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jet breaks in gamma-ray burst (grb) afterglows provide a direct probe of their collimation angle. modeling a jet break requires an understanding of the jet spreading process, whereby the jet transitions from a collimated outflow into the spherical sedov-taylor solution at late times. currently, direct numerical calculations are the most accurate way to capture the deceleration and spreading process, as analytical models have previously given inaccurate descriptions of the dynamics. here (in paper i) we present a new, semi-analytical model built empirically by performing relativistic numerical jet calculations and inferring the relationship between the lorentz factor, opening angle, and shock radius. we then use the analytical model to calculate the lorentz factor and jet opening angle as a function of the shock radius and compare to the numerical solutions. our analytic model provides efficient means of computing synthetic grb afterglow light curves and spectra, which is the focus of paper ii. | on the deceleration and spreading of relativistic jets. i. jet dynamics |
one of the most exciting near-term prospects in physics is the potential discovery of gravitational waves by the advanced ligo and virgo detectors. to maximize both the confidence of the detection and the science return, it is essential to identify an electromagnetic counterpart. this is not trivial, as the events are expected to be poorly localized, particularly in the near-term, with error regions covering hundreds or even thousands of square degrees. in this paper, we discuss the prospects for finding an x-ray counterpart to a gravitational wave trigger with the swift x-ray telescope, using the assumption that the trigger is caused by a binary neutron star merger which also produces a short gamma-ray burst. we show that it is beneficial to target galaxies within the gw error region, highlighting the need for substantially complete galaxy catalogues out to distances of 300 mpc. we also show that nearby, on-axis short grbs are either extremely rare, or are systematically less luminous than those detected to date. we consider the prospects for detecting afterglow emission from an off-axis grb which triggered the gw facilities, finding that the detectability, and the best time to look, are strongly dependent on the characteristics of the burst such as circumburst density and our viewing angle. | optimization of the swift x-ray follow-up of advanced ligo and virgo gravitational wave triggers in 2015-16 |
the external forward shock emitting the gamma-ray burst (grb) afterglow is collisionless, and it is mediated by the weibel instability which generates the magnetic field. the grb afterglow shows that the magnetic field in the large downstream region is much stronger than the shock-compressed preshock field. however, particle-in-cell (pic) simulations of relativistic shocks propagating into homogeneous media show that the weibel generated field decays near the shock front. some grb observations and theoretical studies suggest that the preshock medium is inhomogeneous. we perform the pic simulation of a relativistic shock propagating into inhomogeneous plasma. it is found that the post-shock magnetic field decays slowly compared with the homogeneous case. sound waves and entropy waves are also generated by the shock-wave interaction, and temperature anisotropy is produced by the sound wave in the downstream region. the free energy of the temperature anisotropy is large enough to explain the observed field strength. our results show that the upstream density fluctuation has a significant effect in the downstream region of collisionless shocks even if the wavelength of the upstream inhomogeneity is much larger than the kinetic scale. | weibel-mediated shocks propagating into inhomogeneous electron-positron plasmas |
we model the possible afterglow of the fermi gamma-ray burst monitor (gbm) event associated with ligo detection gw150914, under the assumption that the gamma-rays are produced by a short grb-like relativistic outflow. we model gw150914-gbm as both a weak, on-axis short grb and normal short grb seen far off-axis. given the large uncertainty in the position of gw150914, we determine that the best chance of finding the afterglow is with askap or possibly the murchinson widefield array (mwa), with the flux from an off-axis short grb reaching 0.2-4 mjy (0.12-16 mjy) at 150 mhz (863.5 mhz) by 1-12 months after the initial event. at low frequencies, the source would evolve from a hard to soft spectrum over several months. the radio afterglow would be detectable for several months to years after it peaks, meaning the afterglow may still be detectable and increasing in brightness now (2016 mid-july). with a localization from the mwa or askap, the afterglow would be detectable at higher radio frequencies with the atca and in x-rays with chandra or xmm. | modeling the afterglow of the possible fermi-gbm event associated with gw150914 |
we present optical photometry of the afterglow of the long grb 180205a with the coatli telescope from 217 s to about 5 days after the swift/bat trigger. we analyze this photometry in conjunction with the x-ray light curve from swift/xrt. the late-time light curves and spectra are consistent with the standard forward-shock scenario. however, the early-time optical and x-ray light curves show atypical behavior; the optical light curve exhibits a flat plateau while the x-ray light curve shows a flare. we explore several scenarios and conclude that the most likely explanation for the early behavior is late activity of the central engine. | late central-engine activity in grb 180205a |
multi-wave band synchrotron linear polarization of gamma-ray burst (grb) afterglows is studied under the assumption of an anisotropic turbulent magnetic field with a coherence length of the plasma skin-depth scale in the downstream of forward shocks. we find that for typical grbs, in comparison to optical polarization, the degree of radio polarization shows a similar temporal evolution but a significantly smaller peak value. this results from differences in observed intensity image shapes between the radio and optical bands. we also show that the degree of the polarization spectrum undergoes a gradual variation from the low- to the high-polarization regime above the intensity of the spectral peak frequency, and that the difference in polarization angles in the two regimes is zero or 90°. thus, simultaneous multi-wave band polarimetric observations of grb afterglows would be a new determinative test of the plasma-scale magnetic field model. we also discuss theoretical implications from the recent detection of radio linear polarization in grb 171205a with the atacama large millimeter/submillimeter array and other models of magnetic field configuration. | multi-wave band synchrotron polarization of gamma-ray burst afterglows |
long-duration grb 200829a was detected by fermi-gbm and swift-bat/xrt, and then rapidly observed by other ground-based telescopes. it has a weak γ-ray emission in the very early phase and is followed by a bright spiky γ-ray emission pulse. the radiation spectrum of the very early emission is best fitted by a power-law function with index ~-1.7. however, the bright spiky γ-ray pulse, especially the time around the peak, exhibits a distinct two-component radiation spectrum, i.e., band function combined with a blackbody radiation spectrum. we infer the photospheric properties and reveal a medium magnetization at a photospheric position by adopting the initial size of the outflow as r 0 = 109 cm. it implies that the band component in this pulse may be formed during the dissipation of the magnetic field. the power-law radiation spectra found in the very early prompt emission may imply the external-shock origination of this phase. then, we perform the markov chain monte carlo method fitting on the light curves of this burst, where the jet corresponding to the γ-ray pulse at around 20 s is used to refresh the external shock. it is shown that the light curves of the very early phase and x-ray afterglow after 40 s, involving the x-ray bump at around 100 s, can be well modeled in the external-shock scenario. for the obtained initial outflow, we estimate the minimum magnetization factor of the jet based on the fact that the photospheric emission of this jet is missed in the very early phase. | grb 200829a: external-shock origin of the very early prompt emission? |
in the past decades, an external forward shock model has been successfully developed to explain the main features of the afterglow emission of gamma-ray bursts (grbs). in the numerical modeling of the grb afterglow, some approximations have been made for simplicity, and different groups developed their codes. a robust test of these models/approaches is challenging because of the lack of directly measured physical parameters. fortunately, the viewing angle inferred from the afterglow modeling is widely anticipated to be the same as the inclination angle of the binary neutron star (bns) mergers that can be evaluated with the gravitational wave (gw) data. therefore, in the future, it is possible to calibrate the afterglow modeling with the gw inclination angle measurements. we take three methods, including both analytical estimations and direct simulations, to project the uncertainties of the inclination angle measurements. for some bns mergers accompanied with electromagnetic counterparts detected in the o4/o5 runs of ligo/virgo/kagra/ligo-india detectors, we show that the inclination angle can be determined within an uncertainty of ≤0.1 rad , supposing that the hubble constant is known with an accuracy of ≤3 % and the uncertainty of hubble flow velocity is within ∼1 %. the off-axis grb outflow will give rise to afterglow emission, and the most energetic ones may be detectable at the distance of ∼100 - 200 mpc even for a viewing angle of ≥0.3 rad . such events can thus serve as a robust test of the afterglow modeling approach. we have also evaluated the prospect of resolving the so-called hubble tension with a single gw/grb association event. we find out that a ∼3 % precision hubble constant is obtainable if the uncertainty of the viewing angle can be constrained to be within ∼0.1 rad , which is expected to be the case for some nearby (≤250 mpc ) bright/on-axis grbs with a well-behaved afterglow light curve displaying a clear achromatic break at early times. | prospects of calibrating afterglow modeling of short grbs with gravitational wave inclination angle measurements and resolving the hubble tension with a gw-grb association event |
we collected the optical light-curve data of 227 gamma-ray bursts (grbs) observed with the tarot, coatli, and ratir telescopes. these consist of 133 detections and 94 upper limits. we constructed average light curves in the observer and rest frames in both x-rays (from swift/x-ray telescope) and the optical. our analysis focused on investigating the observational and intrinsic properties of grbs. specifically, we examined observational properties, such as the optical brightness function of the grbs at t = 1000 s after the trigger, as well as the temporal slope of the afterglow. we also estimated the redshift distribution for the grbs within our sample. of the 227 grbs analysed, we found that 116 had a measured redshift. based on these data, we calculated a local rate of ρ0 = 0.2 gpc-3 yr-1 for these events with z < 1. to explore the intrinsic properties of grbs, we examined the average x-ray and optical light curves in the rest frame. we use the afterglowpy library to generate synthetic curves to constrain the parameters typical of the bright grb jet, such as energy (⟨e0⟩ ~ 1053.6 erg), opening angle (⟨θcore⟩ ~ 0.2 rad), and density (⟨n0⟩ ~ 10-2.1 cm-3). furthermore, we analyse microphysical parameters, including the fraction of thermal energy in accelerated electrons (⟨ϵe⟩ ~ 10-1.37) and in the magnetic field (⟨ϵb⟩ ~ 10-2.26), and the power-law index of the population of non-thermal electrons (⟨p⟩ ~ 2.2). | understanding the nature of the optical emission in gamma-ray bursts: analysis from tarot, coatli, and ratir observations |
prompt γ-ray and early x-ray afterglow emissions in gamma-ray bursts (grbs) are characterized by a bursty behavior and are often interspersed with long quiescent times. there is compelling evidence that x-ray flares are linked to prompt γ-rays. however, the physical mechanism that leads to the complex temporal distribution of γ-ray pulses and x-ray flares is not understood. here we show that the waiting time distribution (wtd) of pulses and flares exhibits a power-law tail extending over four decades with an index of about two and can be the manifestation of a common time-dependent poisson process. this result is robust and is obtained on different catalogs. surprisingly, grbs with many (≥slant 8) γ-ray pulses are very unlikely to be accompanied by x-ray flares after the end of the prompt emission (3.1σ gaussian confidence). these results are consistent with a simple interpretation: a hyperaccreting disk breaks up into one or a few groups of fragments, each of which is independently accreted with the same probability per unit time. prompt γ-rays and late x-ray flares are nothing but different fragments being accreted at the beginning and at the end, respectively, following the very same stochastic process and likely the same mechanism. | a common stochastic process rules gamma-ray burst prompt emission and x-ray flares |
the unusual multiwavelength lightcurves of grb 101225a are revisited by assuming that they are from an off-axis grb powered by a newborn magnetar. we show that grb 101225a's optical afterglow lightcurve is fitted with the forward shock model by parameterizing its jet structure as a gaussian function with a half-opening angle of the jet core as 1°67. the derived initial lorentz factor (γ0) is 120, and the viewing angle to the jet axis is θv = 3°7. tentative qpo signatures of p = 488 s and p = 250 ~ 300 s are found with a confidence level of 90% by analyzing its x-ray flares observed in the time interval of [4900, 7500] s. its global gamma-ray/x-ray lightcurve and the qpo signatures are represented with the magnetar dipole radiation (dr) model by considering the magnetar precession motion, assuming that the magnetar spindown is dominated by gw emission. the bulk lorentz factor of the dr ejecta is limited to 8, being much lower than γ0. comparing grb 101225a with the extremely off-axis grb 170817a, we suspect that the nature of the two-component jet in grb 170817a is a combination of a co-axial grb jet and a dr ejecta. grb 101225a would be among the brightest ones of the cdf-s xt2-like x-ray transient population driven by newborn magnetars. a discussion of the detectability of its gravitational wave emission is also presented. | grb 101225a as orphan dipole radiation of a newborn magnetar with precession rotation in an off-axis gamma-ray burst |
the recent ligo/virgo detection of gravitational radiation from gw170817 bears the clear signature of a binary neutron star merger and has validated a number of theoretical predictions. high-energy radiation in the form of a short gamma-ray burst was observed 1.7 s following the merger, and an extended optical/infrared afterglow lasting weeks was observed by dozens of observatories. short gamma-ray bursts had been thought to be produced by mergers, and the afterglow is the predicted signal of radioactive decays from decompressing neutron-rich matter catastrophically ejected from the merging stars. the afterglow provided solid evidence that neutron star mergers are a major, if not primary, source of r-process nuclei, that half of all nuclei heavier than iron formed from the rapid capture of neutrons. although this idea was proposed nearly 45 years ago, it was largely ignored in favor of a supernova mechanism. over the last decade, however, evidence has been accumulating that supernovae are not the primary r-process source. gw170817 may have finally settled this question, which has been one of the thorniest problems in nuclear physics and astrophysics. this article presents my personal perspective of this paradigm shift. | impact of gw170817 for the nuclear physics of the eos and the r-process |
we show the existence of two families of short gamma-ray bursts (grbs), both originating from the merger of binary neutron stars (nss): family-1 with {e}{iso}\lt {10}52 erg, leading to a massive ns as the merged core, and family-2 with {e}{iso}\gt {10}52 erg, leading to a black hole (bh). following the identification of the prototype grb 090227b, we present the details of a new example of family-2 short burst: grb 140619b. from the spectral analysis of the early ∼0.2 s, we infer an observed temperature {kt}=(324+/- 33) kev of the {{{e}}}+{{{e}}}--plasma at transparency (p-grb), a theoretically derived redshift z=2.67+/- 0.37, a total burst energy {e}{{{e}}+{{{e}}}-}{tot}=(6.03+/- 0.79)× {10}52 erg, a rest-frame peak energy {e}p,i=4.7 mev, and a baryon load b=(5.52+/- 0.73)× {10}-5. we also estimate the corresponding emission of gravitational waves. two additional examples of family-2 short bursts are identified: grb 081024b and grb 090510, remarkable for its well determined cosmological distance. we show that marked differences exist in the nature of the afterglows of these two families of short bursts: family-2 bursts, leading to bh formation, consistently exhibit high energy emission following the proper-grb emission; family-1 bursts, leading to the formation of a massive ns, should never exhibit high energy emission. we also show that both the families fulfill an {e}p,i-eiso relation with slope γ =0.59+/- 0.07 and a normalization constant incompatible with the one for long grbs. the observed rate of such family-2 events is {ρ }0=≤ft({2.1}-1.4+2.8\right)× {10}-4 gpc-3 yr-1. | grb 140619b: a short grb from a binary neutron star merger leading to black hole formation |
gamma-ray bursts (grbs) categorically produce broad-band afterglow emission, but in some cases, emission in the optical band is dimmer than expected based on the contemporaneously observed x-ray flux. this phenomenon, aptly dubbed 'optical darkness', has been studied extensively in long grbs (associated with the explosive deaths of massive stars), with possible explanations ranging from host environment extinction to high redshift to possibly unique emission mechanisms. however, investigations into optical darkness in short grbs (associated with the mergers of compact object binaries) have thus far been limited. this work implements a procedure for determining the darkness of grbs based on spectral indices calculated using temporally matched swift-x-ray telescope data and optical follow-up observations; presents a complete and up-to-date catalogue of known short grbs that exhibit optical darkness; and outlines some of the possible explanations for optically dark short grbs. in the process of this analysis, we developed versatile and scalable data processing code that facilitates reproducibility and reuse of our pipeline. these analysis tools and resulting complete sample of dark short grbs enable a systematic statistical study of the phenomenon and its origins, and reveal that optical darkness is indeed quite rare in short grbs, and highly dependent on observing response time and observational effects. | optical darkness in short-duration γ-ray bursts |
afterglows not associated with any gamma-ray bursts (grbs) are called orphan afterglows. the detection of such afterglows is an important goal in many sky survey programs. recently, a promising orphan afterglow candidate, at2021any, was found by the zwicky transient facility. in this work, we performed multi-wavelength fittings of at2021any with two different outflow models, namely, the top-hat jet model and the structured gaussian jet model. although both models can fit the observed light curves well, we found that the structured gaussian jet model presents a better result and is thus preferred by observations. in this framework, the best-fit lorentz factor is about 68, which indicates that at2021any should be a failed grb. the half-opening angle of the jet and the viewing angle were found to be 0.1 and 0.02, respectively, which means that the jet is essentially observed on-axis. we inferred the trigger time of the grb to be about 1000 s before the first detection of the orphan afterglow, and we derived an upper limit of 21.5% for the radiative efficiency, which is typical for grbs. | possible origin of at2021any: a failed gamma-ray burst from a structured jet |
grb 190114c is an unusual gamma-ray burst (grb) due to its detection at sub-tev energies by magic, seen at redshift z = 0.42. this burst is one of the brightest grb detected by fermi. a joint gbm-lat analysis of the prompt emission reveals the presence of sub-gev spectral cutoff when the lat low-energy event (lle) data is also examined. a similar high-energy cutoff was likewise reported in grb 160509a and grb 100724b earlier, as well as a handful of other sources. the cutoff can be explained by the intrinsic opacity due to pair production within the emitting region. grb 190114c shows a transition from nonthermal to a quasi-thermal-like spectrum and a radiation component that can be attributed to afterglow. based on spectral analysis, we constrain the site of the prompt emission and lorentz factor. knowing that sub-tev photons are detected in magic, we perceive that the observed spectrum is indeed an overlap from two emission sites, where the emission observed in fermi is more consistent with prompt emission produced via photospheric dissipation along with a concurrent component from the external shock. | magical grb 190114c: implications of cutoff in the spectrum at sub-gev energies |
the mechanisms of particle acceleration and radiation, as well as magnetic field build-up and decay in relativistic collisionless shocks, are open questions with important implications to various phenomena in high-energy astrophysics. while the weibel instability is possibly responsible for magnetic field build-up and diffusive shock acceleration is a model for acceleration, both have problems and current particle-in-cell simulations show that particles are accelerated only under special conditions and the magnetic field decays on a very short length-scale. we present here a novel model for the structure and the emission of highly relativistic collisionless shocks. the model takes into account (and is based on) non-local energy and momentum transport across the shock front via emission and absorption of high-energy photons. this leads to a pre-acceleration of the fluid and pre-amplification of the magnetic fields in the upstream region. both have drastic implications on the shock structure. the model explains the persistence of the shock-generated magnetic field at large distances from the shock front. the dissipation of this magnetic field results in a continuous particle acceleration within the downstream region. a unique feature of the model is the existence of an `attractor', towards which any shock will evolve. the model is applicable to any relativistic shock, but its distinctive features show up only for sufficiently large compactness. we demonstrate that prompt and afterglow gamma-ray bursts' shocks satisfy the relevant conditions, and we compare their observations with the predictions of the model. | particle acceleration, magnetization and radiation in relativistic shocks |
we present x-ray and multiband optical observations of the afterglow and host galaxy of grb 180418a, discovered by swift/bat and fermi/gbm. we present a reanalysis of the gbm and bat data deriving durations of the prompt emission of t90 ≈ 2.56 and 1.90 s, respectively. modeling the fermi/gbm catalog of 1405 bursts (2008-2014) in the hardness-t90 plane, we obtain a probability of ≈60% that grb 180418a is a short-hard burst. from a combination of swift/xrt and chandra observations, the x-ray afterglow is detected to ≈38.5 days after the burst and exhibits a single power-law decline with fx ∝ t-0.98. late-time gemini observations reveal a faint r ≈ 25.69 mag host galaxy at an angular offset of ≈0"16. at the likely redshift range of z ≈ 1-2.25, we find that the x-ray afterglow luminosity of grb 180418a is intermediate between short and long gamma-ray bursts (grbs) at all epochs during which there are contemporaneous data and that grb 180418a lies closer to the eγ,peak-eγ,iso correlation for short grbs. modeling the multiwavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of θj ≳ 9°-14°. if grb 180418a is a short grb that originated from a neutron star merger, it has one of the brightest and longest-lived afterglows along with an extremely faint host galaxy. if, instead, the event is a long grb that originated from a massive star collapse, it has among the lowest-luminosity afterglows and lies in a peculiar space in terms of the hardness-t90 and eγ,peak-eγ,iso planes. | grb 180418a: a possibly short gamma-ray burst with a wide-angle outflow in a faint host galaxy |
gw170817 was the first ever joint detection of gravitational waves (gw) from a binary neutron star (bns) merger with the detections of short γ-ray burst (sgrb) counterparts. analysis of the multiband afterglow observations of over more than a year revealed that the outflow from the merger end product was consistent with structured relativistic jet models with the core of the jet narrowly collimated to half-opening angles ∼5°. in this work, assuming that all the bns mergers produce gaussian structured jets with properties as inferred for gw170817, we explore the prospects of joint detections of bns mergers and prompt γ-ray emission, expected during the current and upcoming upgrades of ligo-virgo-kagra detectors. we discuss three specific observational aspects: 1) the distribution of detected binary inclination angles, 2) the distance reach, and 3) the detection rates. unlike gw-only detections, the joint detections are greatly restricted at large inclination angles, due to the structure of the jets. we find that at lower inclination angles (say below 20°), the distance reach as well as the detection rates of the joint detections are limited by gw detectability while at larger inclinations (say above 20°), they are limited by the γ-ray detectability. | prospects of joint detections of neutron star mergers and short grbs with gaussian structured jets |
given an increasing number of gamma-ray bursts accompanied by potential kilonovae, there is a growing importance to advance modelling of kilonova afterglows. in this work, we investigate how the presence of two electron populations that follow a maxwellian (thermal) and a power-law (non-thermal) distribution affect kilonova afterglow light curves. we employ semi-analytic afterglow model, pyblastafterglow. we consider kilonova ejecta profiles from ab-initio numerical relativity binary neutron star merger simulations, targeted to gw170817. we do not perform model selection. we find that the emission from thermal electrons dominates at early times. if the interstellar medium density is high (${\simeq }0.1\, \, \text{cm}^{-3}$), it adds an early time peak to the light curve. as ejecta decelerates, the spectral and temporal indexes change in a characteristic way that, if observed, can be used to reconstruct the ejecta velocity distribution. for the low interstellar medium density, inferred for grb 170817a, the emission from the non-thermal electron population generally dominates. we also assess how kilonova afterglow light curves change if the interstellar medium has been partially removed and pre-accelerated by laterally expanding gamma-ray burst ejecta. for the latter, we consider properties informed by observations of grb170817a. we find that the main effect is the emission suppression at early time ${\lesssim }10^{3}\,$ days, and at its maximum it reaches ${\sim }40{{\ \rm per\ cent}}$ when the fast tail of the kilonova ejecta moves subsonically through the wake of laterally spreading gamma-ray burst ejecta. the subsequent rebrightening, when these ejecta break through and shocks form, is very mild (${\lesssim }10{{\ \rm per\ cent}}$) and may not be observable. | modelling kilonova afterglows: effects of the thermal electron population and interaction with grb outflows |
we present the results of an intensive multi-epoch radio frequency campaign on the energetic and nearby grb 171010a with the karl g. janksy very large array and arcminute microkelvin imager large array. we began observing grb 171010a a day after its initial detection, and were able to monitor the temporal and spectral evolution of the source over the following weeks. the spectra and their evolution are compared to the canonical theories for broad-band grb afterglows, with which we find a general agreement. there are, however, a number of features that are challenging to explain with a simple forward shock model, and we discuss possible reasons for these discrepancies. this includes the consideration of the existence of a reverse shock component, potential microphysical parameter evolution, and the effect of scintillation. | a detailed radio study of the energetic, nearby, and puzzling grb 171010a |
we report in this work the creation of new heavy and efficient tb3al5o12:ce (tbag:ce) single crystalline film (scf) scintillators, grown by lpe method from pbo-b2o3 based flux onto y3al5o12 (yag) and gd3ga2.5al2.5o12 (gagg) substrates, for different optoelectronic applications. the luminescent and scintillation properties of the tbag:ce scf screens, grown onto different types of substrates, are studied and compared with the properties of the lu3al5o12:ce (luag:ce) and yag:ce scf counterparts. tbag:ce scfs show very high scintillation light yield (ly) under α-particles excitation, which overcomes by 30% the ly of high-quality luag:ce scf samples. in comparison with yag:ce and luag:ce scfs, tbag:ce scf screens show also significantly lower afterglow (up to 10-4 level at x-ray burst duration of 0.1 s), which is comparable with the afterglow level of the best samples of lso:ce, tb scfs typically being used now for microimaging. together with a high light output of x-ray excited luminescence, such extremely low afterglow of tbag:ce scf is a very good reason for future development of scintillating screens based on the mentioned garnet. we also introduce the possibility to create new types of ;film-substrate; hybrid scintillators using the lpe method for simultaneous registration of different components of ionizing radiation and microimaging based on the tbag:ce scf and gagg:ce substrates. | scintillating screens based on the lpe grown tb3al5o12:ce single crystalline films |
we present very early ground-based optical follow-up observations of gamma-ray burst (grb) 140423a, which was discovered by the swift/burst alert telescope (bat) and by the fermi/gamma-ray burst monitor. its broadband afterglow was monitored by the swift/x-ray telescope and ground-based optical telescopes from t0 + 70.96 s to 4.8 days after the swift/bat trigger. this is one more case of a prompt optical emission observation. the temporal and spectral joint fits of the multiwavelength light curves of grb 140423a reveal that achromatic behavior is consistent with the external shock model, including a transition from a stellar wind to the interstellar medium (ism) and energy injection. in terms of the optical light curves, there is an onset bump in the early afterglow with a rising index ${\alpha }_{{\rm{o}},{\rm{i}}}=-0.59\pm 0.04$ (peaking at ${t}_{\mathrm{peak}}-{t}_{0}\approx 206$ s). it then decays with a steep index ${\alpha }_{{\rm{o}},\mathrm{ii}}=1.78\pm 0.03$ , and shows a steeper to flatter "transition" with ${\alpha }_{{\rm{o}},\mathrm{iii}}=1.13\pm 0.03$ at around t0 + 5000 s. the observed x-ray afterglow reflects achromatic behavior, as does the optical light curve. there is no obvious evolution of the spectral energy distribution between the x-ray and optical afterglows, with an average value of the photon index ${\rm{\gamma }}\approx 1.95$ . this "transition" is consistent with an external shock model having the circumburst medium transition from a wind to the ism, by introducing a long-lasting energy injection with a lorentz factor stratification of the ejecta. the best parameters from monte carlo markov chain fitting are ${e}_{{\rm{k}},\mathrm{iso}}\approx 2.14\times {10}^{55}$ erg, ${{\rm{\gamma }}}_{0}\approx 162$ , ${\varepsilon }_{e}\approx 0.02$ , ${\varepsilon }_{b}\approx 1.7\times {10}^{-6}$ , ${a}_{* }\approx 1.0$ , ${r}_{t}\approx 4.1\times {10}^{17}$ cm, $n\approx 11.0\,\,{\mathrm{cm}}^{-3}$ , ${l}_{0}\approx 3.1\times {10}^{52}\,\mathrm{erg}\ {{\rm{s}}}^{-1}$ , $k\approx 1.98$ , $s\,\approx 1.54$ , and ${\theta }_{j}\gt 0.3\,\mathrm{rad}$ . | grb 140423a: a case of stellar wind to interstellar medium transition in the afterglow |
context. the gamma-ray bursts hosts (grbhs) are excellent targets to study the extinction properties of dust and its effects on the global emission of distant galaxies. the dust extinction curve is measured along the grb afterglow line of sight and the analysis of the spectral energy distribution (sed) of the host galaxy gives access to the global dust attenuation of the stellar light.aims: in this pilot study we gather information on dust extinction in grbhs to compare the properties of the extinction curve to those of the dust obscuration affecting the total stellar light of the host galaxy. assuming the extinction curve to be representative of the dust properties, we aim to investigate which dust-stars geometries and local dust distribution in the inter stellar medium (ism) can reproduce the observed attenuation curve.methods: we selected a sample of 30 grbs for which the extinction curve along the grb afterglow line-of-sight (l.o.s.) is measured in the rest-frame ultraviolet (uv) up to optical and we analysed the properties of the extinction curve as a function of the host galaxy properties. from these 30 grbs, we selected seven grbhs with a good rest-frame uv to near-infrared (nir) spectral coverage for the host. the attenuation curve was derived by fitting the seds of the grbh sample with the cigale sed fitting code. different star formation histories (sfh) were studied to recover the star formation rates (sfr) derived using hα luminosities. implications for the dust-stars geometries in the ism are inferred by a comparison with radiative transfer simulations.results: the most extinguished grbs are preferentially found in the more massive hosts and the uv bump is preferentially found in the most extinguished grb l.o.s. five out of seven hosts are best fitted with a recent burst of star formation, leading to lower stellar mass estimates than previously found. the average attenuation in the host galaxies is about 70% of the amount of extinction along the grb l.o.s. we find a great variety in the derived attenuation curves of grbhs, the uv slope can be similar, flatter or even steeper than the extinction curve slope. half of the attenuation curves are consistent with the calzetti attenuation law and there is evidence of a uv bump in only one grbh. we find that the flatter (steeper) attenuation curves are found in galaxies with the highest (lowest) sfr and stellar masses. the comparison of our results with radiative transfer simulations leads to a uniform distribution of dust and stars in a very clumpy ism for half the grbhs and various dust-stars geometries for the second half of the sample. | investigation of dust attenuation and star formation activity in galaxies hosting grbs |
on 2011 march 28, the swift burst alert telescope triggered on an object that had no analog in over six years of swift operations. follow-up observations by the swift x-ray telescope (xrt) found a new, bright x-ray source covering three orders of magnitude in flux over the first few days, that was much more persistent (and variable) than gamma-ray burst afterglows. ground-based spectroscopy found a redshift of 0.35, implying extremely high luminosity, with integrated isotropic-equivalent energy output in the x-ray band alone exceeding 1053 erg in the first two weeks after discovery. strong evidence for a collimated outflow or beamed emission was found. the observational properties of this object are unlike anything ever before observed. we interpret these unique properties as the result of emission from a relativistic jet produced in the aftermath of the tidal disruption of a main sequence star by a massive black hole (bh) in the center of the host galaxy. the source decayed slowly as the stellar remnants were accreted onto the bh, before abruptly shutting off. here we present the definitive xrt team light curve for swift j164449.3+573451 and discuss its implications. we show that the unabsorbed flux decayed roughly as a {t}-1.5 power law up to 2012 august 17. the steep turnoff of an order of magnitude in 24 hr seems to be consistent with the shutdown of the jet as the accretion disk transitioned from a thick disk to a thin disk. swift continues to monitor this source in case the jet reactivates. | the definitive x-ray light curve of swift j164449.3+573451 |
we present time-resolved spectral analysis of the steep decay segments of 29 bright x-ray flares of gamma-ray bursts (grbs) observed with the swift/x-ray telescope, and model their light curves and spectral index evolution behaviors with the curvature effect model. our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fitted with this model, and the derived characteristic timescales (tc ) are in the range of 23 ∼ 264 s. using an empirical relation between the peak luminosity and the lorentz factor derived from the prompt gamma-rays, we estimate the lorentz factors of the flares (γx). we obtain γx = 17 ∼ 87 with a median value of 52, which is smaller than the initial lorentz factors of prompt gamma-ray fireballs. with the derived tcand γx, we constrain the radiating regions of 13 x-ray flares, yielding r x = (0.2 ∼ 1.1) × 1016 cm, which are smaller than the radii of the afterglow fireballs at the peak times of the flares. a long evolution feature from prompt gamma-ray phase to the x-ray epoch is found by incorporating our results with a sample of grbs whose initial lorentz factors are available in the literature, i.e., {{γ }}\propto {[{t}p/(1+z)]}-0.69+/- 0.06. these results may shed light on the long-term evolution of grb central engines. | the history of grb outflows: ejection lorentz factor and radiation location of x-ray flares |
tidal disruption events (tdes) that occur in active galactic nuclei (agns) with dusty tori are a special class of sources. tdes can generate ultrafast and large opening-angle wind, which will almost inevitably collide with the pre-existing agn dusty tori a few years later after the tde outburst. the wind-torus interactions drive two kinds of shocks: the bow shocks at the windward side of the torus clouds, and the cloud shocks inside the torus clouds. in a previous work, we proved that the shocked clouds will give rise to considerable x-ray emissions which can reach 1041 - 42 erg s-1 (so called years delayed x-ray afterglows). in this work, we focus on the radiations of high energy particles accelerated at both shocks. benefiting from the strong radiation field at the inner edge of the torus, the inverse compton scatterings of agn photons by relativistic electrons at bow shocks dominate the overall gamma-ray radiation. the gamma-ray luminosity can reach $10^{41}~{\rm erg~s^{-1}}(l_{\rm kin}/10^{45}\, {\rm erg \, s^{-1}})$, where lkin is the kinetic luminosity of tde wind. synchrotron radiation at bow shocks contributes to the radio afterglow with a luminosity of 10$^{38-39} ~{\rm erg~s^{-1}}(l_{\rm kin}/10^{45}\, {\rm erg \, s^{-1}})$ at 1-10 ghz if the magnetic field is 100 mgauss, and extends to infrared with a luminosity of $\sim 10^{39-40}~{\rm erg \, s^{-1}} (l_{\rm kin}/10^{45}\, {\rm erg \, s^{-1}})$. our scenario provides a prediction of the years delayed afterglows in multiple wavebands for tdes and reveals their connections. | years delayed gamma-ray and radio afterglows originated from tde wind-torus interactions |
the central engine of gamma-ray bursts (grbs) remains an open and cutting-edge topic in the era of multimessenger astrophysics. x-ray plateaus appear in some grb afterglows, which are widely considered to originate from the spindown of magnetars. according to the stable magnetar scenario of grbs, an x-ray plateau and a decay phase ~t -2 should appear in x-ray afterglows. meanwhile, the "normal" x-ray afterglow is produced by the external shock from a grb fireball. we analyze the neil gehrels swift grb data, then find three gold samples that have an x-ray plateau and a decay phase ~t -2 superimposed on the jet-driven normal component. based on these features of the lightcurves, we argue that the magnetars should be the central engines of these three grbs. future joint multimessenger observations might further test this possibility, which can then be beneficial to constrain grb physics. | evidence of x-ray plateaus driven by the magnetar spindown winds in gamma-ray burst afterglows |
it has been observationally established that supernovae (sne) of type ic produce long duration gamma ray bursts (grbs) and that neutron star mergers generate short hard grbs. sn-less grbs presumably originate in a phase transition of a neutron star in a high mass x-ray binary. how these phenomena actually generate grbs is debated. the fireball and cannonball models of grbs and their afterglows have been widely confronted with the huge observational data, with their defenders claiming success. the claims, however, may reflect multiple choices and the use of many adjustable parameters, rather than the validity of the models. only a confrontation of key falsifiable predictions of the models with solid observational data can test their validity. such critical tests are reviewed in this report. | critical tests of leading gamma ray burst theories ii |
we use joint observations by the swift x-ray telescope (xrt) and the fermi large area telescope (lat) of gamma-ray burst (grb) afterglows to investigate the nature of the long-lived high-energy emission observed by fermi lat. joint broadband spectral modeling of xrt and lat data reveals that lat nondetections of bright x-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the lat and/or xrt energy ranges. such a break is sufficient to suppress the high-energy emission so as to be below the lat detection threshold. by contrast, lat-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the xrt and lat energy ranges. we speculate that the primary difference between grbs with lat afterglow detections and the nondetected population may be in the type of circumstellar environment in which these bursts occur, with late-time lat detections preferentially selecting grbs that occur in low wind-like circumburst density profiles. furthermore, we find no evidence of high-energy emission in the lat-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed x-ray emission. the lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-compton (ssc) component in the lat energy range. alternatively, the peak of the ssc emission could be beyond the 0.1-100 gev energy range considered for this analysis. | investigating the nature of late-time high-energy grb emission through joint fermi/swift observations |
thunderstorms are known to create terrestrial gamma ray flashes (tgfs) which are microsecond-long bursts created by runaway of thermal electrons from propagating lightning leaders, as well as gamma ray glows that possibly are created by relativistic runaway electron avalanches (rrea) that can last for minutes or more and are sometimes terminated by a discharge. in this work we predict a new intermediate thunderstorm radiation mechanism, which we call tgf afterglow, as it is caused by the capture of photonuclear neutrons produced by a tgf. tgf afterglows are milliseconds to seconds long; this duration is caused by the thermalization time of the intermediate neutrons. tgf afterglows indicate that the primary tgf has produced photons in the energy range of 10-30 mev; they are nondirectional in contrast to the primary tgf. gurevich et al. might have reported tgf afterglows in 2011. | tgf afterglows: a new radiation mechanism from thunderstorms |
high-redshift gamma-ray bursts (grbs) beyond redshift {∼}6 are potentially powerful tools to probe the distant early universe. their detections in large numbers and at truly high redshifts call for the next generation of high-energy wide-field instruments with unprecedented sensitivity at least one order of magnitude higher than the ones currently in orbit. on the other hand, follow-up observations of the afterglows of high-redshift grbs and identification of their host galaxies, which would be difficult for the currently operating telescopes, require new, extremely large facilities of at multi-wavelengths. this chapter describes future experiments that are expected to advance this exciting field, both being currently built and being proposed. the legacy of swift will be continued by svom, which is equipped with a set of space-based multi-wavelength instruments as well as and a ground segment including a wide angle camera and two follow-up telescopes. the established lobster-eye x-ray focusing optics provides a promising technology for the detection of faint grbs at very large distances, based on which the theseus, einstein probe and other mission concepts have been proposed. follow-up observations and exploration of the reionization era will be enabled by large facilities such as ska in the radio, the 30 m class telescopes in the optical/near-ir, and the space-borne wfirst and jwst in the optical/near-ir/mid-ir. in addition, the x-ray and γ-ray polarization experiment polar is also introduced. | perspectives on gamma-ray burst physics and cosmology with next generation facilities |
x-ray flares were discovered in the afterglow phase of gamma-ray bursts (grbs) by the swift satellite a decade ago and are known as a canonical component in grb x-ray afterglows. in this paper, we constrain the lorentz factors of grb x-ray flares using two different methods. for the first method, we estimate the lower limit on the bulk lorentz factor with the flare duration and jet break time. in the second method, the upper limit on the lorentz factor is derived by assuming that the x-ray flare jet has undergone saturated acceleration. we also re-estimate the initial lorentz factor with grb afterglow onsets, and find the coefficient of the theoretical lorentz factor is 1.67 rather than the commonly used 2 for the interstellar medium (ism) and 1.44 for the wind case. we find that the correlation between the limited lorentz factor and the isotropic radiation energy of x-ray flares in the ism case is more consistent with that of prompt emission than the wind case in a statistical sense. for a comparison, the lower limit on the lorentz factor is statistically larger than the extrapolation from prompt bursts in the wind case. our results indicate that x-ray flares and prompt bursts are produced by the same physical mechanism. | constraints on the bulk lorentz factors of grb x-ray flares |
we present a detailed prompt emission and early optical afterglow analysis of the two very-high-energy (vhe) detected bursts grb 201015a and grb 201216c, and their comparison with a subset of similar bursts. time-resolved spectral analysis of multistructured grb 201216c using the bayesian binning algorithm revealed that during the entire duration of the burst, the low-energy spectral index (α pt) remained below the limit of the synchrotron line of death. however, statistically some of the bins supported the additional thermal component. additionally, the evolution of spectral parameters showed that both the peak energy (e p) and α pt tracked the flux. these results were further strengthened using the values of the physical parameters obtained by synchrotron modeling of the data. our earliest optical observations of both bursts using the f/photometric robotic atmospheric monitor observatorio del roque de los muchachos and burst observer and optical transient exploring system robotic telescopes displayed a smooth bump in their early optical light curves, consistent with the onset of the afterglow due to synchrotron emission from an external forward shock. using the observed optical peak, we constrained the initial bulk lorentz factors of grb 201015a and grb 201216c to γ0 = 204 and γ0 = 310, respectively. the present early optical observations are the earliest known observations constraining outflow parameters and our analysis indicate that vhe detected bursts could have a diverse range of observed luminosity within the detectable redshift range of present vhe facilities. | prompt emission and early optical afterglow of very-high-energy detected grb 201015a and grb 201216c: onset of the external forward shock |
gamma-ray bursts (grbs), as extremely energetic explosions in the universe, are widely believed to consist of two stages: the prompt phase and the subsequent afterglow. recent studies indicate that some high-energy photons are emitted earlier at source than the prompt phase. due to the light speed variation, these high-energy photons travel more slowly than the low-energy photons, so that they are observed after the prompt low-energy photons at the detector. based on the data from the fermi gamma-ray space telescope (fgst), we analyse the photon distribution before the prompt emission in detail and propose the existence of a hitherto unknown pre-burst stage of grbs by adopting a classification method of machine learning. analysis on the photons automatically selected by machine learning also produces a light speed variation at elv = 3.55 ×1017gev. | novel pre-burst stage of gamma-ray bursts from machine learning |
long-duration gamma-ray bursts (grbs) are generally related to the core-collapse of massive stars. in the collapsar scenario, a rotating stellar-mass black hole (bh) surrounded by a hyperaccretion disk has been considered as one of the plausible candidates of grb central engines. in this paper, we work on a sample including 146 long grbs with significant jet break features in the multiband afterglows. the jet opening angles can then be obtained by the jet break time. by assuming grb jets powered by the blandford-znajek (bz) mechanism in the bh hyperaccretion system, we analyze the distributions of the long grb luminosities and durations in the samples, and constrain the accretion rates for the different bh spins. as a result, we find that the bz mechanism is so powerful that it is possible to interpret the long grb prompt emissions within reasonable accretion rates. | testing blandford-znajek mechanism in black hole hyperaccretion flows for long-duration gamma-ray bursts |
the zwicky transient facility recently announced the detection of an optical transient at2020blt at redshift z = 2.9, consistent with the afterglow of an on-axis gamma-ray burst. however, no prompt emission was observed. we analyse at2020blt with detailed models, showing the data are best explained as the afterglow of an on-axis long gamma-ray burst, ruling out other hypotheses such as a cocoon and a low-lorentz factor jet. we search fermi data for prompt emission, setting deeper upper limits on the prompt emission than in the original detection paper. together with konus-wind observations, we show that the gamma-ray efficiency of at2020blt is $\lesssim 0.3\!-\!4.5{{\ \rm per\, cent}}$. we speculate that at2020blt and at2021any belong to the low-efficiency tail of long gamma-ray burst distributions that are beginning to be readily observed due to the capabilities of new observatories like the zwicky transient facility. | low-efficiency long gamma-ray bursts: a case study with at2020blt |
we present synthetic radio images of the grb 170817a afterglow, computed from moving-mesh hydrodynamic simulations of binary neutron star merger outflows. having expanded for nearly a year, the merger remnant is expected to subtend ∼5 mas on the sky, potentially resolvable by very long baseline radio imaging techniques. any observations revealing the radio centroid to be offset from the line of sight to the merger would be the smoking gun of a jetted outflow. however, our results indicate that a measurement of the centroid position alone cannot independently determine whether that jet escaped successfully from the merger debris cloud, or was “choked,” yielding a quasi-spherical explosion. we find that in both scenarios, the centroid exhibits superluminal proper motion away from the merger site at roughly 4-10 μas per day for at least the first 300 days. we argue that a successful strategy for differentiating among the explosion models will need to include multiple observations over the coming months to years. in particular, we find the time at which the centroid attains its maximum offset, and begins heading back toward the merger site, is considerably later if the jet was choked. detecting a reversal of the centroid trajectory earlier than 600 days would uniquely identify a successful jet. our results indicate that the source might be resolved using very long baseline interferometry (vlbi) radio observing techniques with ∼1 mas resolution starting at roughly 400 days post-merger, and that the the angular extent of a successful jet is significantly smaller than that of a choked jet (4.5 versus 7 mas respectively). | radio sky maps of the grb 170817a afterglow from simulations |
the afterglow emission from gamma-ray bursts (grbs) is a valuable source of information to understand the physics of these energetic explosions. the fireball model has become the standard to describe the evolution of the afterglow emission over time and frequency. because of recent developments in the theory of afterglows and numerical simulations of relativistic outflows, we are able to model the afterglow emission with realistic dynamics and radiative processes. although the models agree with observations remarkably well, the afterglow emission still contains additional physics, instrumental systematics, and propagation effects that make the modelling of these events challenging. in this work, we present a new approach to modelling grb afterglows, using gaussian processes (gps) to take into account systematics in the afterglow data. we show that, using this new approach, it is possible to obtain more reliable estimates of the explosion and microphysical parameters of grbs. we present fit results for five long grbs and find a preliminary correlation between the isotropic energetics and opening angles of grbs, which confirms the idea of a common energy reservoir for the kinetic energy of long grbs. | a new approach to modelling gamma-ray burst afterglows: using gaussian processes to account for the systematics |
the prompt emission of the long, smooth, and single-pulsed gamma-ray burst, grb 141028a, is analyzed under the guise of an external shock model. first, we fit the γ-ray spectrum with a two-component photon model, namely, synchrotron+blackbody, and then fit the recovered evolution of the synchrotron νfνpeak to an analytic model derived considering the emission of a relativistic blast wave expanding into an external medium. the prediction of the model for the νfνpeak evolution matches well with the observations. we observe the blast wave transitioning into the deceleration phase. furthermore, we assume the expansion of the blast wave to be nearly adiabatic, motivated by the low magnetic field deduced from the observations. this allows us to recover within an order of magnitude the flux density at the νfνpeak, which is remarkable considering the simplicity of the analytic model. under this scenario we argue that the distinction between prompt and afterglow emission is superfluous as both early-time emission and late-time emission emanate from the same source. while the external shock model is clearly not a universal solution, this analysis opens the possibility that at least some fraction of grbs can be explained with an external shock origin of their prompt phase. | an external shock origin of grb 141028a |
the central engine and jet composition of gamma-ray bursts (grbs) remain mysterious. here we suggest that observations on the polarization evolution of early optical afterglows may shed light on these questions. we first study the dynamics of a reverse shock and a forward shock that are generated during the interaction of a relativistic jet and its ambient medium. the jet is likely magnetized with a globally large-scale magnetic field from the central engine. the existence of the reverse shock requires that the magnetization degree of the jet should not be high (σ ≤ 1), so that the jet is mainly composed of baryons and leptons. we then calculate the light curves and polarization evolution of early optical afterglows and find that when the polarization position angle changes by 90° during the early afterglow, the polarization degree is zero for a toroidal magnetic field but is very likely to be nonzero for an aligned magnetic field. this result would be expected to provide a probe for the central engine of grbs because an aligned field configuration could originate from a magnetar central engine and a toroidal field configuration could be produced from a black hole via the blandford-znajek mechanism. finally, for such two kinds of magnetic field configurations, we fit the observed data of the early optical afterglow of grb 120308a equally well. | polarization evolution of early optical afterglows of gamma-ray bursts |
we numerically simulate the gamma-ray burst (grb) afterglow emission with a one-zone time-dependent code. the temporal evolutions of the decelerating shocked shell and energy distributions of electrons and photons are consistently calculated. the photon spectrum and light curves for an observer are obtained taking into account the relativistic propagation of the shocked shell and the curvature of the emission surface. we find that the onset time of the afterglow is significantly earlier than the previous analytical estimate. the analytical formulae of the shock propagation and light curve for the radiative case are also different from our results. our results show that even if the emission mechanism is switching from synchrotron to synchrotron self-compton, the gamma-ray light curves can be a smooth power law, which agrees with the observed light curve and the late detection of a 32 gev photon in grb 130427a. the uncertainty of the model parameters obtained with the analytical formula is discussed, especially in connection with the closure relation between spectral index and decay index. | temporal evolution of the gamma-ray burst afterglow spectrum for an observer: gev-tev synchrotron self-compton light curve |
evidence for the central engine of gamma-ray bursts (grbs) has been collected in the neil gehrels swift data. for instance, some grbs show an internal x-ray plateau followed by very steep decay, which is difficult interpret within the framework of a black hole (bh) central engine, but is consistent within a rapidly spinning magnetar engine picture. the very steep decay at the end of the plateau suggests a sudden cessation of the central engine, which is explained as the collapse of a supramassive magnetar into a bh when it spins down. here we propose that some additional evidence, such as a second x-ray plateau feature, would show up if the fallback accretion could activate the newborn bh and sufficient energy could be transferred from the newborn bh to the grb blast wave. with a systematic data analysis for all long grbs, we find three candidates in the swift sample, i.e., grbs 070802, 090111, and 120213a, whose x-ray afterglow lightcurves contain two plateaus, with the first one being an internal plateau. we find that in a fairly loose and reasonable parameter space, the second x-ray plateau data for all 3 grbs could be interpreted with our proposed model. future observations are likely to discover similar events, which could offer more information on the properties of the magnetar, as well as the newborn bh. | the second plateau in x-ray afterglow providing additional evidence for rapidly spinning magnetars as the grb central engine |
the transition from prompt to afterglow emission is one of the most exciting and least understood phases in gamma-ray bursts (grbs). correlations among optical, x-ray, and gamma-ray emission in grbs have been explored, to attempt to answer whether the earliest optical emission comes from internal and/or external shocks. we present optical photometric observations of grb 180325a collected with the tarot and ratir ground-based telescopes. these observations show two strong optical flashes with separate peaks at ∼50 and ∼120 s, followed by a temporally extended optical emission. we also present x-rays and gamma-ray observations of grb 180325a, detected by the burst alert telescope and x-ray telescope, on the neil gehrels swift observatory, which both observed a narrow flash at ∼80 s. we show that the prompt gamma-ray and x-ray early emission shares similar temporal and spectral features consistent with internal dissipation within the relativistic outflow (e.g., by internal shocks or magnetic reconnection), while the early optical flashes are likely generated by the reverse shock that decelerates the ejecta as it sweeps up the external medium. | modeling the prompt optical emission of grb 180325a: the evolution of a spike from the optical to gamma rays |
long gamma-ray bursts show an afterglow emission in the x-rays, optical, and radio wavelengths with luminosities that fade with time with a nearly identical power-law behavior. in this talk, i present an analytic treatment that shows that this afterglow is produced by synchrotron radiation from the supernova ejecta associated with binary-driven hypernovae. | on the origin of the long gamma-ray burst afterglow as synchrotron radiation from binary-driven hypernovae |
we present optical and near-infrared (nir) photometric observations of grb 191016 with the coatli,ddoti, and ratir ground-based telescopes over the first three nights. we present the temporal evolution of the optical afterglow and describe five different stages that were not completely characterized in previous works, mainly due to scarcity of data points to accurately fit the different components of the optical emission. after the end of the prompt gamma-ray emission, we observed the afterglow rise slowly in the optical and nir wavelengths and peak at around t + 1450 s in all filters. this was followed by an early decay, a clear plateau from t + 5000 s to t + 11 000 s, and then a regular late decay. we also present evidence of the jet break at later times, with a temporal index in good agreement with the temporal slope obtained from x-ray observations. although many of the features observed in the optical light curves of gamma-ray bursts are usually well explained by a reverse shock (rs) or forward shock (fs), the shallowness of the optical rise and enhanced peak emission in the grb 191016a afterglow is not well fitted by only a fs or a rs. we propose a theoretical model which considers both of these components and combines an evolving fs with a later embedded rs and a subsequent late energy injection from the central engine activity. we use this model to successfully explain the temporal evolution of the light curves and discuss its implications on the fireball properties. | grb 191016a: the onset of the forward shock and evidence of late energy injection |
we summarize the basic observational properties of gamma-ray bursts (grbs), including prompt emission properties, afterglow properties, and classification schemes. we also briefly comment on the current physical understanding of these properties. | grb observational properties |
polarization measurements from relativistic outflows are a valuable tool to probe the geometry of the emission region and the microphysics of the particle distribution. indeed, the polarization level depends on (i) the local magnetic field orientation, (ii) the geometry of the emitting region with respect to the line of sight and (iii) the electron pitch angle distribution. here we consider optically thin synchrotron emission and we extend the theory of circular polarization from a point source to an extended radially expanding relativistic jet. we present numerical estimates for both linear and circular polarization in such systems. we consider different configurations of the magnetic field, spherical and jetted outflows, isotropic and anisotropic pitch angle distributions, and outline the difficulty in obtaining the reported high level of circular polarization observed in the afterglow of gamma ray burst (grb) 121024a. we conclude that the origin of the observed polarization cannot be intrinsic to an optically thin synchrotron process, even when the electron pitch angle distribution is extremely anisotropic. | linear and circular polarization in ultra-relativistic synchrotron sources - implications to grb afterglows |
we study the ensemble of linear polarization measurements in the optical afterglows of long-duration gamma-ray bursts. we assume a non-sideways-expanding top-hat jet geometry and use the relatively large number of measurements under the assumption that they represent a statistically unbiased sample. this allows us to constrain the ratio between the maximum predicted polarization and the measured one, which is an indicator of the geometry of the magnetic field in the downstream region of the external shock. we find that the measured polarization is substantially suppressed with respect to the maximum possible for either a completely ordered magnetic field parallel to the shock normal or to a field that is entirely contained in the shock plane. the measured polarization is limited, on average, to between 25% and 30% of the maximum theoretically possible value. this reduction requires the perpendicular component of the magnetic field to be dominant in energy with respect to the component parallel to the shock front, as expected for a shock-generated and/or shock-compressed field. we find, however, that the data only marginally support the assumption of a simple top-hat jet, pointing toward a more complex geometry for the outflow. | polarization constraints on the geometry of the magnetic field in the external shock of gamma-ray bursts |
early optical observations of gamma-ray bursts can significantly contribute to the study of the central engine and physical processes therein. however, of the thousands observed so far, only a few have data at optical wavelengths in the first minutes after the onset of the prompt emission. here we report on grb 190106a, whose afterglow was observed in optical bands just 36 s after the swift/bat trigger, i.e., during the prompt emission phase. the early optical afterglow exhibits a bimodal structure followed by a normal decay, with a faster decay after ~t 0 + 1 day. we present optical photometric and spectroscopic observations of grb 190106a. we derive the redshift via metal absorption lines from xinglong 2.16 m/bfosc spectroscopic observations. from the bfosc spectrum, we measure z = 1.861 ± 0.002. the double-peak optical light curve is a significant feature predicted by the reverse-forward external-shock model. the shallow decay followed by a normal decay in both the x-ray and optical light curves is well explained with the standard forward-shock model with late-time energy injection. therefore, grb 190106a offers a case study for grb emission from both reverse and forward shocks. | photometric and spectroscopic observations of grb 190106a: emission from reverse and forward shocks with late-time energy injection |
in this work, we study the distribution of temporal power-law decay indices, α, in the gamma-ray burst (grb) afterglow phase, fitted for 176 grbs (139 long grbs, 12 short grbs with extended emission, and 25 x-ray flashes) with known redshifts. these indices are compared with the temporal decay index, αw , derived with the light-curve fitting using the willingale et al. model. this model fitting yields similar distributions of αwto the fitted α, but for individual bursts a difference can be significant. analysis of (α, la ) distribution, where lais the characteristic luminosity at the end of the plateau, reveals only a weak correlation of these quantities. however, we discovered a significant regular trend when studying grb α values along the dainotti et al. correlation between laand the end time of the plateau emission in the rest frame, {t}a* , hereafter lt correlation. we note a systematic variation of the α parameter distribution with luminosity for any selected {t}a* . we analyze this systematics with respect to the fitted lt correlation line, expecting that the presented trend may allow us to constrain the grb physical models. we also attempted to use the derived correlation of α ({t}a) versus {l}a({t}a) to diminish the luminosity scatter related to the variations of α along the lt distribution, a step forward in the effort of standardizing grbs. a proposed toy model accounting for this systematics applied to the analyzed grb distribution results in a slight increase of the lt correlation coefficient. | study of grb light-curve decay indices in the afterglow phase |
we present new jansky very large array observations of five pre-swift gamma-ray bursts for which an ultraluminous [star formation rate (sfr) >100 m⊙ yr-1] dusty host galaxy had previously been inferred from radio or submillimetre observations taken within a few years after the burst. in four of the five cases, we no longer detect any source at the host location to limits much fainter than the original observations, ruling out the existence of an ultraluminous galaxy hosting any of these gamma-ray bursts (grbs). we continue to detect a source at the position of grb 980703, but it is much fainter than it was a decade ago and the inferred radio sfr (∼80 m⊙) is relatively modest. the radio flattening at 200-1000 d observed in the light curve of this grb may have been caused by a decelerating counterjet oriented 180 deg away from the viewer, although an unjetted wind model can also explain the data. our results eliminate all well-established ultraluminous infrared galaxies (ulirgs) among the pre-swift host population. they also rule out all cases for which an unobscured grb was found in a galaxy dominated by heavily obscured star formation. when grbs do occur in ulirgs, the afterglow is almost always observed to be heavily obscured, consistent with the large dust opacities and high dust covering fractions characteristic of these systems. | late-time vla reobservations rule out ulirg-like host galaxies for most pre- swift long-duration gamma-ray bursts |
we compute the linear polarization during the afterglow phase of gamma-ray bursts, for both on-axis and off-axis observers. we use numerical simulations of the deceleration of a relativistic jet, and compute the polarization by post-processing the results of the numerical simulations. in our simulations, we consider a magnetic field that is chaotic in the plane of the shock, in addition to a magnetic field component that is parallel to the shock velocity. while the linear polarization computed for on-axis observers is consistent with previous analytical estimates, we found that lateral expansion, which is accurately handled in our simulations, plays a crucial role in determining the linear polarization for off-axis observers. our results show that the off-axis linear polarization, as seen by off-axis observers, exhibits a single peak, in contrast to the two peaks inferred by previous analytical studies. the maximum polarization degree is 40 per cent at an observing angle θobs = 0.4 rad, and it decreases as the observing angle increases, which is opposite to that predicted by analytical models, where polarization increases with larger observing angles. from the upper limit of 12 per cent in the linear polarization obtained at 244 d for the grb 170817a, we also infer an anisotropy factor of b∥/b⊥ = 0.5-0.9, consistent with the post-shock magnetic field being amplified by turbulence. | numerical simulations of polarization in gamma-ray burst afterglows |
binary neutron star (ns) mergers may result in remnants of supra-massive or even stable ns, which have been supported indirectly by observed x-ray plateau of some gamma-ray burst (grb) afterglows. recently, xue et al. (2019) discovered an x-ray transient cdf-s xt2 that is powered by a magnetar from merger of double ns via x-ray plateau and following stepper phase. however, the decay slope after the plateau emission is slightly larger than the theoretical value of spin-down in electromagnetic (em) dominated by losing its rotation energy. in this paper, we assume that the feature of x-ray emission is caused by a supra-massive magnetar central engine for surviving thousands of seconds to collapse into a black hole. within this scenario, we present the comparisons of the x-ray plateau luminosity, break time, and the parameters of magnetar between cdf-s xt2 and other short grbs with internal plateau samples. by adopting the collapse time to constrain the equation of state (eos), we find that three eoss (gm1, dd2, and ddme2) are consistent with the observational data. on the other hand, if the most released rotation energy of magnetar is dominated by gw radiation, we also constrain the upper limit of ellipticity of ns for given eos, and its range is [0.32-1.3] × 10-3. its gw signal cannot be detected by advanced ligo or even for more sensitive einstein telescope in the future. | the electromagnetic and gravitational-wave radiations of x-ray transient cdf-s xt2 |
radiation transport codes are often used in astrophysics to construct spectral models. in this work, we demonstrate how producing these models for a time series of data can provide unique information about supernovae (sne). unlike previous work, we specifically concentrate on the method for obtaining the best synthetic spectral fits, and the errors associated with the preferred model parameters. we demonstrate how varying the ejecta mass, bolometric luminosity (lbol) and photospheric velocity (vph), affects the outcome of the synthetic spectra. as an example we analyse the photospheric phase spectra of the grb-sn 2016jca. it is found that for most epochs (where the afterglow subtraction is small) the error on lbol and vph was ∼5 per cent. the uncertainty on ejecta mass and ekin was found to be ∼20 per cent, although this can be expected to dramatically decrease if models of nebular phase data can be simultaneously produced. we also demonstrate how varying the elemental abundance in the ejecta can produce better synthetic spectral fits. in the case of sn 2016jca it is found that a decreasing 56ni abundance as a function of decreasing velocity produces the best-fitting models. this could be the case if the 56ni was synthesized at the side of the grb jet, or dredged up from the centre of the explosion. the work presented here can be used as a guideline for future studies on sne which use the same or similar radiation transfer code. | extracting high-level information from gamma-ray burst supernova spectra |
emission from gamma-ray bursts is thought to be powered mainly by synchrotron radiation from energetic electrons. the same electrons might scatter these synchrotron seed photons to higher (>10 gev) energies, building a distinct spectral component (synchrotron self-compton, ssc). this process is expected to take place, but its relevance (e.g., the ratio between the ssc and synchrotron emitted power) is difficult to predict on the basis of current knowledge of physical conditions at grb emission sites. very high-energy radiation in grbs can be produced also by other mechanisms, such as synchrotron itself (if pev electrons are produced at the source), inverse compton on external seed photons, and hadronic processes. recently, after years of efforts, very high-energy radiation has been finally detected from at least four confirmed long grbs by the cherenkov telescopes h.e.s.s. and magic. in all four cases, the emission has been recorded during the afterglow phase, well after the end of the prompt emission. in this work, i give an overview, accessible also to non-experts of the field, of the recent detections, theoretical implications, and future challenges, with a special focus on why very high-energy observations are relevant for our understanding of gamma-ray bursts and which long-standing questions can be finally answered with the help of these observations. | gamma-ray bursts at the highest energies |
context. a total of four long-duration gamma-ray bursts (grbs) have been confirmed at very high-energy (≥100gev) with high significance, and any possible peculiarities of these bursts will become clearer as the number of detected events increases. multi-wavelength follow-up campaigns are required to extract information on the physical conditions within the jets that lead to the very high-energy counterpart, hence they are crucial to reveal the properties of this class of bursts.aims: grb 201015a is a long-duration grb detected using the magic telescopes from ~40 s after the burst. if confirmed, this would be the fifth and least luminous grb ever detected at these energies. the goal of this work is to constrain the global and microphysical parameters of its afterglow phase, and to discuss the main properties of this burst in a broader context.methods: since the radio band, together with frequent optical and x-ray observations, proved to be a fundamental tool for overcoming the degeneracy in the afterglow modelling, we performed a radio follow-up of grb 201015a over 12 different epochs, from 1.4 days (2020 october 17) to 117 days (2021 february 9) post-burst, with the karl g. jansky very large array, e-merlin, and the european vlbi network. we include optical and x-ray observations, performed respectively with the multiple mirror telescope and the chandra x-ray observatory, together with publicly available data, in order to build multi-wavelength light curves and to compare them with the standard fireball model.results: we detected a point-like transient, consistent with the position of grb 201015a until 23 and 47 days post-burst at 1.5 and 5 ghz, respectively. no emission was detected in subsequent radio observations. the source was also detected in optical (1.4 and 2.2 days post-burst) and in x-ray (8.4 and 13.6 days post-burst) observations.conclusions: the multi-wavelength afterglow light curves can be explained with the standard model for a grb seen on-axis, which expands and decelerates into a medium with a homogeneous density. a circumburst medium with a wind-like profile is disfavoured. notwithstanding the high resolution provided by the vlbi, we could not pinpoint any expansion or centroid displacement of the outflow. if the grb is seen at the viewing angle θ that maximises the apparent velocity βapp (i.e. θ ~ βapp-1), we estimate that the lorentz factor for the possible proper motion is dα ≤ 40 in right ascension and dδ ≤ 61 in declination. on the other hand, if the grb is seen on-axis, the size of the afterglow is ≤5pc and ≤16pc at 25 and 47 days. finally, the early peak in the optical light curve suggests the presence of a reverse shock component before 0.01 days from the burst. | vlbi observations of grb 201015a, a relatively faint grb with a hint of very high-energy gamma-ray emission |
unveiling the mystery of gamma-ray bursts (grbs) has been the target of many multi-waveband observational and theoretical efforts during the last decades. the results collected by current and past space-based instruments have provided important insights into the mechanisms at the origin of their prompt and afterglow phases. on the other hand, many questions, such as the the origin of the multi-gev signal observed in a large number of events, remained unanswered. within this framework, the first firm detections of a very-high-energy (vhe, e≳100 gev) emission component by magic and h.e.s.s. collaborations represented an important, long-awaited result for the vhe astrophysics community. however, while such discoveries opened a new era in the study of grbs, they also provided an unexpected complexity due to the differences between the phenomenology of the observed events. this revealed that we still have an incomplete comprehension of grb physics. in the nearby future, observations by the cherenkov telescope array observatory (ctao), with unprecedented sensitivity in the vhe band, will have a key role in the study of these enigmatic objects and their interactions with the surrounding environment. in this review we will cover the recent grb history, highlighting the efforts of follow-up campaigns by the vhe community that led to the first vhe grb detection, and outlining what we can expect from future facilities in the next decades. | the detection of grbs at vhe: a challenge lasting for more than two decades, what is next? |
grb 030329 displays one clear and, possibly, multiple less intense fast-rising (δt/t ~ 0.3) jumps in its optical afterglow light curve. the decay rate of the optical light curve remains the same before and after the flux jumps. this may be the signature of energy injection into the shocked material at the front of the jet. in this study, we model the gamma-ray burst (grb) ejecta as a series of shells. we follow the dynamical evolution of the ejecta as it interacts with itself (i.e. internal shocks) and with the circumburst medium (i.e. external forward and reverse shocks), and calculate the emission from each shock event assuming synchrotron emission. we confirm the viability of the proposed model in which the jumps in the optical afterglow light curve of grb 030329 are produced via refreshed shocks. the refreshed shocks may be the signatures of collisions between earlier ejected material with an average lorentz factor $\bar{\gamma }\gtrsim 100$ and later ejected material with $\bar{\gamma } \sim 10$ once the early material has decelerated due to interaction with the circumburst medium. we show that even if the late material is ejected with a spread of lorentz factors, internal shocks naturally produce a narrow distribution of lorentz factors (δγ/γ ≲ 0.1), which is a necessary condition to produce the observed quick rise times of the jumps. these results imply a phase of internal shocks at some point in the dynamical evolution of the ejecta, which requires a low magnetization in the outflow. | the signature of refreshed shocks in the afterglow of grb 030329 |
for a sample of swift and fermi gamma-ray bursts, we show that the minimum variability timescale and the spectral lag of the prompt emission is related to the bulk lorentz factor in a complex manner. for small γ's, the variability timescale exhibits a shallow (plateau) region. for large γ's, the variability timescale declines steeply as a function of γ (δ t\propto {{{γ }}-4.05+/- 0.64}). evidence is also presented for an intriguing correlation between the peak times, tp, of the afterglow emission and the prompt emission variability timescale. | gamma-ray bursts: temporal scales and the bulk lorentz factor |
giant flares (gfs) are unusual bursts from soft gamma-ray repeaters (sgrs) that release an enormous amount of energy in a fraction of a second. the afterglow emission of these sgr-gfs or gf candidates is a highly beneficial means of discerning their composition, relativistic speed and emission mechanisms. grb 200415a is a recent gf candidate observed in a direction coincident with the nearby sculptor galaxy at 3.5 mpc. in this work, we searched for transient gamma-ray emission in past observations by fermi-lat in the direction of grb 200415a. these observations confirm that grb 200415a is observed as a transient gev source only once. a pure pair-plasma fireball cannot provide the required energy for the interpretation of gev afterglow emission and a baryonic poor outflow is additionally needed to explain the afterglow emission. a baryonic rich outflow is also viable, as it can explain the variability and observed quasi-thermal spectrum of the prompt emission if dissipation is happening below the photosphere via internal shocks. using the peak energy (ep ) of the time-resolved prompt emission spectra and their fluxes (fp ), we found a correlation between ep and fp or isotropic luminosity l iso for grb 200415a. this supports the intrinsic nature of ep -l iso correlation found in sgrs-gfs, hence favoring a baryonic poor outflow. our results also indicate a different mechanism at work during the initial spike, and that the evolution of the prompt emission spectral properties in this outflow would be intrinsically due to the injection process. | magnetar giant flare originating from grb 200415a: transient gev emission, time-resolved ep - l iso correlation and implications |
correlations between optical flashes and gamma-ray emissions in gamma-ray bursts (grbs) have been searched in order to clarify the question of whether these emissions occur at internal and/or external shocks. among the most powerful grbs ever recorded are grb 080319b and grb 130427a, which at early phases presented bright optical flashes possibly correlated with γ-ray components. additionally, both bursts were fortuitously located within the field of view of the tev γ-ray milagro and hawc observatories, and although no statistically significant excess of counts were collected, upper limits were placed on the gev-tev emission. considering the synchrotron self-compton emission from internal shocks and requiring the gev-tev upper limits, we found that the optical flashes and the γ-ray components are produced by different electron populations. analyzing the optical flashes together with the multiwavelength afterglow observation, we found that these flashes can be interpreted in the framework of the synchrotron reverse shock model when outflows have arbitrary magnetizations. | the origin of the optical flashes: the case study of grb 080319b and grb 130427a |
grb 170817a is the first short gamma-ray burst (grb) with direct detection of the gravitational-wave radiation and also the spectroscopically identified macronova emission (i.e., at 2017gfo). the prompt emission of this burst, however, is underluminous in comparison with the other short grbs with known redshift. in this work, we examine whether grb 170817a is indeed unique. we first show that grb 130603b/macronova may be the on-axis “analogs” of grb 170817a/at 2017gfo, and the extremely dim but long-lasting afterglow emission of grb 170817a may suggest a low number density (∼ {10}-5 {{cm}}-3) of its circumburst medium and a structured outflow. we then discuss whether grb 070923, grb 080121, grb 090417a, grb 111005a, and grb 170817a form a new group of very nearby underluminous grbs originated from neutron star mergers. if the short events grb 070923, grb 080121, and grb 090417a are indeed at a redshift of ∼ 0.076, 0.046, 0.088, respectively, their isotropic energies of the prompt emission are ∼ {10}47 erg and thus comparable to the other two events. the non-detection of optical counterparts of grb 070923, grb 080121, grb 090417a, and grb 111005a, however, strongly suggests that the macronovae from neutron star mergers are significantly diverse in luminosities or, alternatively, there is another origin channel (for instance, the white dwarf and black hole mergers). we finally suggest that gw170817/grb 170817a are likely not alone and similar events will be detected by the upgraded/upcoming gravitational-wave detectors and the electromagnetic monitors. | how special is grb 170817a? |
simultaneous swift and fermi observations of gamma-ray bursts (grbs) offer a unique broadband view of their afterglow emission, spanning more than 10 decades in energy. we present the sample of x-ray flares observed by both swift and fermi during the first three years of fermi operations. while bright in the x-ray band, x-ray flares are often undetected at lower (optical), and higher (mev to gev) energies. we show that this disfavors synchrotron self-compton processes as the origin of the observed x-ray emission. we compare the broadband properties of x-ray flares with the standard late internal shock model, and find that in this scenario, x-ray flares can be produced by a late-time relativistic (γ > 50) outflow at radii r ~ 1013-1014 cm. this conclusion holds only if the variability timescale is significantly shorter than the observed flare duration, and implies that x-ray flares can directly probe the activity of the grb central engine. | swift and fermi observations of x-ray flares: the case of late internal shock |
gamma-ray burst grb 140430a was detected by the swift satellite and observed promptly with the imaging polarimeter ringo3 mounted on the liverpool telescope, with observations beginning while the prompt γ-ray emission was still ongoing. in this paper, we present densely sampled (10-s temporal resolution) early optical light curves (lcs) in 3 optical bands and limits to the degree of optical polarization. we compare optical, x-ray, and gamma-ray properties and present an analysis of the optical emission during a period of high-energy flaring. the complex optical lc cannot be explained merely with a combination of forward and reverse shock emission from a standard external shock, implying additional contribution of emission from internal shock dissipation. we estimate an upper limit for time averaged optical polarization during the prompt phase to be as low as p < 12% (1σ). this suggests that the optical flares and early afterglow emission in this grb are not highly polarized. alternatively, time averaging could mask the presence of otherwise polarized components of distinct origin at different polarization position angles. | limits on optical polarization during the prompt phase of grb 140430a |
context. grb 210731a was a long-duration (t90 = 22.5 s) gamma-ray burst discovered by the burst alert telescope (bat) aboard the neil gehrels swift observatory. swift triggered the wide-field, robotic meerlicht optical telescope in sutherland; it began observing the bat error circle 286 s after the swift trigger and discovered the optical afterglow of grb 210731a in its first 60-s q-band exposure. multi-colour observations of the afterglow with meerlicht revealed a light curve that showed three peaks of similar brightness within the first four hours. the unusual optical evolution prompted multi-wavelength follow-up observations that spanned from x-ray to radio frequencies.aims: we present the results of our follow-up campaign and interpret our observations in the framework of the synchrotron forward shock model.methods: we performed temporal and spectral fits to determine the spectral regime and external medium density profile, and performed detailed multi-wavelength theoretical modelling of the afterglow following the last optical peak at ∼0.2 days to determine the intrinsic blast wave parameters.results: we find a preference for a stellar wind density profile consistent with a massive star origin, while our theoretical modelling results in fairly typical shock microphysics parameters. based on the energy released in γ rays and the kinetic energy in the blast wave, we determine a low radiative efficiency of η ≈ 0.02. the first peak in the optical light curve is likely the onset of the afterglow. we find that energy injection into the forward shock offers the simplest explanation for the subsequent light curve evolution, and that the blast wave kinetic energy increasing by a factor of ∼1000 from the first peak to the last peak is indicative of substantial energy injection. our highest-likelihood theoretical model over-predicts the 1.4 ghz flux by a factor of approximately three with respect to our upper limits, possibly implying a population of thermal electrons within the shocked region. table b.1 is only available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/671/a116 | the triple-peaked afterglow of grb 210731a from x-ray to radio frequencies |
the early optical emission of gamma-ray bursts (grbs) gives an opportunity to understand the central engine and first stages of these events. about 30% of grbs present flares whose origin is still a subject of discussion. we present optical photometry of grb 180620a with the coatli telescope and ratir instrument. coatli started to observe from the end of prompt emission at t + 39.3 s and ratir from t + 121.4 s. we supplement the optical data with the x-ray light curve from swift/xrt. we observe an optical flare from t + 110 s to t + 550 s, with a temporal index decay α o,decay = 1.32 ± 0.01, and δt/t = 1.63, which we interpret as the signature of a reverse shock component. after the initial normal decay the light curves show a long plateau from t + 500 s to t + 7800 s in both x-rays and the optical before decaying again after an achromatic jet break at t + 7800 s. fluctuations are seen during the plateau phase in the optical. adding to the complexity of grb afterglows, the plateau phase (typically associated with the coasting phase of the jet) is seen in this object after the “normal” decay phase (associated with the deceleration phase of the jet), and the jet break phase occurs directly after the plateau. we suggest that this sequence of events can be explained by a rapid deceleration of the jet with td≲ 40 s due to the high density of the environment (≈100 cm-3) followed by reactivation of the central engine, which causes the flare and powers the plateau phase. | grb 180620a: evidence for late-time energy injection |
the current status of observations and theoretical models of gamma-ray bursts and some other related transients, including ultra-long bursts and tidal disruption events, is reviewed. we consider the impact of multi-wavelength data on the formulation and development of theoretical models for the prompt and afterglow emission including the standard fireball model utilizing internal shocks and external shocks, photospheric emission, the role of the magnetic field and hadronic processes. in addition, we discuss some of the prospects for non-photonic multi-messenger detection and for future instrumentation, and comment on some of the outstanding issues in the field. | gamma-ray bursts and fast transients. multi-wavelength observations and multi-messenger signals |
gw170817/grb 170817a probably marks a double neutron star (ns) coalescence. extended emission {t}s≃ (0.67+/- 0.03) s post-merger shows an estimated energy output { \mathcal e }≃ (3.5+/- 1) % {m}⊙ {c}2 determined by response curves to power-law signal injections, where c is the velocity of light. it provides calorimetric evidence for a rotating black hole of ∼ 3{m}⊙ , inheriting the angular momentum j of the merged hyper-massive ns in the immediate aftermath of gw170817 following core-collapse about or prior to ts . core-collapse greatly increases the central energy reservoir to {e}j≲ 1{m}⊙ {c}2, accounting for { \mathcal e } even at modest efficiencies in radiating gravitational waves through a non-axisymmetric thick torus. the associated multi-messenger output in ultra-relativistic outflows and sub-relativistic mass-ejecta is consistent with observational constraints from the gamma-ray burst afterglow emission of grb 170817a and accompanying kilonova. | multi-messenger extended emission from the compact remnant in gw170817 |
grb 160325a is the only bright burst detected by astrosat czt imager in its primary field of view to date. in this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using astrosat, fermi, and niel gehrels swift observations. the prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. the first emission episode shows a thermal component as well as a low polarization fraction of pf < 37 per cent at 1.5 σ confidence level. on the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarized with pf > 43 per cent at 1.5σ confidence level. we also study the afterglow properties of the jet using swift/xrt data. the observed jet break suggests that the jet is pointed towards the observer and has an opening angle of 1.2° for an assumed redshift, z = 2. with composite modelling of polarization, spectrum of the prompt emission, and the afterglow, we infer that the first episode of emission originates from the photosphere with localized dissipation happening below it, and the second from the optically thin region above the photosphere. the photospheric emission is generated mainly by inverse compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. the low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet. | spectropolarimetric analysis of prompt emission of grb 160325a: jet with evolving environment of internal shocks |
we analyze the early growth stage of direct-collapse black holes (dcbhs) with ∼105 m ⊙, which are formed by collapse of supermassive stars in atomic-cooling halos at z ≳ 10. a nuclear accretion disk around a newborn dcbh is gravitationally unstable and fragments into clumps with a few × 10 m ⊙ at ∼0.01-0.1 pc from the center. such clumps evolve into massive population iii stars with a few × 10-102 m ⊙ via successive gas accretion, and a nuclear star cluster is formed. radiative and mechanical feedback from an inner slim disk and the star cluster will significantly reduce the gas accretion rate onto the dcbh within ∼106 yr. some of the nuclear stars can be scattered onto the loss cone orbits also within ≲106 yr and tidally disrupted by the central dcbh. the jet luminosity powered by such tidal disruption events can be lj≳ 1050 erg s-1. the prompt emission will be observed in x-ray bands with a peak duration of δt obs ∼ 105-6(1 + z) s followed by a tail ∝t obs -5/3, which can be detectable by swift bat and erosita even from z ∼ 20. follow-up observations of the radio afterglows with, e.g., evla and the host halos with james webb space telescope could probe the earliest active galactic nucleus feedback from dcbhs. | stellar tidal disruption events by direct-collapse black holes |
grb 160625b is an extremely bright outburst with well-monitored afterglow emission. the geometry-corrected energy is high, up to ∼5.2 × 1052 erg or even ∼8 × 1052 erg, rendering it the most energetic grb prompt emission recorded so far. we analyzed the time-resolved spectra of the prompt emission and found that in some intervals there were likely thermal-radiation components and the high energy emission was characterized by significant cutoff. the bulk lorentz factors of the outflow material are estimated accordingly. we found out that the lorentz factors derived in the thermal-radiation model are consistent with the luminosity-lorentz factor correlation found in other bursts, as well as in grb 090902b for the time-resolved thermal-radiation components, while the spectral cutoff model yields much lower lorentz factors that are in tension with the constraints set by the electron pair compton scattering process. we then suggest that these spectral cutoffs are more likely related to the particle acceleration process and that one should be careful in estimating the lorentz factors if the spectrum cuts at a rather low energy (e.g., ∼tens of mev). the nature of the central engine has also been discussed, and a stellar-mass black hole is favored. | evaluating the bulk lorentz factors of outflow material: lessons learned from the extremely energetic outburst grb 160625b |
during a compact binary merger involving at least one neutron star (ns), a small fraction of the gravitational energy could be liberated in such a way to accelerate a small fraction (∼10-6) of the ns mass in an isotropic or quasi-isotropic way. in presence of certain conditions, a pair-loaded fireball can form, which undergoes accelerated expansion reaching relativistic velocities. as in the standard fireball scenario, internal energy is partly transformed into kinetic energy. at the photospheric radius, the internal radiation can escape, giving rise to a pulse that lasts for a time equal to the delay time since the merger. the subsequent interaction with the interstellar medium can then convert part of the remaining kinetic energy back into radiation in a weak isotropic afterglow at all wavelengths. this scenario does not require the presence of a jet: the associated isotropic prompt and afterglow emission should be visible for all ns-ns and bh-ns mergers within 90 mpc, independent of their inclination. the prompt emission is similar to that expected from an off-axis jet, either structured or much slower than usually assumed (γ ∼ 10), or from the jet cocoon. the predicted afterglow emission properties can discriminate among these scenarios. | jet-driven and jet-less fireballs from compact binary mergers |
this paper presents data on the simultaneous and complementary observations of the gamma-ray burst (grb) grb 161017a for optical, x-ray, and gamma wavelengths obtained by the russian multi-messenger lomonosov space observatory and supplemented by additional data from the swift satellite as well as the ground-based master global robotic net and the 10 m gran telescopio canarias. multifrequency spectra of this very powerful explosion indicate that it originated at a distance of 10 billion light years from earth. here, we present the results of the prompt, early, and afterglow optical observations. the light curves and spectra suggest that the prompt optical and high-energy emissions occur in the same region near the grb source. | prompt and follow-up multi-wavelength observations of the grb 161017a |
intense flares that occur at late times relative to the prompt phase have been observed by the swift satellite in the x-ray afterglows of gamma-ray bursts (grbs). here, we present a detailed analysis on the fall back accretion process to explain the intense flare phase in the very early x-ray afterglow light curves. to reproduce the afterglow at late times, we resort to the external shock by engaging energy injections. by applying our model to grbs 080810, 081028 and 091029, we show that their x-ray afterglow light curves can be reproduced well. we then apply our model to the ultralong swift grb 111209a, which is the longest burst ever observed. the very early x-ray afterglow of grb 111209a showed many interesting features, such as a significant bump observed at around 2000 s after the swift/bat trigger. we assume two constant energy injection processes in our model. these can explain the observed plateau at x-ray wavelength in the relatively early stage (8.0 × 103 s) and a second x-ray plateau and optical rebrightening at about 105 s. our analysis supports the scenario that a significant amount of material may fall back towards the central engine after the prompt phase, causing an enhanced and long-lived mass accretion rate powering a poynting-flux-dominated outflow. | fall back accretion and energy injections in gamma-ray bursts |
gamma-ray bursts (grbs) central engines and jet production mechanisms are still open questions. assuming that the shallow decay segments of canonical x-ray afterglow light curves of swift grbs are attributed to the magnetic dipole (md) radiations of newly born magnetars, we derive the parameters of the magnetars and explore their possible relations to jet and md wind emission. we show that the magnetar initial spin period (p 0) are tightly correlated with the jet energy (e jet), which is almost proportional to the wind energy (e wind). our least-squares fits yield {p}0\propto {e}jet}-0.36+/- 0.03 and {e}wind}\propto {e}jet}0.91+/- 0.07. these relations may imply that a magnetar with faster rotating speed can power a more energetic grb, and energy partition between the jet and wind may be quasi-universal. although the p 0-e jet relation is driven by a few sub-energetic grbs in our sample, our monte carlo simulation analysis shows that sample selection biases from instrumental flux limits and contaminations of the bright jet afterglows cannot make this correlation. within this jet-wind paradigm, we propose that grb 101225a-like ultra-long grbs, whose prompt gamma-ray/x-ray light curves are featured as a long-lasting plateau with a sharp drop, may be the orphan md wind emission being due to misalignment of their jet axis to the light of sight. we briefly discuss the orphan md wind emission and its association with the gravitational-wave radiation of newly born magnetars. | magnetar as central engine of gamma-ray bursts: central engine-jet connection, wind-jet energy partition, and origin of some ultra-long bursts |
we report our observations of very bright prompt optical and reverse shock (rs) optical emission of grb 140512a and analyze its multi-wavelength data observed with the swift and fermi missions. it is found that the joint optical-x-ray-gamma-ray spectrum with our first optical detection (r = 13.09 mag) at {t}0+136 s during the second episode of the prompt gamma-rays can be fit by a single power law with an index of -1.32 ± 0.01. our empirical fit to the afterglow light curves indicates that the observed bright optical afterglow with r = 13.00 mag at the peak time is consistent with predictions of the rs and forward shock (fs) emission of external shock models. a joint optical-x-ray afterglow spectrum is well fit with an absorbed single power law, with an index evolving with time from -1.86 ± 0.01 at the peak time to -1.57 ± 0.01 at a late epoch, which could be due to the evolution of the ratio of the rs to fs emission fluxes. we fit the light curves with standard external models, and derive the physical properties of the outflow. it is found that the ratio {r}{{b}}\equiv {ɛ }{{b},{{r}}}/{ɛ }{{b},{{f}}} is 8187, indicating a high magnetization degree in the rs region. measuring the relative radiation efficiency with {r}{{e}}\equiv {ɛ }{{e},{{r}}}/{ɛ }{{e},{{f}}}, we have r e = 0.02, implying that the radiation efficiency of the rs is much lower than that in fs. we also show that the r b of grbs 990123, 090102, and 130427a are similar to that of grb 140512a and their apparent difference may be mainly attributed to the difference of the jet kinetic energy, initial lorentz factor, and medium density among them. | very bright prompt and reverse shock emission of grb 140512a |
grb 140903a, a short duration γ-ray burst (sgrb) detected by swift, is characterized by its long-lasting radio emission among sgrbs. in addition to the ∼ {10}6 s radio afterglow emission, the afterglow of grb 140903a displays a plateau from 103 s to 7× {10}3 {{s}} in the x-rays. in this work, we attribute the x-ray plateau to the energy injection into the decelerating blast wave and then model the later radio/optical/x-ray afterglow emission within the standard fireball afterglow model. the afterglow emission has been well reproduced with reasonable physical parameters, including a jet half-opening angle of ∼0.05. | the interpretation of the multi-wavelength afterglow emission of short grb 140903a |
the emission region of γ-ray bursts (grbs) is poorly constrained. the uncertainty on the size of the dissipation site spans over 4 orders of magnitude (1012-1017 cm) depending on the unknown energy composition of the grb jets. the joint multiband analysis from soft x-rays to high energies (up to ~1 gev) of one of the most energetic and distant grbs, grb 220101a (z = 4.618), allows us to make an accurate distinction between prompt and early afterglow emissions. the enormous amount of energy released by grb 220101a (e iso ≈ 3 × 1054 erg) and the spectral cutoff at ${e}_{\mathrm{cutoff}}={85}_{-26}^{+16}$ mev observed in the prompt emission spectrum constrain the parameter space of the grb dissipation site. we put stringent constraints on the prompt emission site, requiring 700 < γ0 < 1160 and rγ~ 4.5 × 1013 cm. our findings further highlight the difficulty of finding a simple self-consistent picture in the electron-synchrotron scenario, favoring instead a proton-synchrotron model, which is also consistent with the observed spectral shape. deeper measurements of the time variability of grbs, together with accurate high-energy observations (mev-gev), would unveil the nature of the prompt emission. | constraints on the physics of the prompt emission from distant and energetic gamma-ray burst grb 220101a |
following the detection of a long grb 190919b by integral (international gamma-ray astrophysics laboratory), we obtained an optical photometric sequence of its optical counterpart. the light curve of the optical emission exhibits an unusually steep rise ∼100 s after the initial trigger. this behaviour is not expected from a `canonical' grb optical afterglow. as an explanation, we propose a scenario consisting of two superimposed flares: an optical flare originating from the inner engine activity followed by the hydrodynamic peak of an external shock. the inner-engine nature of the first pulse is supported by a marginal detection of flux in hard x-rays. the second pulse eventually concludes in a slow constant decay, which, as we show, follows the closure relations for a slow cooling plasma expanding into the constant interstellar medium and can be seen as an optical afterglow sensu stricto. | grb 190919b: rapid optical rise explained as a flaring activity |
a large-scale ordered magnetic field plays a very important role in the formation and acceleration of a gamma-ray burst (grb) jet. during the grb prompt phase, some dissipation processes may occur and disturb the magnetic field, making the field become random to some extent. therefore, a mixed magnetic field consisting of an ordered component and a random component is plausible for the early afterglow phase. here we investigate the polarization evolution and light curve of an afterglow under such a three-dimensional mixed magnetic field. three kinds of ordered component (i.e., aligned, toroidal, and radial) are discussed. we find that the three cases are distinguishable through jet polarization evolution. the polarization angle for a 3d mixed magnetic field with an aligned ordered component can evolve gradually but only changes abruptly by 90° in the toroidal and radial cases. furthermore, during the reverse shock crossing time, the polarization degree (pd) can be non-zero for the toroidal case but roughly zero for the radial case. since an aligned component in a jet corresponds to a magnetar central engine and a toroidal component corresponds to a black hole, grb central engines are distinguishable through polarization observations even if the magnetic field is mixed in a jet. in addition, our polarization calculation can be applied to grb 170817a associated with gw170817. using the recently observed pd upper limit 12% of grb 170817a at t = 244 days, the magnetic field strength ratio of the ordered to random components in this afterglow is constrained to be ≲0.9. | polarization with a three-dimensional mixed magnetic field and its application to grb 170817a |
bimodal spectral energy distributions (seds) of gamma-ray burst (grb) afterglow of grbs 190114c, 130427a, and 180720b confirm that they are originated from the synchrotron emission (syn) and synchrotron self-compton scattering (ssc) process of electrons accelerated in the jets. the radiation mechanism and the physics of the observed spectrum-luminosity/energy relations of grbs remain as open questions. by extracting the syn component through fitting their early afterglow seds with the syn+ssc model, we find that their luminosity (lsyn), peak energy (ep,syn,z), and the lorentz factor of the afterglow fireball (γt) follow the lp,iso-ep,z-γ0 relation of prompt gamma rays, where lp,iso is the isotropic luminosity, ep,z is the peak energy of the $\nu {f}_{\nu }$ spectrum in the burst frame, and γ0 is the initial lorentz factor of the fireball. to examine whether late afterglows are consistent with this relation, we calculate the synchrotron component at late afterglows. it is found that they also follow the same lp,iso-ep,z-γ0 relation, albeit they are not consistent with the lp,iso-ep,z relation. our results may imply that the lp,iso-ep,z-γ0 relation would be a universal feature of synchrotron radiations of electrons accelerated in grb jets throughout the prompt and afterglow phases among grbs. its origin is not fully understood, and possible explanations are briefly discussed. | do afterglow synchrotron radiations follow the lp,iso-ep,z-γ0 relation of gamma-ray bursts? the cases of grbs 190114c, 130427a, and 180720b |
early afterglow observations of gamma-ray bursts (grbs) are valuable for exploring the properties of their jets and ambient medium. we report our photometric and spectroscopic observations of grb 210104a and discuss its jet properties with multiwavelength data. our spectroscopic observation reveals several absorption features and a tentative redshift of 0.46 is identified. a bright optical flare that has a peak brightness of r = 13 mag at 112 ± 7 s was observed in the r band during 67 ~ 165 s post the grb trigger. the flux of the r-band afterglow decays with a slope of α o = - 0.91 ± 0.03 at t > 650 s. the early x-ray afterglow lightcurve is a smooth bump, and it decays with a slope of α x = -1.18 ± 0.01 at late epoch. our joint spectral fit to the optical-x-ray afterglows during (1.1-1.3) × 104 s yields a photon index γo,x = -1.82 ± 0.04. the derived host galaxy extinction is ar= 0.87. attributing the early optical flare to the reverse-shock (rs) emission and the late optical-x-ray emission to the forward shock emission, the optical and x-ray lightcurves at t < 3 × 104 s can be well fit adopting a markov chain monte carlo algorithm. comparing the properties of grb 210104a with other grbs that have detection of bright rs emission, we show that its jet is mildly magnetized (r b = 28), with high radiation efficiency (77%), is subenergetic (e k,iso = 4.5 × 1051 erg), and moderately relativistic (γ0 ~ 35) in a density medium (n 0 ~ 417 cm-3). it follows the tight lγ,iso-e p,z-γ0 relation as with typical grbs. | photometric and spectroscopic observations of grb 210104a: bright reverse-shock emission and dense circumburst environment |
bright x-ray flares are routinely detected by the swift satellite during the early afterglow of gamma-ray bursts, when the explosion ejecta drives a blast wave into the external medium. we suggest that the flares are produced as the reverse shock propagates into the tail of the ejecta. the ejecta is expected to contain a few dense shells formed at an earlier stage of the explosion. we show an example of how such dense shells form and describe how the reverse shock interacts with them. a new reflected shock is generated in this interaction, which produces a short-lived x-ray flare. the model provides a natural explanation for the main observed features of the x-ray flares - the fast rise, the steep power-law decline and the characteristic peak duration δt/t ≃ 0.1-0.3. | x-ray flares from dense shells formed in gamma-ray burst explosions |
we observed the afterglow of grb 130427a with the reionization and transients infrared camera (ratir) instrument on the 1.5 m harold l. johnson telescope of the observatorio astronómico nacional in sierra san pedro mártir. our homogenous grizy jh photometry extends from the night of the burst to three years later. we fit a model for the afterglow. there is a significant positive residual that matches the behavior of sn 1998bw in the griz filters; we suggest that this is a photometric signature of the supernova sn 2013cq associated with the gamma-ray burst. the peak absolute magnitude of the supernova is {m}r=-18.43+/- 0.11. | photometric observations of supernova 2013cq associated with grb 130427a |
we present the light curves and spectra of 24 afterglows that have been monitored by fermi-lat at 0.1-100 gev over more than a decade. all light curves (except 130427) are consistent with a single power law starting from their peaks, which occur in most cases before the burst end. the light curves display a brightness-decay rate correlation, with all but one (130427) of the bright afterglows decaying faster than the dimmer afterglows. we attribute this dichotomy to the quick deposition of relativistic ejecta energy in the external shock for the brighter/faster-decaying afterglows and to an extended energy injection in the afterglow shock for the dimmer/slower-decaying light curves. the spectra of six afterglows (090328, 100414, 110721, 110731, 130427, 140619b) indicate the existence of a harder component above a spectral dip or ankle at energies of 0.3-3 gev, offering evidence for inverse-compton emission at higher energies and suggesting that the harder power-law spectra of five other lat afterglows (130327b, 131231, 150523, 150627, 160509) could also be inverse-compton, while the remaining, softer lat afterglows should be synchrotron emission. marginal evidence for a spectral break and softening at higher energies is found for two afterglows (090902b and 090926). | seven-year collection of well-monitored fermi-lat gamma-ray burst afterglows |
the multimessenger observation of gamma-ray burst (grb) 170817a from the nearby binary neutron star merger gw170817 demonstrated that low-energy gamma-ray emission can be observed at relatively large angles from grb jet axes. if such structured emission is typical, then the currently known sample of short grbs with no distance measurements may contain multiple nearby off-axis events whose delayed afterglows could have gone undetected. these nearby neutron star mergers may produce telltale radio flares peaking years after the prompt grb emission that could still be observable. here, we show that several short grbs observed by the burst alert telescope on the neil gehrels swift satellite, with no identified afterglow and no distance measurement, could potentially be associated with radio flares detectable by sensitive cm-wavelength radio facilities such as the karl g. jansky very large array. we also examine optical follow-up observations that have been carried out for these events, and find that a nearby gw170817-like kilonova is ruled out for only a third of them. | radio forensics could unmask nearby off-axis gamma-ray bursts |
the broadband afterglow of gw170817/grb 170817a is believed to be from an off-axis structured jet. the central engine of a gamma-ray burst usually launches a pair of outflows that move oppositely. it is reasonable to consider the emission from double-sided structured jets, with a near-jet moving toward us and a counter-jet moving away from us. assuming that the two branches of the jet have the same physical parameters, we have calculated their radio emission. it is found that the counter-jet component will emerge in the radio light curves ∼2500 days post-merger. it typically leads to a plateau in the light curve, thus it could be revealed by high accuracy radio observations. we have also considered the possibilities that both branches have different parameters, and found that if some of the physical parameters of the counter-jet are evaluated more favorably than those of the near-jet, then the counter-jet emission will be enhanced and will clearly show up as an obvious plateau or even a rebrightening. for example, when the circumburst medium encountered by the counter-jet is assumed to be 100 times denser than that of the near-jet, a remarkable radio plateau will appear at ∼600 days. however, in x-ray bands, the counter-jet component is generally too faint to be discerned. it is argued that the late radio observations of gw170817/grb 170817a can help determine the key parameters and diagnose the environment of the event. | late-time afterglow from double-sided structured jets: application to grb 170817a |
grb 201015a is a peculiarly low luminosity, spectrally soft gamma-ray burst (grb), with t90 = 9.8 ± 3.5 s (time interval of detection of 90 per cent of photons from the grb), and an associated supernova (likely to be type ic or ic-bl). grb 201015a has an isotropic energy $e_{\gamma , \rm iso}$$= 1.75 ^{+0.60} _{-0.53} \times 10^{50}$ erg, and photon index $\gamma = 3.00 ^{+0.50} _{-0.42}$ (15-150 kev). it follows the amati relation, a correlation between $e_{\gamma , \rm iso}$ and spectral peak energy ep followed by long grbs. it appears exceptionally soft based on γ, the hardness ratio of hr = 0.47 ± 0.24, and low-ep, so we have compared it to other grbs sharing these properties. these events can be explained by shock breakout, poorly collimated jets, and off-axis viewing. follow-up observations of the afterglow taken in the x-ray, optical, and radio reveal a surprisingly late flattening in the x-ray from t = (2.61 ± 1.27) × 104 s to $t = 1.67 ^{+1.14} _{-0.65} \times 10^6$ s. we fit the data to closure relations describing the synchrotron emission, finding the electron spectral index to be $p = 2.42 ^{+0.44} _{-0.30}$ and evidence of late-time energy injection with coefficient $q = 0.24 ^{+0.24} _{-0.18}$. the jet half opening angle lower limit (θj ≥ 16°) is inferred from the non-detection of a jet break. the launch of svom and einstein probe in 2023 should enable detection of more low-luminosity events like this, providing a fuller picture of the variety of grbs. | grb 201015a and the nature of low-luminosity soft gamma-ray bursts |
on 2017 august 17, advanced ligo and virgo observed gw170817, the first gravitational-wave (gw) signal from a binary neutron star merger. it was followed by a short-duration gamma-ray burst, grb 170817a, and by a non-thermal afterglow emission. in this work, a combined simultaneous fit of the electromagnetic (em, specifically, afterglow) and gw domains is implemented, both using the posterior distribution of a gw standalone analysis as prior distribution to separately process the em data, and fitting the em and gw domains simultaneously. these approaches coincide mathematically, as long as the actual posterior of the gw analysis, and not an approximation, is used as prior for the em analysis. we treat the viewing angle, θv, as shared parameter across the two domains. in the afterglow modelling with a gaussian structured jet this parameter and the jet core angle, θc, are correlated, leading to high uncertainties on their values. the joint em + gw analysis relaxes this degeneracy, reducing the uncertainty compared to an em-only fit. we also apply our methodology to hypothetical gw170817-like events occurring in the next gw observing run at ~140 and 70 mpc. at 70 mpc the existing em degeneracy is broken, thanks to the inclusion of the gw domain in the analysis. at 140 mpc, the em-only fit cannot constrain θv nor θc because of the lack of detections in the afterglow rising phase. folding the gw data into the analysis leads to tighter constraints on θv, still leaving θc unconstrained, requiring instruments with higher sensitivities, such as athena. | joint analysis of gravitational-wave and electromagnetic data of mergers: breaking an afterglow model degeneracy in gw170817 and in future events |
the brightness of the multi-wavelength afterglow of grb 170817a is increasing unexpectedly even ∼160 days after the associated gravitational burst. here we suggest that the brightening can be caused by a late-time energy injection process. we use an empirical expression to mimic the evolution of the injection luminosity, which consists of a power-law rising phase and a power-law decreasing phase. it is found that the power-law indices of the two phases are 0.92 and -2.8, respectively, with the peak time of the injection being ∼110 days. the energy injection could be due to some kind of accretion, with the total accreted mass being ∼0.006 m ⊙. however, normal fall-back accretion, which usually lasts for a much shorter period, cannot provide a natural explanation. our best-fit decay index of -2.8 is also at odds with the expected value of -5/3 for normal fall-back accretion. noting that the expansion velocities of the kilonova components associated with gw170817 are 0.1-0.3 c, we argue that there should also be some ejecta with correspondingly lower velocities during the coalescence of the double neutron star (ns) system. they are bound by the gravitational well of the remnant central compact object and might be accreted at a timescale of about 100 days, providing a reasonable explanation for the energy injection. detailed studies on the long-lasting brightening of grb 170817a thus may provide useful information on matter ejection during the merger process of binary neutron stars. | continued brightening of the afterglow of gw170817/grb 170817a as being due to a delayed energy injection |
gamma-ray bursts have been widely argued to originate from binary compact object mergers or core collapse of massive stars. jets from these systems may have two components: an inner, narrow sub-jet and an outer, wider sub-jet. such a jet subsequently interacts with its ambient gas, leading to a reverse shock (rs) and a forward shock. the magnetic field in the narrow sub-jet is very likely to be mixed by an ordered component and a random component during the afterglow phase. in this paper, we calculate light curves and polarization evolution of optical afterglows with this mixed magnetic field in the rs region of the narrow sub-jet in a two-component jet model. the resultant light curve has two peaks: an early peak arising from the narrow sub-jet and a late-time rebrightening due to the wider sub-jet. we find the polarization degree (pd) evolution under such a mixed magnetic field confined in the shock plane is very similar to that under the purely ordered magnetic field condition. the two-dimensional “mixed” magnetic fields confined in the shock plane are essentially the ordered magnetic fields only with different configurations. the position angle (pa) of the two-component jet can change gradually or abruptly by 90°. in particular, an abrupt 90° change of the pa occurs when the pd changes from its decline phase to the rise phase. | gamma-ray burst optical afterglows with two-component jets: polarization evolution revisited |
recent detection of sub-tev emission from gamma-ray bursts (grbs) represents a breakthrough in the grb study. the multiwavelength data of the afterglows of grb 190114c support the synchrotron self-compton (ssc) origin for its sub-tev emission. we present a comparative analysis on the ssc emission of grb afterglows in the homogeneous and wind environments in the framework of the forward shock model. the γγ absorption of very-high-energy photons due to pair production within the source and the klein-nishina effect on the inverse compton scattering are considered. generally a higher ssc flux is expected for a larger circum-burst density due to a larger compton parameter, but meanwhile the internal γγ absorption is more severe for sub-tev emission. the flux ratio between the ssc component and the synchrotron component decreases more quickly with time in the wind medium case than in the homogeneous density medium case. the light curves of the ssc emission are also different for the two types of media. we also calculate the cascade emission resulting from the absorbed high-energy photons. in the ism environment with n ≳ 1 cm-3, the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band, which may flatten the optical afterglow light curve at an early time (t < 1 hr). in the wind medium with a* ≳ 0.1, the cascade emission in the ev-gev band is comparable or even larger than the emission of the primary electrons at the early time. | very-high-energy emission and cascade radiation of gamma-ray burst afterglows: homogeneous versus wind external media |
a stellar-mass black hole (bh) surrounded by a neutrino-dominated accretion flow (ndaf) has been discussed in a number of works as the central engine of gamma-ray bursts (grbs). it is widely believed that ndaf cannot liberate enough energy for bright grbs. however, these works have been based on the assumption of a “no torque” boundary condition, which is invalid when the disk is magnetized. in this paper, we present both numerical and analytical solutions for ndafs with non-zero boundary stresses and reexamine their properties. we find that an ndaf with such a boundary torque can be powerful enough to account for those bright short grbs, energetic long grbs, and ultra-long grbs. the disk becomes viscously unstable, which makes it possible to interpret the variability of grb prompt emission and the steep decay phase in the early x-ray afterglow. finally, we study the gravitational waves radiated from a processing bh-ndaf. we find that the effects of the boundary torque on the strength of the gravitational waves can be ignored. | numerical and analytical solutions of neutrino-dominated accretion flows with a non-zero torque boundary condition and its applications in gamma-ray bursts |
the gamma-ray burst grb 180720b is very peculiar. on one hand, some interesting features have been found by performing the detailed time-resolved spectral analysis in the prompt phase. first, the “flux-tracking” pattern is exhibited both for the low energy spectral index α and the peak energy epin the band function. second, some parameter relations show strong monotonous positive correlations, include ep -f, α-f, ep -α, and ep -lγ,iso for all time-resolved spectra. lastly, it should be noted that the values of α do not exceed the synchrotron limits ≤ft({from} -\tfrac{3}{2} {to} -\tfrac{2}{3}\right). on the other hand, the photons with the energy of ≳100 mev were detected by lat both in the prompt phase and afterglow. notably, the 5 gev photon was observed at 142 s after the gbm trigger. the spectrum of this burst in the lat range can be described as fν∝ ν -1.3 t -1.54±0.02 in the afterglow phase. and there are six gev photons during the x-ray flare when the lower energy emission is fading to a weaker level. we try to give reasonable interpretations of the mechanism for prompt emission and the high energy emission (100 mev to gev) in the afterglow. the interpretations suggesting that synchrotron origin can account for the prompt emission and synchrotron self-compton radiation can account for both the spectrum and temporal behavior of the 100 mev to gev afterglow emission that have been accepted by us. | origin for the prompt spectral evolution characteristics and high energy emission during the x-ray flare in grb 180720b |
tidal disruption events (tdes) occurring in active galactic nuclei (agns) are a special class of sources with outstanding scientific significance. tdes can generate ultrafast winds, which should almost inevitably collide with the preexisting agn dusty tori. we perform analytical calculations and simulations on the wind-torus interactions and find such a process can generate considerable x-ray afterglow radiation several years or decades later after the tde outburst. this provides a new origin for the years delayed x-rays in tdes. the x-ray luminosity can reach 1041-42 erg s-1, and the light curve characteristics depend on the parameters of winds and tori. we apply the model to two tde candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. our results suggest that the observations of the time delay, spectral shape, luminosity, and the light curve of the x-ray afterglow can be used to constrain the physical parameters of both tde winds and tori, including the wind velocity, wind density, cloud density, and cloud size. | years-delayed x-ray afterglows of tdes originated from wind-torus interactions |
we report the detections of molecular hydrogen (h2), vibrationally-excited h2 (h2∗), and neutral atomic carbon (c i), an efficient tracer of molecular gas, in two new afterglow spectra of grbs 181020a (z = 2.938) and 190114a (z = 3.376), observed with x-shooter at the very large telescope (vlt). both host-galaxy absorption systems are characterized by strong damped lyman-α absorbers (dlas) and substantial amounts of molecular hydrogen with logn(h i, h2) = 22.20 ± 0.05, 20.40 ± 0.04 (grb 181020a) and logn(h i, h2) = 22.15 ± 0.05, 19.44 ± 0.04 (grb 190114a). the dla metallicites, depletion levels, and dust extinctions are within the typical regimes probed by grbs with [zn/h] = -1.57 ± 0.06, [zn/fe] = 0.67 ± 0.03, and av = 0.27 ± 0.02 mag (grb 181020a) and [zn/h] = -1.23 ± 0.07, [zn/fe] = 1.06 ± 0.08, and av = 0.36 ± 0.02 mag (grb 190114a). in addition, we examine the molecular gas content of all known h2-bearing grb-dlas and explore the physical conditions and characteristics required to simultaneously probe c i and h2∗. we confirm that h2 is detected in all c i- and h2∗-bearing grb absorption systems, but that these rarer features are not necessarily detected in all grb h2 absorbers. we find that a large molecular fraction of fh2 ≳ 10-3 is required for c i to be detected. the defining characteristic for h2∗ to be present is less clear, though a large h2 column density is an essential factor. we also find that the observed line profiles of the molecular-gas tracers are kinematically "cold", with small velocity offsets of δv < 20 km s-1 from the bulk of the neutral absorbing gas. we then derive the h2 excitation temperatures of the molecular gas and find that they are relatively low with tex ≈ 100-300 k, however, there could be evidence of warmer components populating the high-j h2 levels in grbs 181020a and 190114a. finally, we demonstrate that even though the x-shooter grb afterglow campaign has been successful in recovering several h2-bearing grb-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured h2 absorbers from identification. c i and h2∗ could open a potential route to identify molecular gas even in low-metallicity or highly dust-obscured bursts, though they are only efficient tracers for the most h2-rich grb-host absorption systems. reduced spectra are also available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/629/a131based on observations collected at the european southern observatory, paranal, chile, under the stargate consortium with program id: 0102.d-0662. | new constraints on the physical conditions in h2-bearing grb-host damped lyman-α absorbers |
we investigate signatures of population iii (popiii) stars in the metal-enriched environment of gamma-ray bursts (grbs) originating from population ii-i (popii/i) stars by using abundance ratios derived from numerical simulations that follow stellar evolution and chemical enrichment. we find that at z > 10 more than 10 per cent of popii/i grbs explode in a medium previously enriched by popiii stars (we refer to them as grbii→iii). although the formation of grbii→iii is more frequent than that of pristine popiii grbs (grbiiis), we find that the expected grbii→iii observed rate is comparable to that of grbiiis, due to the usually larger luminosities of the latter. grbii→iii events take place preferentially in small protogalaxies with stellar masses m⋆ ∼ 104.5-107 m⊙, star formation rates sfr ∼ 10^{-3}-10^{-1} m_{⊙} yr^{-1} and metallicities z ∼ 10- 4-10- 2 z⊙. on the other hand, galaxies with z < 10- 2.8 z⊙ are dominated by metal enrichment from popiii stars and should preferentially host grbii→iii. hence, measured grb metal content below this limit could represent a strong evidence of enrichment by pristine stellar populations. we discuss how to discriminate popiii metal enrichment on the basis of various abundance ratios observable in the spectra of grbs' afterglows. by employing such analysis, we conclude that the currently known candidates at redshift z ≃ 6 - i.e. grb 050904 and grb 130606a - are likely not originated in environments pre-enriched by popiii stars. abundance measurements for grbs at z ≃ 5 - such as grb 100219a and grb 111008a - are still poor to draw definitive conclusions, although their hosts seem to be dominated by popii/i pollution and do not show evident signatures of massive popiii pre-enrichment. | popiii signatures in the spectra of popii/i grbs |
the recently discovered rapid transient gn-z11-flash has been suggested to be the prompt-emission ultraviolet (uv) flash associated with a gamma-ray burst (grb) serendipitously exploding in the ultra-high-z galaxy gn-z11. we here place the flash into the context of the early uv emission of grbs, and find it is in agreement with the luminosity distribution of these events. | gn-z11-flash in the context of gamma-ray burst afterglows |
we report the insight-hxmt observations on both the prompt emission and the afterglow of the record-breaking grb 221009a (gcn 32632, atel #15650). insight-hxmt is mainly composed of three x-ray telescopes: the high energy x-ray telescope (he, liu et al. 2022, scpma, 63, 249503), the medium energy x-ray telescope (me, cao et al. 2022, scpma, 63, 249504) and the low energy x-ray telescope (le, chen et al. 2022, scpma, 63, 249505). | insight-hxmt observation on the prompt emission and afterglow of grb 221009a |
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