellanic cloud (smc)-type extinction curve. in different empirical extinction laws, the total-to-selective extinction, rv, is an important quantity because of its relation to dust grain sizes and compositions. we here analyse a sample of 17 grbs (0.34 < z < 7.84) where the ultraviolet to near-infrared spectroscopic observations are available through the vlt/x-shooter instrument, giving us an opportunity to fit individual extinction curves of grbs for the first time. our sample is compiled on the basis of the availability of multiband photometry around the x-shooter observations. the x-shooter data are combined with the swift x-ray data and a single or broken power law together with a parametric extinction law is used to model the individual seds. we find 10 cases with significant dust, where the derived extinction, av, ranges from 0.1-1.0 mag. in four of those, the inferred extinction curves are consistent with the smc curve. the grb individual extinction curves have a flat rv distribution with an optimal weighted combined value of rv = 2.61 ± 0.08 (for seven broad coverage cases). the `average grb extinction curve' is similar to, but slightly steeper than the typical smc, and consistent with the smc bar extinction curve at ∼95 per cent confidence level. the resultant steeper extinction curves imply populations of small grains, where large dust grains may be destroyed due to grb activity. another possibility could be that young age and/or lower metallicities of grbs environments are responsible for the steeper curves. | vlt/x-shooter grbs: individual extinction curves of star-forming regions |
grb 110721a was detected by the gamma-ray burst monitor and the large area telescope (lat) on board the fermi satellite and the gamma-ray burst polarimeter on board the ikaros solar mission. previous analysis of this burst showed: (i) a linear polarization signal with position angle stable ({φ }p=160^\circ +/- 11) and high degree of {{\pi }}={84}-28+16 % , (ii) an extreme peak energy of a record-breaking 15 ± 2 mev, and (iii) a subdominant prompt thermal component observed right after the onset of this burst. in this paper, the lat data around the reported position of grb 110721a are analyzed with the most recent software and then, the lat light curve above 100 mev is obtained. the lat light curve is modeled in terms of adiabatic early-afterglow external shocks when the outflow propagates into a stellar wind. additionally, we discuss the possible origins and also study the implications of the early-afterglow external shocks on the extreme peak energy observed at 15 ± 2 mev, the polarization observations, and the subdominant prompt thermal component. | modeling the high-energy emission in grb 110721a and implications on the early multiwavelength and polarimetric observations |
very recently xue et al. reported an important detection of the x-ray transient, cdf-s xt2, whose light curve is analogous to x-ray plateau features of gamma-ray burst afterglows. they suggested that this transient is powered by a remnant stable magnetar from a binary neutron star merger because several pieces of evidence (host galaxy, location, and event rate) all point toward such an assumption. in this letter, we revisit this scenario and confirm that this x-ray emission can be well explained by the internal gradual magnetic dissipation process in an ultra-relativistic wind of the newborn magnetar. we show that both the light curve and spectral evolution of cdf-s xt2 can be well fitted by such a model. furthermore, we can probe some key properties of the central magnetar, such as its initial spin period, surface magnetic field strength, and wind saturation lorentz factor. | on the properties of a newborn magnetar powering the x-ray transient cdf-s xt2 |
many gamma-ray bursts are followed by periods of extended emission. at least in some cases, the burst afterglow may be powered by a rapidly rotating, highly magnetized neutron star, which spins down due to electromagnetic and gravitational-wave emission. such a remnant is likely to strongly precess in the early stages of its life, which would lead to modulations in the x-ray luminosity as the triaxiality of the system evolves over time. using a radiation profile appropriate for a precessing, oblique rotator, we find that swift-xrt data of a long (080602) and a short (090510) burst matches the model with significantly higher accuracy (mean-square residuals dropping by ≳200% in the early stages of the extended emission) than for an orthogonal rotator. we interpret this as evidence for precession in newborn magnetars. | evidence for magnetar precession in x-ray afterglows of gamma-ray bursts |
the origin of the x-ray afterglows of gamma-ray bursts has regularly been debated. we fit both the fireball-shock and millisecond-magnetar models of gamma-ray bursts to the x-ray data of grb 130603b and 140903a. we use bayesian model selection to answer the question of which model best explains the data. this is dependent on the maximum allowed non-rotating neutron star mass m tov, which depends solely on the unknown nuclear equation of state. we show that the data for grb 140903a favors the millisecond-magnetar model for all possible equations of state, while the data for grb 130603b favors the millisecond-magnetar model if m tov ≳ 2.3 m ⊙. if m tov ≲ 2.3 m ⊙, the data for grb 130603b supports the fireball-shock model. we discuss implications of this result in regards to the nuclear equation of state and the prospect of gravitational-wave emission from newly born millisecond magnetars. | x-ray afterglows of short gamma-ray bursts: magnetar or fireball? |
we study an extensive sample of 87 gamma-ray bursts (grbs) for which there are well-sampled and simultaneous optical and x-ray light curves. we extract the cleanest possible signal of the afterglow component and compare the temporal behaviors of the x-ray light curve, observed by swift xrt, and optical data, observed by uvot and ground-based telescopes for each individual burst. overall we find that 62% of the grbs are consistent with the standard afterglow model. when more advanced modeling is invoked, up to 91% of the bursts in our sample may be consistent with the external-shock model. a large fraction of these bursts are consistent with occurring in a constant interstellar density medium (61%) while only 39% of them occur in a wind-like medium. only nine cases have afterglow light curves that exactly match the standard fireball model prediction, having a single power-law decay in both energy bands that are observed during their entire duration. in particular, for the bursts with chromatic behavior, additional model assumptions must be made over limited segments of the light curves in order for these bursts to fully agree with the external-shock model. interestingly, for 54% of the x-ray and 40% of the optical band observations, the end of the shallow decay (t∼-0.5) period coincides with the jet-break (t∼-p) time, causing an abrupt change in decay slope. the fraction of the burst that is consistent with the external-shock model is independent of the observational epochs in the rest frame of grbs. moreover, no cases can be explained by the cooling frequency crossing the x-ray or optical band. | a correlated study of optical and x-ray afterglows of grbs |
the afterglows to gamma-ray bursts (grbs) are due to synchrotron emission from shocks generated as an ultrarelativistic outflow decelerates. a forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave-detected neutron star mergers the reverse shock has been neglected. here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. the off-axis reverse shock will appear as a brightening feature in the rising afterglow at radio frequencies. for bursts at ∼100 mpc, the system should be inclined ≲20° for the reverse shock to be observable at ∼0.1-10 d post-merger. for structured outflows, enhancement of the reverse shock emission by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. early radio photometry of the afterglow could reveal the presence of a strong magnetic field associated with the central engine. | reverse shocks in the relativistic outflows of gravitational wave-detected neutron star binary mergers |
short gamma-ray bursts (grbs) are believed to be produced by relativistic jets from mergers of neutron stars (nss) or nss and black-holes (bhs). if the lorentz-factors γ of jets from compact stellar mergers follow a similar power-law distribution to those observed for other high-energy astrophysical phenomena (e.g., blazars, active galactic nuclei), the population of jets should be dominated by low-γ outflows. these jets will not produce prompt gamma-rays, but jet energy will be released as x-ray/optical/radio transients when they collide with the ambient medium. using monte carlo simulations, we study the properties of such transients. approximately 78% of merger jets \lt 300 mpc result in failed grbs if the jet γ follows a power-law distribution of index -1.75. x-ray/optical transients from failed grbs will have broad distributions of their characteristics: light-curves peak {t}p∼ 0.1{--}10 days after a merger; flux peaks for x-ray {10}-6 {mjy}≲ {f}x≲ {10}-2 mjy; and optical flux peaks at 14≲ {m}g≲ 22. x-ray transients are detectable by swift xrt, and ∼ 85 % of optical transients will be detectable by telescopes with limiting magnitude {m}g≳ 21, for well localized sources on the sky. x-ray/optical transients are followed by radio transients with peak times narrowly clustered around {t}p∼ 10 days, and peak flux of ∼10-100 mjy at 10 ghz and ∼0.1 mjy at 150 mhz. by considering the all-sky rate of short grbs within the ligo/virgo range, the rate of on-axis orphan afterglows from failed grbs should be 2.6(26) per year for ns-ns(ns-bh) mergers, respectively. since merger jets from gravitational-wave (gw) trigger events tend to be directed to us, a significant fraction of gw events could be associated with the on-axis orphan afterglow. | low-γ jets from compact stellar mergers: candidate electromagnetic counterparts to gravitational wave sources |
grb 130427a was the brightest gamma-ray burst detected in the last 30 yr. with an equivalent isotropic energy output of 8.5 × 1053 erg and redshift z = 0.34, it uniquely combined very high energetics with a relative proximity to earth. as a consequence, its x-ray afterglow has been detected by sensitive x-ray observatories such as xmm-newton and chandra for a record-breaking baseline longer than 80 million seconds. we present the x-ray light curve of this event over such an interval. the light curve shows a simple power-law decay with a slope α = 1.309 ± 0.007 over more than three decades in time (47 ks-83 ms). we discuss the consequences of this result for a few models proposed so far to interpret grb 130427a, and more in general the significance of this outcome in the context of the standard forward shock model. we find that this model has difficulty in explaining our data, in both cases of constant density and stellar-wind circumburst media, and requires far-fetched values for the physical parameters involved. | the 80 ms follow-up of the x-ray afterglow of grb 130427a challenges the standard forward shock model |
the electromagnetic (em) follow-up of a gravitational-wave (gw) event requires scanning a wide sky region, defined by the so-called “skymap,” to detect and identify a transient counterpart. we propose a novel method that exploits the information encoded in the gw signal to construct a “detectability map,” which represents the time-dependent (“when”) probability of detecting the transient at each position of the skymap (“where”). focusing on the case of a neutron star binary inspiral, we model the associated short gamma-ray burst afterglow and macronova emission using the probability distributions of binary parameters (sky position, distance, orbit inclination, mass ratio) extracted from the gw signal as inputs. the resulting family of possible light curves is the basis for constructing the detectability map. as a practical example, we apply the method to a simulated gw signal produced by a neutron star merger at 75 mpc whose localization uncertainty is very large (∼1500 deg2). we construct observing strategies for optical, infrared, and radio facilities based on the detectability maps, taking vst, vista, and meerkat as prototypes. assuming limiting fluxes of r∼ 24.5, j∼ 22.4 (ab magnitudes), and 500 μ {jy} (1.4 {ghz}) for ∼1000 s of exposure each, the afterglow and macronova emissions are successfully detected with a minimum observing time of 7, 15, and 5 hr respectively. | where and when: optimal scheduling of the electromagnetic follow-up of gravitational-wave events based on counterpart light-curve models |
the detection of grb 180722b and grb 190114c in sub-tev gamma-rays has opened up a new window to study gamma-ray bursts in high-energy gamma-rays. recently it has been shown that the synchrotron and inverse compton processes are responsible for the production of these high-energy gamma-rays during the afterglow. here, for the first time we demonstrate that the photohadronic scenario that is successful in explaining the multi-tev flaring in high-energy blazars is also applicable for gamma-ray bursts. we show that the sub-tev spectra of grb 190114c and grb 180720b are due to the interaction of high-energy protons with the background photons in the synchrotron self-compton region and synchrotron region, respectively. the nature of the background photon distributions help us to constrain their bulk lorentz factors. | origin of sub-tev afterglow emission from gamma-ray bursts grb 190114c and grb 180720b |
the x-ray afterglow of many gamma-ray bursts (grbs) exhibits a plateau phase before the normal power-law decay stage, which may be related to continued activities of the central engine. tang et al. collected 174 such grbs and confirmed the so-called l-t-e correlation which involves three key parameters, i.e., the isotropic γ-ray energy eγ,iso of the prompt phase, the end time t a of the plateau phase, and the corresponding x-ray luminosity l x. in this study, the l-t-e correlation is confirmed and updated as ${l}_{{\rm{x}}}\propto {t}_{{\rm{a}}}^{-0.99}{e}_{\gamma ,\mathrm{iso}}^{0.86}$ with a large sample consisting of 210 plateau grbs with known redshifts. the tight correlation is then applied to derive the pseudo-redshift of other 130 plateau grbs whose redshifts are not directly measured. statistical analysis is also carried out on this pseudo-redshift sample. | pseudo-redshifts of gamma-ray bursts derived from the l-t-e correlation |
optical rebrightenings in the afterglows of some gamma-ray bursts (grbs) are unexpected within the framework of the simple external shock model. while it has been suggested that the central engines of some grbs are newly born magnetars, we aim to relate the behaviors of magnetars to the optical rebrightenings. a newly born magnetar will lose its rotational energy in the form of poynting-flux, which may be converted into a wind of electron-positron pairs through some magnetic dissipation processes. as proposed by dai, this wind will catch up with the grb outflow and a long-lasting reverse shock (rs) would form. by applying this scenario to grb afterglows, we find that the rs propagating back into the electron-positron wind can lead to an observable optical rebrightening and a simultaneous x-ray plateau (or x-ray shallow decay). in our study, we select four grbs (i.e., grb 080413b, grb 090426, grb 091029, and grb 100814a), of which the optical afterglows are well observed and show clear rebrightenings. we find that they can be well interpreted. in our scenario, the spin-down timescale of the magnetar should be slightly smaller than the peak time of the rebrightening, which can provide a clue to the characteristics of the magnetar. | imprints of electron-positron winds on the multiwavelength afterglows of gamma-ray bursts |
we report early optical linear polarization observations of two gamma-ray bursts made with the master robotic telescope network. we found the minimum polarization for grb 150301b to be 8 per cent at the beginning of the initial stage, whereas we detected no polarization for grb 150413a either at the rising branch or after the burst reached the power-law afterglow stage. this is the earliest measurement of the polarization (in cosmological rest frame) of gamma-ray bursts. the primary intent of the paper is to discover optical emission and publish extremely rare (unique) high-quality light curves of the prompt optical emission of gamma-ray bursts during the non-monotonic stage of their evolution. we report that our team has discovered the optical counterpart of one of the bursts, grb 150413a. | early polarization observations of the optical emission of gamma-ray bursts: grb 150301b and grb 150413a |
we present observations by the fermi gamma-ray space telescope gamma-ray burst monitor (gbm) of the nearby (z = 0.55) grb 101219b. this burst is a long grb, with an associated supernova and with a blackbody (bb) component detected in the early afterglow observed by the swift x-ray telescope (xrt). here we show that the prompt gamma-ray emission has a bb spectrum, making this the second such burst observed by fermi gbm. the properties of the bb, together with the redshift and our estimate of the radiative efficiency makes it possible to calculate the absolute values of the properties of the outflow. we obtain an initial lorentz factor γ = 138 ± 8, a photospheric radius {{r}phot}=4.4+/- 1.9× {{10}11} cm, and a launch radius {{r}0}=2.7+/- 1.6× {{10}7} cm. the latter value is close to the black hole and suggests that the jet has a relatively unobstructed path through the star. there is no smooth connection between the bb components seen by gbm and xrt, ruling out the scenario that the late emission is due to high-latitude effects. in the interpretation that the xrt bb is prompt emission due to late central engine activity, the jet either has to be very wide or have a clumpy structure where the emission originates from a small patch. other explanations for this component, such as emission from a cocoon surrounding the jet, are also possible. | evidence for jet launching close to the black hole in grb 101219b—a fermi grb dominated by thermal emission |
grb 160625b is an extremely bright grb with three distinct emission episodes. by analyzing its data observed with the gamma-ray burst monitor (gbm) and large area telescope (lat) on board the fermi mission, we find that a multicolor blackbody (mbb) model can be used to fit very well the spectra of the initial short episode (episode i) within the hypothesis of photosphere emission of a fireball model. the time-resolved spectra of its main episode (episode ii), which was detected with both gbm and lat after a long quiescent stage (∼180 s) following the initial episode, can be fitted with a model comprising an mbb component plus a cutoff power-law (cpl) component. this grb was detected again in the gbm and lat bands with a long extended emission (episode iii) after a quiescent period of ∼300 s. the spectrum of episode iii is adequately fitted with cpl plus single power-law models, and no mbb component is required. these features may imply that the emission of the three episodes are dominated by distinct physics processes, i.e., episode i is possible from the cocoon emission surrounding the relativistic jet, episode ii may be from photosphere emission and internal shock of the relativistic jet, and episode iii is contributed by internal and external shocks of the relativistic jet. on the other hand, both x-ray and optical afterglows are consistent with the standard external shocks model. | extremely bright grb 160625b with multiple emission episodes: evidence for long-term ejecta evolution |
context. short-duration gamma-ray bursts (grbs) with extended emission form a subclass of short grbs, comprising about 15% of the short-duration sample. afterglow detections of short grbs are also rare (about 30%) because of their lower luminosity.aims: we present a multiband data set of the short burst with extended emission, grb 150424a, comprising of grond observations, complemented with data from swift/uvot, swift/xrt, hst, keck/lris, and data points from the literature. the grb 150424a afterglow shows an extended plateau phase, lasting about 8 h. the analysis of this unique grb afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate.methods: we present a phenomenological analysis made by applying fireball closure relations and interpret the findings in the context of the fireball model. we discuss the plausibility of a magnetar as a central engine, which would be responsible for additional and prolonged energy injection into the fireball.results: we find convincing evidence for energy injection into the afterglow of grb 150424a. we find that a magnetar spin-down as the source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted into radiation. | long optical plateau in the afterglow of the short grb 150424a with extended emission. evidence for energy injection by a magnetar? |
we report the lowest-frequency measurements of gamma-ray burst (grb) 171205a with the upgraded giant metrewave radio telescope (ugmrt) covering a frequency range of 250-1450 mhz and a period of 4-937 days. it is the first grb afterglow detected in the 250-500 mhz frequency range and the second brightest grb detected with the ugmrt. even though the grb was observed for nearly 1000 days, there is no evidence of a transition to a nonrelativistic regime. we also analyzed the archival chandra x-ray data on day ∼70 and day ∼200. we also found no evidence of a jet break from the analysis of combined data. we fit synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock breakout of a wide-angle cocoon. our data also allowed us to discern the nature and the density of the circumburst medium. we found that the density profile deviates from a standard constant density medium and suggests that the grb exploded in a stratified wind-like medium. our analysis shows that the lowest-frequency measurements covering the absorbed part of the light curves are critical to unraveling the grb environment. our data combined with other published measurements indicate that the radio afterglow has a contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with the results of izzo et al. the cocoon emission likely dominates at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio light curves. | 1000 days of the lowest-frequency emission from the low-luminosity grb 171205a |
we present the first results from a recently concluded study of grbs at z ≳ 5 with the karl g. jansky very large array (vla). spanning 1 to 85.5 ghz and 7 epochs from 1.5 to 82.3 days, our observations of grb 140311a are the most detailed joint radio and millimeter observations of a grb afterglow at z ≳ 5 to date. in conjunction with optical/near-ir and x-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy {e}{{k},{iso}}≈ 8.5× {10}53 erg expanding into a constant density environment with density, {n}0≈ 8 {cm}}-3. the x-ray and radio observations require a jet break at {t}jet}≈ 0.6 days, yielding an opening angle of {θ }jet}≈ 4^\circ and a beaming-corrected blast wave kinetic energy of {e}{{k}}≈ 2.2× {10}50 erg. the results from our radio follow-up and multiwavelength modeling lend credence to the hypothesis that detected high-redshift grbs may be more tightly beamed than events at lower redshift. we do not find compelling evidence for reverse shock emission, which may be related to fast cooling driven by the moderately high circumburst density. | a vla study of high-redshift grbs. i. multiwavelength observations and modeling of grb 140311a |
we use gamma-ray burst (grb) afterglow spectra observed with the vlt/x-shooter spectrograph to measure rest-frame extinction in grb lines-of-sight by modelling the broadband near-infrared (nir) to x-ray afterglow spectral energy distributions (seds). our sample consists of nine swift grbs, of which eight belong to the long-duration and one to the short-duration class. dust is modelled using the average extinction curves of the milky way and the two magellanic clouds. we derive the rest-frame extinction of the entire sample, which fall in the range 0 ≲ av ≲ 1.2. moreover, the smc extinction curve is the preferred extinction curve template for the majority of our sample, a result that is in agreement with those commonly observed in grb lines of sights. in one analysed case (grb 120119a), the common extinction curve templates fail to reproduce the observed extinction. to illustrate the advantage of using the high-quality, x-shooter afterglow seds over the photometric seds, we repeat the modelling using the broadband seds with the nir-to-uv photometric measurements instead of the spectra. the main result is that the spectroscopic data, thanks to a combination of excellent resolution and coverage of the blue part of the sed, are more successful in constraining extinction curves and therefore dust properties in grb hosts with respect to photometric measurements. in all cases but one the extinction curve of one template is preferred over the others. we show that themodelled values of the extinction av and the spectral slope, obtained through spectroscopic and photometric sed analysis, can differ significantly for individual events, though no apparent trend in the differences is observed. finally we stress that, regardless of the resolution of the optical-to-nir data, the sed modelling gives reliable results only when the fit is performed on a sed covering a broader spectral region (in our case extending to x-rays). based on observations collected at the european southern observatory, paranal, chile, under programs 084.a-0260(b), 085.a-0009(b), 088.a-0051(b), 089.a-0067(b) and 091.c-0934(c).appendix a is available in electronic form at http://www.aanda.org | spectrophotometric analysis of gamma-ray burst afterglow extinction curves with x-shooter |
we present a catalog of early-time (∼ {10}2-{10}4 s) photometry and polarimetry of all gamma-ray burst (grb) optical afterglows observed with the ringo2 imaging polarimeter on the liverpool telescope. of the 19 optical afterglows observed, the following nine were bright enough to perform photometry and attempt polarimetry: grb 100805a, grb 101112a, grb 110205a, grb 110726a, grb 120119a, grb 120308a, grb 120311a, grb 120326a, and grb 120327a. we present multiwavelength light curves for these 9 grbs, together with estimates of their optical polarization degrees and/or limits. we carry out a thorough investigation of detection probabilities, instrumental properties, and systematics. using two independent methods, we confirm previous reports of significant polarization in grb 110205a and 120308a, and report the new detection of p={6}-2+3% in grb101112a. we discuss the results for the sample in the context of the reverse- and forward-shock afterglow scenario, and show that grbs with detectable optical polarization at early time have clearly identifiable signatures of reverse-shock emission in their optical light curves. this supports the idea that grb ejecta contain large-scale magnetic fields, and it highlights the importance of rapid-response polarimetry. | polarimetry and photometry of gamma-ray bursts with ringo2 |
studies of gamma-ray bursts (grbs) and their multiwavelength afterglows have led to insights in electron acceleration and emission properties from relativistic, high-energy astrophysical sources. broad-band modelling across the electromagnetic spectrum has been the primary means of investigating the physics behind these sources, although independent diagnostic tools have been developed to inform and corroborate assumptions made in particle acceleration simulations and broad-band studies. we present a methodology to constrain three physical parameters related to electron acceleration in grb blast waves: the fraction of shock energy in electrons, ϵe; the fraction of electrons that gets accelerated into a power-law distribution of energies, ξe; and the minimum lorentz factor of the accelerated electrons, γm. these parameters are constrained by observations of the peaks in radio afterglow light curves and spectral energy distributions. from a sample of 49 radio afterglows, we are able to find narrow distributions for these parameters, hinting at possible universality of the blast wave microphysics, although observational bias could play a role in this. using radio peaks and considerations related to the prompt gamma-ray emission efficiency, we constrain the allowed parameter ranges for both ϵe and ξe to within about one order of magnitude, 0.01 ≲ ϵe ≲ 0.2 and 0.1 ≲ ξe ≲ 1. such stringent constraints are inaccessible for ξe from broad-band studies due to model degeneracies. | constraints on electron acceleration in gamma-ray bursts afterglows from radio peaks |
gamma-ray bursts (grbs) are very energetic cosmological transients. long grbs are usually associated with type ib/c supernovae (sne), and we refer to them as grb-sne. since the associated sn for a given grb is observed only at low redshift, a possible selection effect exists when we consider intrinsically faint sources that cannot be observed at high redshift. thus, it is important to explore the possible relationships between grb and sn parameters after these have been corrected for astrophysical biases due to the instrumental selection effects and redshift evolution of the variables involved. so far, only grb prompt emission properties have been checked against the sn ib/c properties without considering the afterglow (ag). this work investigates the existence of relationships among grbs' prompt and ag and associated sn properties. we investigate 91 bidimensional correlations among the sn and grb observables before and after their correction for selection biases and evolutionary effects. as a result of this investigation, we find hints of a new correlation with a pearson correlation coefficient >0.50 and a probability of being drawn by chance <0.05. this correlation is between the luminosity at the end of the grb optical plateau emission and the rest-frame peak time of the sn. according to this relation, the brightest optical plateaus are accompanied by the largest peak times. this correlation is corrected for selection biases and redshift evolution and may provide new constraints for the astrophysical models associated with the grb-sn connection. | the quest for new correlations in the realm of the gamma-ray burst-supernova connection |
gamma-ray bursts (grbs) are the strongest explosions in the universe, which due to their extreme character likely involve some of the strongest magnetic fields in nature. this review discusses the possible roles of magnetic fields in grbs, from their central engines, through the launching, acceleration and collimation of their ultra-relativistic jets, to the dissipation and particle acceleration that power their γ-ray emission, and the powerful blast wave they drive into the surrounding medium that generates their long-lived afterglow emission. an emphasis is put on particular areas in which there have been interesting developments in recent years. | gamma-ray bursts as sources of strong magnetic fields |
grb 170817a, the first short gamma-ray burst (sgrb) to be detected in coincidence with a gravitational wave signal, demonstrated that merging binary neutron star (bns) systems can power collimated ultra-relativistic jets, and in turn, produce sgrbs. moreover, it revealed that sgrb jets possess an intrinsic angular structure that is imprinted in the observable prompt and afterglow emission. advanced numerical simulations represent the leading approach to investigate the physical processes underlying the evolution of sgrb jets breaking out of post-merger environments, and thus connect the final angular structure and energetics with specific jet launching conditions. in a previous paper, we carried out the first 3d special-relativistic hydrodynamic simulations of incipient (top-hat) sgrb jets propagating across the realistic environment resulting from a general-relativistic (gr) hydrodynamic bns merger simulation. while the earlier work marked an important step toward a consistent end-to-end description of sgrb jets from bns mergers, those simulations did not account for the presence of magnetic fields, which are expected to play a key role. here, we overcome this limitation, reporting the first 3d special-relativistic magnetohydrodynamic (mhd) simulation of a magnetized (structured and rotating) sgrb jet piercing through a realistic magnetized post-merger environment, wherein the initial conditions of the latter are directly imported from the outcome of a previous grmhd bns merger simulation. | jet-environment interplay in magnetized binary neutron star mergers |
it was proposed that a remnant stable magnetar could be formed in a binary neutron-star merger, leading to a fast x-ray transient (fxt) that can last for thousands of seconds. recently, xue et al. suggested that cdf-s xt2 was exactly such a kind of source. if confirmed, such emission can be used to search for electromagnetic counterparts to gravitational wave events from binary neutron-star mergers that have short gamma-ray bursts and the corresponding afterglows seen off-axis and thus too weak to be detected. here we report the discovery of three new fxts, xrt 170901, xrt 030511, and xrt 110919, from a preliminary search over chandra archival data. similar to cdf-s xt2, these new fxts had a very fast rise (less than a few tens of seconds) and a plateau of x-ray flux of ~1.0 × 10-12 erg s-1 cm-2 lasting for 1-2 ks, followed by a steep decay. their optical/ir counterparts, if present, are very weak, arguing against a stellar flare origin for these fxts. for xrt 170901, we identified a faint host galaxy with the source at the outskirts, very similar to cdf-s xt2. therefore, our newly discovered fxts are also strong candidates for magnetar-powered x-ray transients resulting from binary neutron star mergers. | discovery of three candidate magnetar-powered fast x-ray transients from chandra archival data |
we report the early discovery of the optical afterglow of gamma-ray burst (grb) 140801a in the 137 deg2 3-σ error-box of the fermi gamma-ray burst monitor (gbm). master is the only observatory that automatically reacts to all fermi alerts. grb 140801a is one of the few grbs whose optical counterpart was discovered solely from its gbm localization. the optical afterglow of grb 140801a was found by master global robotic net 53 s after receiving the alert, making it the fastest optical detection of a grb from a gbm error-box. spectroscopy obtained with the 10.4-m gran telescopio canarias and the 6-m big telescope alt-azimuth of the special astrophysical observatory of the russian academy of sciences reveals a redshift of z = 1.32. we performed optical and near-infrared photometry of grb 140801a using different telescopes with apertures ranging from 0.4 to 10.4 m. grb 140801a is a typical burst in many ways. the rest-frame bolometric isotropic energy release and peak energy of the burst are e_iso = 5.54_{-0.24}^{+0.26} {×} 10^{52} erg and ep, rest ≃ 280 kev, respectively, which is consistent with the amati relation. the absence of a jet break in the optical light curve provides a lower limit on the half-opening angle of the jet θ = 6.1°. the observed epeak is consistent with the limit derived from the ghirlanda relation. the joint fermi gbm and konus-wind analysis show that grb 140801a could belong to the class of intermediate duration. the rapid detection of the optical counterpart of grb 140801a is especially important regarding the upcoming experiments with large coordinate error-box areas. | the optical identification of events with poorly defined locations: the case of the fermi gbm grb 140801a |
since the gravitational wave event gw170817 and gamma-ray burst gw170817a, there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle (θ obs/θj ) between 4 and 6. we use a parameterized model and broadband synchrotron data up to ~800 days post-merger to constrain parameters of the burst. to reproduce the hydrodynamics of a gamma-ray burst outflow, we use a two-parameter "boosted fireball" model. the structure of a boosted fireball is determined by the specific internal energy, η 0, and the bulk lorentz factor, γb ( ~ 1/θj ), with shapes varying smoothly from a quasi-spherical outflow for low values of γbto a highly collimated jet for high values. we run simulations with γbin the range 1-20 and η 0 in the range 2-15. to calculate light curves, we use a synchrotron radiation model characterized by f peak, νm , and νc , and calculate millions of spectra at different times and θ obs values using the boxfit radiation code. we can tabulate the spectral parameter values from our spectra and rapidly generate arbitrary light curves for comparison to data in mcmc analysis. we find that our model prefers a gamma-ray burst with jet energy ej~ 1050 erg and with an observer angle of ${\theta }_{\mathrm{obs}}={0.65}_{-0.14}^{+0.13}$ radians and ratio to the jet opening angle of (θ obs/θj ) = ${5.4}_{-0.38}^{+0.53}$ . | revisiting the parameter space of binary neutron star merger event gw170817 |
gamma ray bursts (grbs) are a powerful probe of the high-redshift universe. we present a tool to estimate the detection rate of high-z grbs by a generic detector with defined energy band and sensitivity. we base this on a population model that reproduces the observed properties of grbs detected by swift, fermi and cgro in the hard x-ray and γ-ray bands. we provide the expected cumulative distributions of the flux and fluence of simulated grbs in different energy bands. we show that scintillator detectors, operating at relatively high energies (e.g. tens of kev to the mev), can detect only the most luminous grbs at high redshifts due to the link between the peak spectral energy and the luminosity (epeak-liso) of grbs. we show that the best strategy for catching the largest number of high-z bursts is to go softer (e.g. in the soft x-ray band) but with a very high sensitivity. for instance, an imaging soft x-ray detector operating in the 0.2-5 kev energy band reaching a sensitivity, corresponding to a fluence, of ∼10-8 erg cm-2 is expected to detect ≈40 grbs yr-1 sr-1 at z ≥ 5 (≈3 grbs yr-1 sr-1 at z ≥ 10). once high-z grbs are detected the principal issue is to secure their redshift. to this aim we estimate their nir afterglow flux at relatively early times and evaluate the effectiveness of following them up and construct usable samples of events with any forthcoming grb mission dedicated to explore the high-z universe. | accessing the population of high-redshift gamma ray bursts |
we work on a grb sample whose initial lorentz factors (γ0) are constrained by the afterglow onset method and the jet opening angles (θj) are determined by the jet break time. we confirm the γ0-eγ,iso correlation by liang et al. (2010), and the γ0-lγ,iso correlation by lü et al. (2012). furthermore, we find correlations between γ0 and the beaming corrected γ-ray energy (eγ) and mean γ-ray luminosity (lγ). by also including the kinetic energy of the afterglow, we find rough correlations (with larger scatter) between γ0 and the total (γ-ray plus kinetic) energy and the total mean luminosity, both for isotropic values and beaming corrected values: these correlations allow us to test the data with grb central engine models. limiting our sample to the grbs that likely have a black hole central engine, we compare the data with theoretical predictions of two types of jet launching mechanisms from bhs, i.e. the non-magnetized ν ν bar -annihilation mechanism, and the strongly magnetized blandford-znajek (bz) mechanism. we find that the data are more consistent with the latter mechanism, and discuss the implications of our findings for grb jet composition. | lorentz factor - beaming corrected energy/luminosity correlations and grb central engine models |
the neil gehrels swift observatory (swift) has been in operation for 18 years. the ultra-violet/optical telescope (uvot) onboard swift was designed to capture the earliest optical/uv emission from gamma-ray bursts (grbs), spanning the first few minutes to days after the prompt gamma-ray emission. in this article, we provide an overview of the long grbs (whose prompt gamma-ray duration is >2 s) observed by the swift/uvot, and review the major discoveries that have been achieved by the swift/uvot over the last 18 years. we discuss where improvements have been made to our knowledge and understanding of the optical/uv emission, particularly the early optical/uv afterglow. | swift/uvot: 18 years of long grb discoveries and advances |
the various stages of baryonic gamma-ray burst (grb) afterglow blast waves are reviewed. these are responsible for the afterglow emission from which much of our understanding of gamma-ray bursts derives. initially, the blast waves are confined to the dense medium surrounding the burster (stellar envelope or dense wind), giving rise to a jet-cocoon structure. a massive ejecta is released and potentially fed by ongoing energy release from the burster and a forward-reverse shock system is set up between ejecta and ambient density. ultimately the blast wave spreads sideways and slows down, and the dominant afterglow emission shifts from x-rays down to radio. over the past years significant progress has been made both observationally and theoretically/numerically in our understanding of these blast waves, unique in the universe due to their often incredibly high initial lorentz factors of 100-1000. the recent discovery of a short gamma-ray burst counterpart to a gravitational wave detection (gw 170817) brings the promise of a completely new avenue to explore and constrain the dynamics of gamma-ray burst blast waves. | gamma-ray burst afterglow blast waves |
the synchrotron external shock model predicts the evolution of the spectral (β) and temporal (α) indices during the gamma-ray burst (grb) afterglow for different environmental density profiles, electron spectral indices, electron cooling regimes, and regions of the spectrum. we study the relationship between α and β, the so-called “closure relations” with grbs detected by fermi large area telescope (fermi-lat) from 2008 august to 2018 august. the spectral and temporal indices for the >100 mev emission from the fermi-lat as determined in the second fermi-lat gamma-ray burst catalog (2flgc) are used in this work. we select grbs whose spectral and temporal indices are well constrained (58 long-duration grbs and 1 short-duration grbs) and classify each grb into the best-matched relation. as a result, we found that a number of grbs require a very small fraction of the total energy density contained in the magnetic field (ɛb≲ 10-7). the estimated mean and standard deviation of electron spectral index p are 2.40 and 0.44, respectively. the grbs satisfying a closure relation of the slow cooling tend to have a softer p value compared to those of the fast cooling. moreover, the kolmogorov-smirnov test of the two p distributions from the fast and slow coolings rejects a hypothesis that the two distributions are drawn from the single reference distribution with a significance of 3.2σ. lastly, the uniform density medium is preferred over the medium that decreases like the inverse of distance squared for long-duration grbs. | closure relations of gamma-ray bursts in high energy emission |
the spectrum of gamma-ray burst (grb) afterglows can be studied with color indices. here, we present a large comprehensive catalog of 70 grbs with multiwavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. we categorize them into two samples based on how well the color indices are evaluated. the golden sample includes 25 bursts mostly observed by grond, and the silver sample includes 45 bursts observed by other telescopes. for the golden sample, we find that 96% of the color indices do not vary over time. however, the color indices do vary during short periods in most bursts. the observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43%) and in a constant-density medium (30%), (ii) early dust extinction (12%), (iii) transition from reverse-shock to forward-shock emission (5%), or (iv) an emergent sn emission (10%). we also study the evolutionary properties of the mean color indices for different emission episodes. we find that 86% of the color indices in the 70 bursts show constancy between consecutive ones. the color index variations occur mainly during the late grb-sn bump, the flare, and early reverse-shock emission components. we further perform a statistical analysis of various observational properties and model parameters (spectral index {β }o{ci}, electron spectral indices p ci, etc.) using color indices. overall, we conclude that ∼90% of colors are constant in time and can be accounted for by the simplest external forward-shock model, while the varying color indices call for more detailed modeling. | a large catalog of multiwavelength grb afterglows. i. color evolution and its physical implication |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.