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gamma-ray bursts (grbs) are the brightest explosions in the universe, yet the properties of their energy sources are far from understood. very important clues, however, can be deduced by studying the afterglows of these events. we present observations of grb 130831a and its afterglow obtained with swift, chandra, and multiple ground-based observatories. this burst shows an uncommon drop in the x-ray light curve at about 100 ks after the trigger, with a decay slope of α ≃ 7. the standard forward shock (fs) model offers no explanation for such a behaviour. instead, a model in which a newly born magnetar outflow powers the early x-ray emission is found to be viable. after the drop, the x-ray afterglow resumes its decay with a slope typical of fs emission. the optical emission, on the other hand, displays no clear break across the x-ray drop and its decay is consistent with that of the late x-rays. using both the x-ray and optical data, we show that the fs model can explain the emission after ≃ 100 ks. we model our data to infer the kinetic energy of the ejecta and thus estimate the efficiency of a magnetar "central engine" of a grb. furthermore, we break down the energy budget of this grb into prompt emission, late internal dissipation, kinetic energy of the relativistic ejecta, and compare it with the energy of the accompanying supernova, sn 2013fu. | grb 130831a: rise and demise of a magnetar at z = 0.5 |
we report on the observations of gamma-ray burst (grb) 190114c by the fermi gamma-ray space telescope and the neil gehrels swift observatory. the early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 mev in the first few seconds of the burst. this power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed at later times. this afterglow component is clearly identifiable in the gbm and bat light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. as a result, we are able to constrain the transition from internal shock to external shock dominated emission. we find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment and find that high-energy photons observed by fermi lat are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. these violations of the maximum synchrotron energy are further compounded by the detection of very high energy (vhe) emission above 300 gev by magic concurrent with our observations. we conclude that the observations of vhe photons from grb 190114c necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the lat energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process. | fermi and swift observations of grb 190114c: tracing the evolution of high-energy emission from prompt to afterglow |
we present the discovery of the radio afterglow of the short $\gamma$-ray burst (grb) 210726a, localized to a galaxy at a photometric redshift of $z\sim 2.4$. while radio observations commenced $\lesssim 1~$day after the burst, no radio emission was detected until $\sim11$~days. the radio afterglow subsequently brightened by a factor of $\sim 3$ in the span of a week, followed by a rapid decay (a ``radio flare''). we find that a forward shock afterglow model cannot self-consistently describe the multi-wavelength x-ray and radio data, and underpredicts the flux of the radio flare by a factor of $\approx 5$. we find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of $\approx 4$, or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. at $z\sim 2.4$, grb\,210726a is among the highest redshift short grbs discovered to date as well as the most luminous in radio and x-rays. combining and comparing all previous radio afterglow observations of short grbs, we find that the majority of published radio searches conclude by $\lesssim 10~$days after the burst, potentially missing these late rising, luminous radio afterglows. | a radio flare in the long-lived afterglow of the distant short grb 210726a: energy injection or a reverse shock from shell collisions? |
the ultrarelativistic jets responsible for prompt and afterglow emission in gamma ray bursts are presumably driven by a central engine that consists of a dense accretion disk around a spinning black hole. we consider such engine, composed of free nucleons, electron-positron pairs, helium nuclei, and cooled by neutrino emission. a significant number density of neutrons in the disk provide conditions for neutron rich plasma in the outflows and jets. heavy nuclei are also formed in the accretion flow, at the distances 150-250 gravitational radii from the black hole. we study the process of nucleosynthesis in the grb engine, depending on its physical properties. our results may have important observational implications for the jet deceleration process and heavy elements observed in the spectra of grb afterglows. | nucleosynthesis of heavy elements in gamma ray bursts |
since 2009 x-shooter has collected spectroscopy of over 80 gamma-ray burst afterglows with redshifts ranging from 0.05 to 6.3. thanks to its efficiency, broad wavelength coverage (3,000 to 24,800 å), and intermediate spectral resolution (r~8,000) it has become the most efficient tool for gamma-ray burst afterglow spectroscopy. in this focus meeting we presented the sample and some preliminary results of the analysis of absorption systems. | absorption features in the x-shooter grb afterglow sample |
we present a possible theoretical interpretation of the observed afterglow emission of long gamma-ray burst grb 080710. while its prompt grb emission properties are normal, the afterglow light curves in the optical and infrared bands are exceptional in two respects. one is that the observed light curves of different wavelengths have maximum at the same time, and that the achromatic peak time, $2.2\times10^3$ s after the burst trigger, is about an order of magnitude later than typical events. the other is that the observed flux before the peak increases more slowly than theoretically expected so far. assuming that the angular distribution of the outflow energy is top-hat or gaussian-shaped, we calculate the observed light curves of the synchrotron emission from the relativistic jets and explore the model parameters that explain the observed data. it is found that a narrowly collimated gaussian-shaped jet with large isotropic-equivalent energy is the most plausible model for reproducing the observed afterglow behavior. namely, an off-axis afterglow scenario to the achromatic peak is unlikely. the inferred values of the opening angle and the isotropic equivalent energy of the jet are possibly similar to those of grb 221009a, but the jet of grb 080710 has a much smaller efficiency of the prompt gamma-ray emission. our results indicate a greater diversity of the grb jet properties than previously thought. | grb 080710: a narrow, structured jet showing a late, achromatic peak in the optical and infrared afterglow? |
a newly born millisecond magnetar is thought to be the central engine of some gamma-ray bursts (grbs), especially those that present long-lasting x-ray plateau emissions. by solving the field equations, we find that when the rotational speed of the magnetar is approaching the breakup limit, its radius r and moment of inertia i undergo an obvious evolution as the magnetar spins down. meanwhile, the values of r and i sensitively depend on the adoption of a neutron star (ns) equation of state (eos) and the ns baryonic mass. with different eoss and baryonic mass considered, the magnetic dipole radiation luminosity (ldip) could be variant within one to two orders of magnitude. we thus suggest that when using the x-ray plateau data of grbs to diagnose the properties of the nascent nss, eos and ns mass information should be invoked as simultaneously constrained parameters. on the other hand, due to the evolution of r and i, the temporal behavior of ldip becomes more complicated. for instance, if the spin-down process is dominated by gravitational wave emission due to the ns asymmetry caused by magnetic field distortion ( $\epsilon \propto {b}_{p}^{2}$ ), the segment ldip ∝ t0 could be followed by ldip ∝ t-γ with γ larger than 3. this case could naturally interpret the so-called internal x-ray plateau feature shown in some grb afterglows, which means the sharp decay following the plateau unnecessarily corresponds to the collapse of the ns. this may explain why some internal x-ray plateaus are followed by late-time central engine activity, manifested through flares and second shallow plateaus. | the evolutionary effects of the radius and moment of inertia of rapidly rotating neutron stars |
we examine grbs with both fermi-lat and x-ray afterglow data. assuming that the 100mev (lat) emission is radiation from cooled electrons accelerated by external shocks, we show that the kinetic energy of the blast wave estimated from the 100mev flux is 50 times larger than the one estimated from the x-ray flux. this can be explained if either: i) electrons radiating at x-rays are significantly cooled by ssc (suppressing the synchrotron flux above the cooling frequency) or ii) if the x-ray emitting electrons, unlike those emitting at 100mev energies, are in the slow cooling regime. in both cases the x-ray flux is no longer an immediate proxy of the blast wave kinetic energy. we model the lat, x-ray and optical data and show that in general these possibilities are consistent with the data, and explain the apparent disagreement between x-ray and lat observations. all possible solutions require weak magnetic fields: $10^{-6}< \epsilon_b < 10^{-3}$ (where $\epsilon_b$ is the fraction of shocked plasma energy in magnetic fields). using the lat emission as a proxy for the blast wave kinetic energy we find that the derived prompt efficiencies are of order 15%. this is considerably lower compared with previous estimates (87% and higher for the same bursts). this provides at least a partial solution to the "prompt high efficiency paradox". | shedding light on the prompt high efficiency paradox - self consistent modeling of grb afterglows |
we analyze extragalactic extinction profiles derived through gamma-ray burst afterglows, using a dust model specifically constructed on the assumption that dust grains are not immutable but respond, time-dependently, to the local physics. such a model includes core-mantle spherical particles of mixed chemical composition (silicate core, sp2, and sp3 carbonaceous layers), and an additional molecular component in the form of free-flying polycyclic aromatic hydrocarbons. we fit most of the observed extinction profiles. failures occur for lines of sight, presenting remarkable rises blueward of the bump. we find a tendency for the carbon chemical structure to become more aliphatic with the galactic activity, and to some extent with increasing redshifts. moreover, the contribution of the molecular component to the total extinction is more important in younger objects. the results of the fitting procedure (either successes and failures) may be naturally interpreted through an evolutionary prescription based on the carbon cycle in the interstellar medium of galaxies. | modeling extragalactic extinction through gamma-ray burst afterglows |
measuring grb properties in their rest-frame is crucial to understand the physics at work in gamma-ray bursts. this can only be done for grbs with known redshift. since redshifts are usually measured from the optical spectrum of the afterglow, correlations between prompt and afterglow emissions may introduce biases in the distribution of rest-frame properties of the prompt emission, especially considering that we measure the redshift of only one third of swift grbs. in this paper we study the brightness of optical grb afterglows and the role of optical selection effects in the distribution of various intrinsic properties of grbs and on the epi - eiso relation discovered by amati et al. (2002). our analysis is based on a sample of 85 grbs with good optical follow-up and well measured prompt emission. 71 of them have a measure of redshift and 14 have no redshift. we discuss the connection between the location of grbs in the epi-eiso plane and their optical brightness measured two hours after the trigger in the grb rest frame. we show that the brightness of grbs in our sample is mainly driven by their intrinsic luminosity and depends only slightly on their redshift. we also show that grbs with faint afterglows are preferentially located in the upper part of the epi-eiso plane. this optical selection effect favors the detection of grbs with bright afterglows located below the best fit epi - eiso relation whose redshift is easily measurable. we conclude that the distributions of prompt grb properties in the rest frame undergo selection effects due to the need to measure the redshift from the optical afterglow emission. these biases put significant uncertanties when interpreting the statistical studies of grb properties in the rest frame. we show that the epi - eiso relation is not immune to these selection effects. | connecting prompt and afterglow grb emission i. investigating the impact of optical selection effects in the epi - eiso plane |
we observed gamma-ray burst (grb) 221009a using very long baseline interferomety (vlbi) with the european vlbi network (evn) and the very long baseline array (vlba), over a period spanning from 40 to 262 days after the initial grb. the high angular resolution (~mas) of our observations allowed us, for the second time ever after grb 030329, to measure the projected size, $s$, of the relativistic shock caused by the expansion of the grb ejecta into the surrounding medium. our observations support the expansion of the shock with a 3.6 $\sigma$-equivalent significance, and confirm its relativistic nature by revealing an apparently superluminal expansion rate. fitting a power law expansion model, $s\propto t^a$, to the observed size evolution, we find a slope $a=1.9_{-0.6}^{+0.7}$, which is steeper than expected from either a forward shock (fs) or reverse shock (rs) model, implying an apparent acceleration of the expansion. fitting the data at each frequency separately, we find different expansion rates, pointing to a frequency-dependent behaviour. we show that the observed size evolution can be reconciled with a rs plus fs in the case of a wind-like circum-burst medium, provided that the two shocks dominate the emission at different frequencies and, possibly, at different times. | the expansion of the grb 221009a afterglow |
the tev observations of grb~221009a provided us with a unique opportunity to analyze the contemporaneous phase in which both prompt and afterglow emissions are seen simultaneously. to describe this initial phase of gamma-ray burst afterglows, we suggest a model for a blast wave with an intermittent energy supply. we treat the blast wave as a two-element structure. the central engine supplies energy to the inner part (shocked ejecta material) via the reverse shock. as the shocked ejecta material expands, its internal energy is transferred to the shocked external matter. we take into account the inertia of the shocked external material so that the pressure difference across this region determines the derivative of the blast wave's lorentz factor. applied to grb~221009a, the model yields a very good fit to the observations of the entire tev lightcurve except for three regions where there are excesses in the data with respect to the model. those are well correlated with the three largest episodes of the prompt activity, and thus, we interpret them as the reverse shock emission. our best-fit solution for grb~221009a is an extremely narrow jet with an opening angle theta_j approx 0.07^o (500/\gamma_0) propagating into a wind-like external medium. this extremely narrow angle is consistent with the huge isotropic equivalent energy of this burst, and its inverse jet break explains the very rapid rise of the afterglow. interestingly, photon-photon annihilation doesn't play a decisive role in the best-fit model. | the contemporaneous phase of grb afterglows -- application to grb~221009a |
particle-in-cell simulations have unveiled that shock-accelerated electrons do not follow a pure power-law distribution, but have an additional low-energy "thermal" part, which owns a considerable portion of the total energy of electrons. investigating the effects of these thermal electrons on gamma-ray burst (grb) afterglows may provide valuable insights into the particle acceleration mechanisms. we solve the continuity equation of electrons in the energy space, from which multi-wavelength afterglows are derived by incorporating processes including synchrotron radiation, synchrotron self-absorption, synchrotron self-compton scattering, and gamma-gamma annihilation. first, there is an underlying positive correlation between temporal and spectral indices due to the cooling of electrons. moreover, thermal electrons would result in the simultaneous non-monotonic variation in both spectral and temporal indices at multi-wavelength, which could be individually recorded by the 2.5-meter wide field survey telescope and vera rubin observatory legacy survey of space and time (lsst). the thermal electrons could also be diagnosed from afterglow spectra by synergy observation in the optical (with lsst) and x-ray bands (with the microchannel x-ray telescope on board the space variable objects monitor). finally, we use monte carlo simulations to obtain the distribution of peak flux ratio ($r_{\rm x}$) between soft and hard x-rays, and of the time delay ($\delta t$) between peak times of soft x-ray and optical light curves. the thermal electrons significantly raise the upper limits of both $r_{\rm x}$ and $\delta t$. thus the distribution of grb afterglows with thermal electrons is more dispersive in the $r_{\rm x} - \delta t$ plane. | probing thermal electrons in grb afterglows |
gamma-ray bursts could serve as probes to the extreme physics which power them, yet the origin of their observed prompt emission and their jet structure is still shrouded in mystery. the compton spectrometer and imager (cosi), designed to survey the gamma-ray sky at 0.2-5 mev, could demystify the physics at play through its ability to localize and characterize grbs as a small explorer satellite mission slated for launch in 2027. the detector array of cosi serves as a powerful wide-field spectrometer and polarimeter, and the surrounding active shields provide background rejection as well as allow for grb triggering. this presentation will discuss cosi's contribution to multimessenger studies by its ability to localize grbs with sub-degree positions and enable afterglow follow-ups. it will also delve into grb characterization through maximum likelihood method analysis tools for joint spectral-polarization studies. | observing gamma-ray bursts with the compton spectrometer and imager |
five long gamma-ray bursts (grbs) have been found to have very high energy (vhe, > 100gev) counterparts. interestingly, more than one emission mechanism has been invoked to explain the vhe counterpart from different events. as a result of this discovery, it has become apparent that we have been missing half of the energy produced in the afterglow of grbs. we have been studying the radio afterglows in order to investigate whether these vhe grbs have unusual jet properties. studying these events in the radio waveband is advantageous as the emission at lower frequencies is brighter for longer enabling detailed, long term study of the jet evolution. the jet properties and environments of these grbs vary hugely in a similar manner to that seen in the `regular' long grb population with evidence of bright reverse shock emission and multiple jet components. this work is presented on behalf of a much larger collaboration. | radio afterglows of very high energy gamma-ray bursts |
the physical mechanism of fast radio bursts (frbs) is still unknown. on 2020 april 28, a special radio burst, frb 200428, was detected and believed to be associated with the galactic magnetar sgr 1935+2154. it confirms that at least some of the frbs were generated by magnetars, although the radiation mechanism continues to be debated. to this end, we study in detail the multiband afterglows of frb 200428 described by the synchrotron fireball shock model. we find the prediction for the optical and radio afterglows of frb 200428 is consistent with the observations when considering the post-frb optical and radio upper limits from the literature. we also show that the follow up detection of the afterglows from fast radio bursts like-frb 200428 is possible at the radio band, though challenging. based on our model, one can obtain information about the energy of the fireball, the radiation zone, and the nature of the surrounding medium. that may shed light on the physical mechanism of frbs. | prediction for the multi-band afterglows of frb 200428 and its implication |
a large fraction of γ-ray bursts (grbs) show a plateau phase during the x-ray afterglow emission, which has a physical origin that is still debated. in this work, we select a sample of 30 grbs with simultaneous x-ray and optical data during and after the plateau phase. through a time-resolved spectral analysis of the x-ray plateaus, we test the consistency of the unabsorbed optical fluxes with those obtained via x-ray-to-optical spectral extrapolation by assuming a synchrotron spectrum. by combining x-ray with optical data, we find that 63% (19/30) of grbs are compatible with a single synchrotron spectrum, thus suggesting that both the optical and x-ray radiations are produced from a single emitting region. for these grbs, we derive the temporal evolution of the break frequency, and we compare it with the expectations predicted by several models. for 11 of the 30 grbs, the optical emission is above the predicted range of values extrapolated from the x-rays in at least one temporal bin of the light curve. these 11 grbs may not be explained with a single-zone emission, indicating the necessity of invoking two co-operating processes in order to explain the broad-band spectral behaviour. we discuss our findings in the framework of different scenarios that are invoked to explain the plateau feature, including the energy injection from a spinning-down magnetar and the high latitude emission from a structured jet. | combined x-ray and optical analysis to probe the origin of the plateau emission in γ-ray burst afterglows |
the environment of a γ-ray burst has an important influence on the evolution of jet dynamics and its afterglow. here we investigate the afterglow polarizations in a stratified medium with the equal arrival time surface (eats) effect. polarizations of multiband afterglows are predicted. the effects of the parameters of the stratified medium on the afterglow polarizations are also investigated. we found the influences of the eats effect on the afterglow polarizations become important for off-axis detections and polarization degree (pd) bumps move to later times with the eats effect. even if the magnetic field configurations, jet structure, and observational angles are fixed, polarization properties of the jet emission could still evolve. here, we assume a large-scale ordered magnetic field in the reverse-shock region and a two-dimensional random field in the forward-shock region. then, pd evolution is mainly determined by the evolution of the f 32 parameter (the flux ratio between the reverse-shock region and forward-shock region) at the early stage and by the evolution of the bulk lorentz factor γ at the late stage. through the influences on the f 32 or γ, the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations. | afterglow polarizations in a stratified medium with effect of the equal arrival time surface |
gamma-ray bursts (grbs), both long and short, are explosive events whose inner engine is generally expected to be a black hole or a highly magnetic neutron star (magnetar) accreting high-density matter. recognizing the nature of grb central engines, and in particular the formation of neutron stars (nss), is of high astrophysical significance. a possible signature of nss in grbs is the presence of a plateau in the early x-ray afterglow. here we carefully select a subset of long and short grbs with a clear plateau, and look for an additional ns signature in their prompt emission, namely a transition between the accretion and propeller phases in analogy with accreting, magnetic compact objects in other astrophysical sources. we estimate from the prompt emission the minimum accretion luminosity below which the propeller mechanism sets in, and the ns magnetic field and spin period from the plateau. we demonstrate that these three quantities obey the same universal relation in grbs as in other accreting compact objects switching from accretion to propeller. this relation provides also an estimate of the radiative efficiency of grbs, which we find to be several times lower than radiatively efficient accretion in x-ray binaries and in agreement with theoretical expectations. these results provide additional support to the idea that at least some grbs are powered by magnetars surrounded by an accretion disk. | magnetar central engines in gamma-ray bursts follow the universal relation of accreting magnetic stars |
the use of high energy transients such as gamma ray bursts (grbs) as probes of the distant universe relies on the close collaboration between space and ground facilities. in this context, the sino-french mission svom has been designed to combine a space and a ground segment and to make the most of their synergy. on the ground, the 1.3 meter robotic telescope colibri, jointly developed by france and mexico, will quickly point the sources detected by the space hard x-ray imager eclairs, in order to detect and localise their visible/nir counterpart and alert large telescopes in minutes. colibri is equipped with two visible cameras, called ddrago-blue and ddrago-red, and an infrared camera, called cagire, designed for the study of high redshift grbs candidates. being a low-noise nir camera mounted at the focus of an alt-azimutal robotic telescope imposes specific requirements on cagire. we describe here the main characteristics of the camera: its optical, mechanical and electronics architecture, the alfa detector, and the operation of the camera on the telescope. the instrument description is completed by three sections presenting the calibration strategy, an image simulator incorporating known detector effects, and the automatic reduction software for the ramps acquired by the detector. this paper aims at providing an overview of the instrument before its installation on the telescope. | cagire: a wide-field nir imager for the colibri 1.3 meter robotic telescope |
to demonstrate the magnetic energy dissipation via relativistic shocks, we carry out spherically symmetrical one-dimensional special relativistic magnetohydrodynamic simulations of highly magnetized outflows with an adaptive mesh refinement method. we first investigate the details of the dynamical energy dissipation via interaction between a single ejecta and an external medium. the energy dissipation time-scales, which affect the early behaviour of the afterglow emission in gamma-ray bursts, are estimated for a wide range of magnetization. in addition, we demonstrate the internal shock dissipation in multiple interactions between magnetically dominated relativistic ejecta and kinetically dominated non-relativistic winds. our numerical results show that ~10 per cent of the magnetic energy in the ejecta can be converted into the thermal energy of the relativistic and low-magnetized outflows via shocks in the rarefaction waves or the winds. such hot and less magnetized outflows are relevant for observed non-thermal emissions in blazars or gamma-ray bursts. | dynamical energy dissipation of relativistic magnetic bullets |
the australia telescope compact array (atca) observed grb 220627a (fermi gbm team, gcn circ. 32278) at multiple frequencies starting at 04:00 ut on 2022 july 5 (7.3 days post-burst). we detected a radio source with a preliminary flux density of ~0.4 mjy at a central frequency of 17 ghz at ra = 13:25:28.48 dec = -32:25:32.14 with positional uncertainty of ~1.5 arcsec. the position of the radio source is consistent with the candidate optical (de wet et al., gcn circ. 32289) and x-ray afterglow (gropp et al., gcn circ. 32296) positions. further observations with several facilities at different frequencies are ongoing. we thank csiro staff for rapidly scheduling and supporting these observations. | grb 220627a: atca detection of radio counterpart |
at 19:21:43 ut, the swift burst alert telescope (bat) triggered and located grb 200205b (trigger=954519 and 954520). swift slewed to the burst after a short delay. the bat on-board calculated location is ra, dec 107.861, -56.506 which is ra(j2000) = 07h 11m 27s dec(j2000) = -56d 30' 21" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat light curve showed a peak about 20 seconds long (trigger 954519) followed by a brighter more complex peak structure 350 s later (trigger 954520) with a peak countrate of 3500 counts/s. due to the double trigger, the complete bat lightcurve is not immediately available. the xrt began observing the field at 19:27:26.3 ut, 342.7 seconds after the bat trigger. using promptly downlinked data we find a bright, uncatalogued x-ray source with an enhanced position: ra, dec 107.7881, -56.4875 which is equivalent to: ra(j2000) = 07h 11m 09.14s dec(j2000) = -56d 29' 15.0" with an uncertainty of 2.8 arcseconds (radius, 90% containment). this location is 37 arcseconds from the bat onboard position, within the bat error circle. this position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. we cannot determine whether the source is fading at the present time. a power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the galactic value of 1.04 x 10^21 cm^-2 (willingale et al. 2013). the initial flux in the 2.5 s image was 1.26e-09 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 351 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 07:11:09.15 = 107.78811 dec(j2000) = -56:29:15.1 = -56.48753 with a 90%-confidence error radius of about 0.61 arc sec. this position is 0.8 arc sec. from the center of the xrt error circle. the estimated magnitude is 16.43 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.11. burst advocate for this burst is p. a. evans (pae9 at star.le.ac.uk). please contact the ba by email if you require additional information regarding swift followup of this burst. in extremely urgent cases, after trying the burst advocate, you can contact the swift pi by phone (see swift too web site for information: http://www.swift.psu.edu/) | grb 200205b: swift detection of a burst with optical counterpart |
i summarize our recent calculations on quark stars (qss), for the purpose of explaining some short gamma-ray bursts characterized by internal plateau in their early x-ray afterglow. according to the present plateau sample, the qs central engine model is demonstrated to more preferred than the original neutron star (ns) one. new qs equation of states (pmqs1, pmqs2, pmqs3) are then proposed, respecting fully the observed burst data and the mass distribution of the galactic ns-ns systems. | internal plateau in short grbs and quark stars |
nustar's focusing x-ray optics provide unprecedented sensitivity above 10 kev, enabling late-time observations of gamma-ray burst (grb) afterglows. to date nustar has observed six grbs: grb130427a, grb130925a, grb150201a, grb180720b, grb190114c, and grb190829a. while most nustar data are consistent with the power-law spectra seen at lower energies, the multiple components seen in grb130925a indicate the potential of late-time x-ray observations to reveal unexpected surprises. | nustar observations of gamma-ray burst afterglows |
gamma-ray bursts (grbs) are the most energetic explosions in the universe. they are collimated, ultra-relativistic outflows which are initially detected as brief flashes of gamma-rays. as the relativistic ejecta starts to interact with the circumburst medium, a pair of shocks are formed where magnetic fields are amplified and charged particles are accelerated. these particles emit synchrotron radiation observable throughout the whole electromagnetic spectrum. this long-lived emission is called the afterglow of the grb. the afterglow emission contains valuable information regarding the dynamics of the outflow, energetics and environments of grbs, and microphysics of relativistic shocks. in this work, we improve the method we have developed in aksulu et al. (2020) where we use gaussian processes (gps) to account for the systematics in the observed afterglow emission. this method enables us to obtain reliable estimates of the grb parameters even when the dataset contains systematics. we gather a sample of multiwavelength short and long grb afterglow datasets and perform bayesian inference using the improved gp model in order to probe the physics of grbs. finally, we present our findings regarding the dynamics, energetics, and environments of these catastrophic events. | exploring the population of grbs: robust afterglow modelling using gaussian processes |
the shock system that produces the afterglow to grbs consists of a forward- and a reverse-shock. for short grbs, observational evidence for a reverse-shock has been sparse, however, the afterglow to grb 160821b requires a reverse-shock at early times to explain the radio observations. grb 160821b is additionally accompanied by the best-sampled macronova without a gravitational-wave detection, and an interesting late-time x-ray afterglow behaviour indicative of a refreshed-shock. the presence of an observed reverse-shock in an on-axis short grb means that the reverse-shock should be considered as a potential counterpart to gravitational-wave detected mergers. as a gravitational-wave counterpart, the afterglow to an off-axis grb jet can reveal the jet structure -- a reverse-shock will exist in these structured jet systems and the signature of these reverse-shocks, if observed, can indicate the degree of magnetisation in the outflow. here we show the case of grb 160821b, and how a reverse-shock will appear for an off-axis observer to a structured jet. | reverse shocks in short gamma-ray bursts -- the case of grb 160821b and prospects as gravitational-wave counterparts |
at 06:53:27 ut, the swift burst alert telescope (bat) triggered on a burst from the soft gamma-ray repeater swift j1555.2-5402 (trigger=1070298). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 238.836, -54.070 which is ra(j2000) = 15h 55m 21s dec(j2000) = -54d 04' 12" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat light curve showed a single structure with a duration of about 0.1 sec. the peak count rate was ~2700 counts/sec (15-350 kev), at ~0 sec after the trigger. the xrt began observing the field at 06:54:47.7 ut, 80.4 seconds after the bat trigger. using promptly downlinked data we find an uncatalogued x-ray source with an enhanced position: ra, dec 238.7859, -54.0617 which is equivalent to: ra(j2000) = 15h 55m 08.61s dec(j2000) = -54d 03' 42.2" with an uncertainty of 1.9 arcseconds (radius, 90% containment). this location is 109 arcseconds from the bat onboard position, within the bat error circle. this position is consistent with the previously-reported position of swift j1555.2-5402 (gcn 30122). this position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. we cannot determine whether the source is fading at the present time. a power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. uvot took a finding chart exposure of 150 seconds with the white filter starting 85 seconds after the bat trigger. no credible afterglow candidate has been found in the initial data products. the 2.7'x2.7' sub-image covers none of the xrt error circle. the typical 3-sigma upper limit has been about 19.6 mag. the 8'x8' region for the list of sources generated on-board covers 100% of the xrt error circle. the list of sources is typically complete to about 18 mag. no correction has been made for the large, but uncertain, extinction expected. | swift detection of an outburst from sgr swift j1555.2-5402 |
swift-xrt has performed follow-up observations of the swift/bat-guano-detected burst grb 211106a, collecting 5.0 ks of photon counting (pc) mode data between t0+39.2 ks and t0+58.2 ks. eight uncatalogued x-ray sources have been detected, however none of them is above the rass limit or shows definitive signs of fading. therefore, at the present time we cannot identify which, if any, is the afterglow. details of these sources are given below: source 2: ra (j2000.0): 343.5852 = 22:54:20.45 dec (j2000.0): -53.2303 = -53:13:49.0 error: 3.4 arcsec (radius, 90% conf. [enhanced position]) count-rate: (8.7 +/- 1.6)e-3 ct s^-1 distance: 126 arcsec from swift/bat-guano position. flux: (4.42 +/- 0.81)e-13 erg cm^-2 s^-1 (observed, 0.3-10 kev) source 3: ra (j2000.0): 343.7780 = 22:55:6.71 dec (j2000.0): -53.3150 = -53:18:53.9 error: 5.9 arcsec (radius, 90% conf.) count-rate: (2.24 [+1.04, -0.81])e-3 ct s^-1 distance: 406 arcsec from swift/bat-guano position. flux: (9.8 [+4.5, -3.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 kev) source 4: ra (j2000.0): 343.7223 = 22:54:53.36 dec (j2000.0): -53.1215 = -53:07:17.5 error: 5.5 arcsec (radius, 90% conf.) count-rate: (3.18 [+1.21, -0.98])e-3 ct s^-1 distance: 445 arcsec from swift/bat-guano position. flux: (1.42 [+0.54, -0.44])e-13 erg cm^-2 s^-1 (observed, 0.3-10 kev) the source position is 1" away from an agn candidate (assef et al, 2018, apjs, 234, 23; guo et al., 2018, a&a, 618, 144). source 5: ra (j2000.0): 343.6827 = 22:54:43.86 dec (j2000.0): -53.1372 = -53:08:14.1 error: 5.8 arcsec (radius, 90% conf. [enhanced position]) count-rate: (5.5 +/- 1.5)e-3 ct s^-1 distance: 365 arcsec from swift/bat-guano position. flux: (2.23 +/- 0.59)e-13 erg cm^-2 s^-1 (observed, 0.3-10 kev) source 6: ra (j2000.0): 343.7423 = 22:54:58.14 dec (j2000.0): -53.2260 = -53:13:33.6 error: 7.3 arcsec (radius, 90% conf.) count-rate: (1.62 [+0.86, -0.66])e-3 ct s^-1 distance: 216 arcsec from swift/bat-guano position. flux: (7.5 [+4.0, -3.0])e-14 erg cm^-2 s^-1 (observed, 0.3-10 kev) the source position is 7" away from an agn candidate (assef et al, 2018, apjs, 234, 23). source 7: ra (j2000.0): 343.7343 = 22:54:56.22 dec (j2000.0): -53.3579 = -53:21:28.5 error: 6.0 arcsec (radius, 90% conf.) count-rate: (2.98 [+1.18, -0.94])e-3 ct s^-1 distance: 481 arcsec from swift/bat-guano position. the source position is 5" away from an agn candidate (assef et al, 2018, apjs, 234, 23). source 8: ra (j2000.0): 343.7230 = 22:54:53.51 dec (j2000.0): -53.2861 = -53:17:10.0 error: 6.9 arcsec (radius, 90% conf.) count-rate: (2.72 [+1.12, -0.90])e-3 ct s^-1 distance: 249 arcsec from swift/bat-guano position. flux: (9.7 [+4.0, -3.2])e-14 erg cm^-2 s^-1 (observed, 0.3-10 kev) the source position is 9.5" away from an agn candidate (assef et al, 2018, apjs, 234, 23). source 9: ra (j2000.0): 343.8707 = 22:55:28.96 dec (j2000.0): -53.2028 = -53:12:10.2 error: 5.3 arcsec (radius, 90% conf.) count-rate: (2.17 [+1.02, -0.79])e-3 ct s^-1 distance: 504 arcsec from swift/bat-guano position. flux: (6.3 [+3.0, -2.3])e-14 erg cm^-2 s^-1 (observed, 0.3-10 kev) a catalogued source was also detected. the results of the xrt-team automatic analysis of the xrt observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/too_grbs/00021466. this circular is an official product of the swift-xrt team. | grb 211106a: swift-xrt observations |
we analyse the additional effect on planetary atmospheres of recently detected gamma-ray burst afterglow photons in the range up to 1 tev. for an earth-like atmosphere, we find that there is a small additional depletion in ozone versus that modeled for only prompt emission. we also find a small enhancement of muon flux at the planet surface. overall, we conclude that the additional afterglow emission, even with tev photons, does not result in a significantly larger impact over that found in past studies. | gamma-ray bursts: not so much deadlier than we thought |
the blackcat cubesat will survey the soft x-ray sky, searching for distant gamma-ray bursts (grbs) and other transient phenomena. we present simulations of blackcat observations that illustrate the capabilities of this mission. blackcat will identify and localize bursts to sub-arcminute precision, sending rapid alerts to enable follow-up observations and identification of coincident events from multi-messenger networks of observatories. blackcat's sensitivity in the 0.5-20 kev band makes it a particularly valuable detector of high-redshift (z > 5) grbs, which can serve as powerful probes of the reionization era, when the first stars and galaxies were forming. optical and near-infrared follow-up observations of afterglows of grbs identified by blackcat will enable measurements of their redshifts and characterization of the environments in which they occurred, in turn allowing measurements of the star formation rate and neutral hydrogen fraction of the intergalactic medium as a function of redshift. we use burst redshift and luminosity functions derived by fits to the swift burst alert telescope (bat) grb sample, and the light curves and spectra of those bursts (redshifted when necessary), to simulate observations of bursts by blackcat. from these simulations, we estimate the expected properties of the burst population observed by blackcat, including the expected redshift distribution. these simulations will be used to inform the operations, analysis, and follow-up techniques of this mission. we also discuss prospects for observations of x-ray flares and other x-ray transients with blackcat, including possible x-ray counterparts to gravitational-wave events, x-ray blazar monitoring, and potential x-ray flare counterparts to neutrino events. | planned observations and science with the blackcat cubesat x-ray transient monitor |
we present the earliest observation and first detection of polarized millimeter emission in a γ-ray burst with alma cycle 7 band 3 (97.5 ghz) observations of grb 190114c. with observations spanning 2.2 to 5.2 hours after the burst, we detect linear polarization in the grb afterglow at 5σ, decreasing from π=(0.87±0.13)% to (0.60±0.19)%, and evolving in polarization position angle from (10±5) degrees to (-44±12) degrees. we show that the optical and x-ray observations between 0.03 days and ∼0.3 days are consistent with a fast cooling forward shock expanding into a wind environment. however, the optical observations at ≲0.03 days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked jet. using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the grb jet to an angular size of θ ≍0.001 radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular the popular toroidal magnetic field model for grbs outflows. | alma detection of a linearly polarised reverse shock in grb 190114c |
at 13:59:34 ut, the swift burst alert telescope (bat) triggered and located grb 200829a (trigger=993768). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 251.212, +72.335 which is ra(j2000) = 16h 44m 51s dec(j2000) = +72d 20' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat light curve showed a complex structure with a duration of about 30 sec. the peak count rate was ~1335 counts/sec (15-350 kev), at ~6 sec after the trigger. the xrt began observing the field at 14:01:43.1 ut, 128.7 seconds after the bat trigger. xrt found a bright, uncatalogued x-ray source located at ra, dec 251.2096, 72.3273 which is equivalent to: ra(j2000) = 16h 44m 50.30s dec(j2000) = +72d 19' 38.3" with an uncertainty of 4.7 arcseconds (radius, 90% containment). this location is 27 arcseconds from the bat onboard position, within the bat error circle. no event data are yet available to determine the column density using x-ray spectroscopy. the initial flux in the 2.5 s image was 9.30e-09 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 138 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 16:44:49.05 = 251.20439 dec(j2000) = +72:19:45.3 = 72.32924 with a 90%-confidence error radius of about 0.61 arc sec. this position is 9.0 arc sec. from the center of the xrt error circle. the estimated magnitude is 14.28 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.04. burst advocate for this burst is m. h. siegel (siegel at swift.psu.edu). please contact the ba by email if you require additional information regarding swift followup of this burst. in extremely urgent cases, after trying the burst advocate, you can contact the swift pi by phone (see swift too web site for information: http://www.swift.psu.edu/) | grb 200829a: swift detection of a burst with an optical afterglow |
we investigate the scenario of a binary system composed by a neutron star (ns) and a main sequence star which has been stripped to be a naked carbon-oxygen core (co core). at a given time, the co core collapses as the supernova (sn) explosion, the hypercritical accretion of the sn ejecta onto the companion ns induces the gamma-ray burst (grb) and gives birth to a new ns, which powers the afterglow later on. the examples of grb 180728a is analyzed and demonstrated. | gamma-ray burst from binary star: neutron star and carbon-oxygen core |
we further observed the bright grb 211023a (fermi gbm team 2021, gcn 30958; lesage et al. 2021, gcn 30965; n. di lalla et al. 2021, gcn 30961; ursi et al. 2021, gcn 30969; and ridnaia et al. 2021, gcn 31022) with 1.3m devasthal fast optical telescope (dfot) located at devasthal observatory of aryabhatta research institute of observational sciences (aries), india, starting on 2021-10-29 at 19:19:13 ut, i.e., ~ 6.26 days after the fermi trigger. we obtained 24 and 18 frames having an exposure time of 300 sec in the r and i filters, respectively. we detected the optical afterglow of grb 211023a (lipunov et al. 2021, gcn 30970; zhirkov et al. 2021, gcn 30977; kann et al. 2021, gcn 30982; hu et al. 2021, gcn 30990; belkin et al. 2021, gcn 31004, 31018, 31020; vinko et al. 2021, gcn 31008; and kumar et al. 2021, gcn 31023) in our stacked images of both the filters. the preliminary photometric estimate of the afterglow in the r filter is following : date start ut t-t0 (mid, days) filter exp time (sec) magnitude =========================================================2021-10-29 19:19:13 ~6.28 r 300*24 22.58 +/- 0.33 based on the preliminary photometry reported by using various ground-based optical telescopes (kann et al. 2021, gcn 30982; hu et al. 2021, gcn 30990; and belkin et al. 2021, gcn 31004, 31018, 31020) and our first epoch observations using 1.3m dfot (kumar et al. 2021, gcn 31023), we measure the optical flux decay power-law index of the afterglow light curve (galactic extinction corrected) of grb 211023a equal to 1.49 +/- 0.04, consistent with those reported earlier (belkin et al. 2021, gcn 31004, 31020; and vinko et al. 2021, gcn 31008). however, we noticed that our second epoch observations are fainter than the extrapolation of the early temporal decay index of ~1.5, suggesting a possible break in the light curve. however, due to the unavailability of near-simultaneous x-ray afterglow observations, we could not confirm if this optical steeping is due to the jet break. further deeper and multiwavelength observations with larger telescopes will be helpful to establish the possible jet break. the reported magnitude value is not corrected for the galactic extinction in the direction of the burst. photometric calibration is performed using the standard stars from the usno-b1.0 catalog. this circular may be cited. | grb 211023a: continued 1.3m dfot optical observations and possible jet break |
neutron-star (ns) mergers provide unique environments for mass accretion, ejection, and r-process nucleosynthesis. information about the ns merger and its outcome is encoded in the observed electromagnetic (em) signatures, such as gamma ray bursts, kilonova and afterglow emissions. to boost the scientific return from these observations, we need first-principle models to quantitatively interpret them. here, i will present results of the first long-duration (~ few seconds) high resolution 3d general relativistic magnetohydrodynamic (grmhd) simulation of the merger aftermath including (a) accurate neutrino transport with (b) realistic equation of state and (c) realistically weak seed magnetic fields, which will allow me to the predict em emission from from first principles. moreover, i will enhance the realism of the simulations by initializing them with an outcome of a fully general relativistic merger simulation. | simulating neutron star merger remnant outflows with a two-moment neutrino scheme |
at 07:53:05 ut, the swift burst alert telescope (bat) triggered and located emission from the source 4u 1630-472 (a.k.a. swift j1633.8-4724a) (trigger=1160641). swift slewed immediately to the source. the bat on-board calculated location is ra, dec 248.508, -47.365 which is ra(j2000) = 16h 34m 02s dec(j2000) = -47d 21' 54" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). as is typical for long image triggers, there is no obvious variation in immediately available bat lightcurve. the xrt began observing the field at 08:01:45.3 ut, 520.3 seconds after the bat trigger. xrt found a bright, x-ray source located at ra, dec 248.5015, -47.3926 which is equivalent to: ra(j2000) = 16h 34m 0.36s dec(j2000) = -47d 23' 33.4" with an uncertainty of 4.7 arcseconds (radius, 90% containment). this location is 100 arcseconds from the bat onboard position, within the bat error circle. no event data are yet available to determine the column density using x-ray spectroscopy. uvot took a finding chart exposure of 150 seconds with the white filter starting 531 seconds after the bat trigger. no credible afterglow candidate has been found in the initial data products. the 2.7'x2.7' sub-image covers 100% of the xrt error circle. the typical 3-sigma upper limit has been about 19.6 mag. the 8'x8' region for the list of sources generated on-board covers 100% of the xrt error circle. the list of sources is typically complete to about 18 mag. no correction has been made for the large, but uncertain, extinction expected. bat transient monitor shows 4u 1630-472 has brighten up significantly in the last few weeks: https://swift.gsfc.nasa.gov/results/transients/weak/4u1630-472/ the on-going outburst activity was also reported in bouchet et al. 2023 (atel #15924) and jiang et al. 2022 (atel #15575). | trigger 1160641: swift detection of 4u 1630-472 (a.k.a. swift j1633.8-4724a) |
as we further our studies on gamma-ray bursts (grbs), both on theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. as transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by grbs should present some degree of polarization throughout the evolution of the burst. whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short grbs, it is paramount that the groundwork is laid for the day we have abundant data. in this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile $n(r)\propto r^ {-k}$. we present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the grb literature for four degrees of stratification $k=0,1,1.5 {\rm \, and\,} 2$ and two magnetic field configurations with high extreme anisotropy. we apply this model and predict polarization from a set of grbs exhibiting off-axis afterglow emission. in particular, for grb 170817a, we use the available polarimetric upper limits to rule out the possibility of a extremely anisotropic configuration for the magnetic field. | afterglow polarization from off-axis grb jets |
we analyzed 500 grb optical afterglows with known redshifts, thus building the most comprehensive sample of optical light curves (lcs) to date by searching the literature for all grbs detected between 1997 may and 2021 may by several satellites, e.g., the swift ultraviolet/optical telescope (uvot), and ground-based telescopes/detectors, e.g., the subaru telescope, gamma-ray burst optical/near-ir detector (grond), re-ionization and transients infrared camera/telescope (ratir), the mitsume, etc. (2 data files). | vizier online data catalog: optical lc fit parameters for 179 grbs (dainotti+, 2022) |
dark grbs constitute a significant fraction of the grb population. in this paper, we present the multiwavelength analysis of an intense two-episodic grb 150309a observed early on to ~114 days post-burst. despite the strong gamma-ray emission, no optical afterglow was detected for this burst. however, we discovered near-infrared afterglow ($k_{\rm s}$-band), ~5.2 hours post burst, with the circe instrument mounted at the 10.4m gtc. we used fermi observations of grb 150309a to understand the prompt emission mechanisms and jet composition. we performed the early optical observations using the bootes robotic telescope and late-time afterglow observations using the gtc. a potential faint host galaxy is also detected at optical wavelength using the gtc. we modelled the potential host galaxy of grb 150309a in order to explore the environment of the burst. the time-resolved spectral analysis of fermi data indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated poynting flux. gtc observations of the afterglow revealed that the counterpart of grb 150309a was very red, with h-$k_{\rm s}$ > 2.1 mag (95 $\%$ confidence). the red counterpart was not discovered in any bluer filters of swift uvot, indicative of high redshift origin. this possibility was discarded based on multiple arguments, such as spectral analysis of x-ray afterglow constrain z < 4.15 and a moderate redshift value obtained using spectral energy distribution modelling of the potential galaxy. the broadband afterglow sed implies a very dusty host galaxy with deeply embedded grb (suggesting $a_{\rm v}$ $\gtrsim$ 35 mag). the environment of grb 150309a demands a high extinction towards the line of sight, demanding dust obscuration is the most probable origin of optical darkness and the very red afterglow of grb 150309a. this result makes grb 150309a the highest extinguished grb known to date. | revealing characteristics of dark grb 150309a: dust extinguished or high-z? |
"at 03:27:45.98 ut on 16 september 2023, the fermi gamma-ray burst monitor (gbm) triggered on trigger 716527670/230916144 as a grb with 64% confidence). this trigger was later followed up by goto (vail et al., gcn 34730), identifying a candidate afterglow (at 2023sva) and suggested it was the counterpart. careful inspection of the gbm trigger by the fermi-gbm team identified a weak c1-class solar flare occurring contemporaneously at the trigger time, which dominates the spectrum below 50 kev. there is approximately several hundred counts in the 50-300 kev band over 12s, which the team was able to localize. the on-ground calculated location using the fermi gbm trigger data over this energy range is ra = 143.0, dec = 70.5 (j2000 degrees, equivalent to j2000 9h 32m, +/- 70d 30'), with a statistical uncertainty of 8.0 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of grbs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error. [connaughton et al. 2015, apjs, 216, 32] ). we note that this localization is 71 degrees from the sun, and is quite far from the ra and dec of the afterglow candidate reported by vail et al. in gcn 34730. we therefore believe these two events to be unrelated and suggest that at2023sva is an orphan afterglow. we also note that this weak burst is contaminated by the solar flare and that any meaningful data regarding the nature of this transient is likely unrecoverable. the skymap can be found here: https://heasarc.gsfc.nasa.gov/ftp/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_skymap_all_bn230916144.png the healpix fits file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/ftp/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_healpix_all_bn230916144.fit the gbm light curve can be found here: https://heasarc.gsfc.nasa.gov/ftp/fermi/data/gbm/triggers/2023/bn230916144/quicklook/glg_lc_medres34_bn230916144.gif" | fermi gbm statement for trigger 716527670/230916144 |
in this work, we have modeled the hydrodynamic evolution of the grbs fireball in violent interaction with the external medium (the ism) surrounding the burst source by assuming a power law distribution of the accelerated relativistic electrons. for this purpose, a computer code based only on the contribution of the predominant synchrotron radiation mechanism was developed. light curves for the afterglow emissions following several grbs over the x-ray and the visible r frequency bands were calculated. their comparison to observed data by the xrt/swift satellite and earth telescopes, respectively, points out fair overall agreements, thus confirming the validity of our hydrodynamic simulation based on the model of feng et al. (2002). | grb afterglow light curves from a hydrodynamic model |
the large area polarimeter, or leap, is a proposed mission concept that will radically improve our understanding of some of the most energetic phenomena in our universe by exposing the underlying physics governing astrophysical jets and the extreme environment surrounding newborn compact objects. leap will do this by making the most sensitive polarization measurements to date of the prompt gamma-ray emission from a large sample of gamma-ray bursts (grbs). the leap science objectives are met with a single, proven instrument—a wide field-of-view (fov) compton polarimeter that measures grb polarization over the energy range from 50-500 kev and simultaneously performs spectroscopy from 20 kev to 5 mev. mounted as an external payload on the international space station (iss), leap's large fov of ±75° (1.5π sr) allows it to continuously monitor the sky for grbs, ensuring that a significant number of grbs are observed over the 3-year mission lifetime. the large fov combined with leap's intrinsic ability to provide source localization and the iss's ability for swift downlink permits rapid follow-up by the community, which is essential to providing important context information (counterpart identifications, redshift, afterglow studies, etc.) and to maximizing the science output for the mission. an overview of leap's science objectives and mission design will be presented. | the leap gamma-ray burst polarimeter: an overview |
high-redshift gamma-ray bursts (grbs) are useful to probe the early universe, but only a few candidates have been detected so far. here, we report the optical and near-infrared observations of the afterglow of a relatively high-redshift event grb 220101a, which was triggered on new year's day of 2022, and therefore referred to as the "new year's burst." with the optical spectra obtained by xl2.16/bfosc and not/alfosc, we determine the redshift of the burst to be z = 4.615. we find that the optical afterglow of grb 220101a is one of the most luminous ever detected. based on our optical and near-infrared data, and combined with the x-ray observations, we perform a multiband fit with the python package afterglowpy. the jet opening angle is constrained to ~3.°4, which is consistent with the jet-break time at ~0.7 day. we also determine the circumburst density of n 0 = 0.15 cm-3 and kinetic energy e k,iso = 3.5 × 1054 erg. in the prompt phase of the burst, we find a "mirror" feature in the lightcurve from 80 s to 120 s. the physical origin of such a mirror feature is unclear. | optical and near-infrared observations of the distant but bright "new year\'s burst" grb 220101a |
the plateau phase in radio afterglows has been observed in very few gamma-ray bursts (grbs), and in this paper, 27 radio light curves with plateau phases were acquired from the published literature. we obtain the related parameters of the radio plateau, such as temporal indexes during the plateau phase (α 1 and α 2), break time (t b,z), and the corresponding radio flux (f b). the two-parameter dainotti relation between the break time of the plateau and the corresponding break luminosity (l b,z) in the radio band is ${l}_{{\rm{b}},{\rm{z}}}\propto {t}_{{\rm{b}},{\rm{z}}}^{-1.20\pm 0.24}$ . including the isotropic energy eγ,iso and peak energy e p,i, the three-parameter correlations for the radio plateaus are written as ${l}_{{\rm{b}},{\rm{z}}}\propto {t}_{{\rm{b}},{\rm{z}}}^{-1.01\pm 0.24}{e}_{\gamma ,\mathrm{iso}}^{0.18\pm 0.09}$ and ${l}_{{\rm{b}},{\rm{z}}}\propto {t}_{{\rm{b}},{\rm{z}}}^{-1.18\pm 0.27}{e}_{{\rm{p}},{\rm{i}}}^{0.05\pm 0.28}$ , respectively. the correlations are less consistent with those of the x-ray and optical plateaus, implying that radio plateaus may have a different physical mechanism. the typical frequencies crossing the observational band may be a reasonable hypothesis that causes the breaks of the radio afterglows. we calibrate the grb empirical luminosity correlations as a standard candle for constraining cosmological parameters and find that our samples can constrain the flat λcdm model well but are not sensitive to the nonflat λcdm model. by combining grbs with other probes, such as supernovae and the cmb, the constraints on the cosmological parameters are ωm = 0.297 ± 0.006 for the flat λcdm model and ωm = 0.283 ± 0.008, ωλ = 0.711 ± 0.006 for the nonflat λcdm model. | radio plateaus in gamma-ray burst afterglows and their application in cosmology |
this poster is meant to complement the presentation about the ixpe observation of grb 221009a. here, we provide more details on data analysis and interpretation. grb 221009a is an exceptionally bright gamma-ray burst (grb) that reached earth on 2022 october 9 after traveling through the dust of the milky way. the imaging x-ray polarimetry explorer (ixpe) pointed at grb 221009a on october 11 to observe, for the first time, the 2-8 kev x-ray polarization of a grb afterglow. we set an upper limit to the polarization degree of the afterglow emission of 13.8% at a 99% confidence level. this result provides constraints on the jet opening angle and the viewing angle of the grb, or alternatively, other properties of the emission region. additionally, ixpe captured halo-rings of dust-scattered photons which are echoes of the grb prompt emission. the 99% confidence level upper limit of the prompt polarization degree is about 55%, consistent with a scenario involving synchrotron emission in an ordered magnetic field. this single ixpe pointing provides both the first assessment of x-ray polarization of a grb afterglow and the first grb study with polarization observations of both the prompt and afterglow phases. | the ixpe view of grb221009a |
despite 40 years of observations, grb prompt emission is still subject to intense debate. several dissipation models are employed to explain the observed non-thermal spectrum, like internal shocks, sub-photospheric dissipation, magnetic reconnection. all these models are indistinguishable for the available data. the dissipation distance is uncertain, spanning several orders of magnitude (10^12-10^17 cm) depending on the model. moreover, a precise estimation of the bulk lorenz factor is still very challenging. the recent joint multi-wavelength detection of the very energetic (eiso ~ 3e+54 erg) and distant (z = 4.61) grb 220101a allows an accurate discrimination between prompt and afterglow emission. this source has been observed through several instruments, revealing a spectrum that spans from soft x-rays up to high energies (~ 1 gev). this allowed us to measure the spectral cutoff and to put the strictest constrains on the r-gamma parameter space. with the additional constraints from the time variability measure of grb220101a (dt ~ 0.2s) and afterglow modelling, we found that the jet is accelerated up to high lorentz factors (400 < gamma < 700) and dissipated at radii of order 10^14 cm. these results are in contrast with a prompt emission through magnetic reconnection or photosphere, but consistent with the proton synchrotron model. in this work we propose a robust method of the estimation of the radius of the prompt emission with the early high energy (100 mev - gev) observations. this work highlights the importance of precise constraints on time-variability and high energy part of the spectrum in order to better understand the physics behind prompt emission. | constrains on the physics of prompt emission from a distant and energetic gamma-ray burst. |
in 50 years of observations, grb 221009a is unequivocally the brightest gamma-ray burst (grb) every detected. due to its nearby distance, at redshift z=0.151, it is an incredibly rare event with a rate of once in a century. the nearby distance combined with its fantastic energy release provided an unprecedented opportunity to observe its emission across the electromagnetic spectrum. in this talk i will present our multi-wavelength observations and analysis of the early afterglow of grb 221009a. | the early afterglow of the brightest gamma-ray burst of all time |
at 02:48:59 ut, the swift burst alert telescope (bat) triggered and located grb 201024a (trigger=1001514). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 125.948, +3.377 which is ra(j2000) = 08h 23m 47s dec(j2000) = +03d 22' 38" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat light curve showed a single peak structure with a duration of about 5 sec. the peak count rate was ~1200 counts/sec (15-350 kev), at ~0 sec after the trigger. the xrt began observing the field at 02:50:14.0 ut, 74.9 seconds after the bat trigger. using promptly downlinked data we find a bright, uncatalogued x-ray source with an enhanced position: ra, dec 125.9521, 3.3537 which is equivalent to: ra(j2000) = 08h 23m 48.50s dec(j2000) = +03d 21' 13.4" with an uncertainty of 2.1 arcseconds (radius, 90% containment). this location is 85 arcseconds from the bat onboard position, within the bat error circle. this position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. a power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the galactic value of 3.64 x 10^20 cm^-2 (willingale et al. 2013). the initial flux in the 2.5 s image was 6.41e-10 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 495 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 08:23:48.55 = 125.95229 dec(j2000) = +03:21:14.5 = 3.35403 with a 90%-confidence error radius of about 0.62 arc sec. this position is 0.7 arc sec. from the center of the xrt error circle. the estimated magnitude is 17.85 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.03. burst advocate for this burst is f. e. marshall (marshall at milkyway.gsfc.nasa.gov). please contact the ba by email if you require additional information regarding swift followup of this burst. in extremely urgent cases, after trying the burst advocate, you can contact the swift pi by phone (see swift too web site for information: http://www.swift.psu.edu/) | grb 201024a: swift detection of a burst with an optical afterglow |
during its four year long all sky survey, the erosita instrument on spectrum-x-gamma will perform the deepest all sky survey of the x-ray (0.2-8 kev) sky. the survey consists of a total of eight scans of the whole celestial sphere, which last six months each. each survey is designed as a slew survey where the spectrum-x-gamma probe rotates with a period of four hours around an axis that is (roughly) pointing at the sun while the probe moves around the l2-point of the sun-earth system. due to erosita's large field of view of 1.03 degrees, sources are visible during multiple of such rotations for about 40s each. as a result, erosita will also be able to measure the flux evolution of several million sources on a hourly, and semi-annual cadence, as well as discover new transient sources. in this presentation i will present initial results on transient phenomena that were found during the first four erosita all sky surveys, covering source classes from stars, novae, galactic x-ray binaries, to tidal disruption events, active galactic nuclei, and gamma-ray burst afterglows. | erosita observations of transient sources |
in this work we study the afterglow light curve of grb 171205a; this long burst was discovered by the burst alert telescope (bat) on board neil gehrels swift and konus-wind, respectively. the x-ray telescope (xrt) began observing the burst at 144.7 seconds and ended at 6748135.04 seconds after the bat trigger. the whole contour of the xrt light curve of grb 171205a consists of three parts, as follows: steep decay, shallow decay and normal decay. margutti wrote two proposals for two observations of this source using chandra satellites on february 14 and june 29, 2018. combined with the chandra data, a detailed analysis of the normal decay reveals that there are four steps in the substructure. this phenomenon is unusual. it occurs in the late stages and does not undergo intense light changes. so it is not caused by the central engine, but may be caused by the external environment. | a stepped afterglow light curve of grb 171205a |
the swift/uvot began settled observations of the field of grb 220627a 130224 s after the lat trigger (di lalla, et al., gcn circ. 32283). no optical afterglow consistent with the optical position (de wet et al., gcn circ. 32289, izzo et al., gcn circ. 33391) or x-ray afterglow (gropp et al., gcn circ. 32296) is detected in the initial uvot exposures. preliminary 3-sigma upper limits using the uvot photometric system (breeveld et al. 2011, aip conf. proc. 1358, 373) for the initial exposures are: filter t_start(s) t_stop(s) exp(s) mag u 130224 148828 4854 >22.0 the magnitudes in the table are not corrected for the galactic extinction due to the reddening of e(b-v) = 0.047 in the direction of the burst (schlegel et al. 1998). | grb 220627a: swift/uvot upper limits |
observations of the long-lived x-ray plateau in short gamma-ray burst (sgrb) afterglow suggest that a portion of binary neutron star mergers would leave behind a rapidly spinning, strongly magnetized neutron star (millisecond magnetar). the new-born magnetar may undergo large deformation due to magnetic distortion or unstable oscillation, which would emit the extended gravitational wave (gw) associated with the sgrb x-ray plateau. in this work, we focus on the spin-down luminosity evolution of magnetar by considering the spin energy loss due to the gw and magnetic dipole radiation, and systematically analyse the sgrb light curves of our magnetar sample. the results show that gw emission signatures have existed in the spin-down stage of grb 090426 and grb 150424a. we also present constraints on the ellipticity of the new-born magnetar as ɛ < 1.58 × 10-3(b/1015 g)(p/1 ms). the magnetar can lose significant spin energy via gw radiation if the ellipticity ɛ ≥ 10-3 and magnetic field strength b ~ 1015 g. in addition, we derive the evolution of gw strain for magnetars through their spin-down processes. this result shows that the gw signals from these magnetars may be detectable for the einstein telescope (et). for a rapidly spinning magnetar (p ~ 1 ms), the detection horizons for aligo o3, aligo o5, and et detectors are ~60, ~210, and ~900 mpc, respectively. the detection of the gw emission from new-born millisecond magnetar may reveal the interior composition of magnetar in the near future. | identifying gravitational wave emission signature in electromagnetic observations of short gamma-ray bursts |
we present the broadband numerical modeling of afterglows for two remarkably bright long gamma-ray bursts (grbs), grb 050820a and grb 070125, with a wide range of observations from the radio band to the x-ray band. in our work, we fit light curves and constrain physical parameters using a standard forward shock model from the afterglowpy python package, considering different jet structures and the jet lateral expansion. for grb 050820a, the constrained jet is close to a top-hat jet with an extremely small half opening angle of about 0.015 rad, and the circumburst matter density is as small as 10-7 cm-3, which suggests that this peculiar long grb might originate from metal-poor stars with low mass-loss rates. to explain the late time optical light curves of grb 070125, the effects of the lateral expansion and the participation factor of electrons that are accelerated by the shock have to be taken into account. the constrained results for grb 070125 show that the jet is also close to a top-hat jet with a half opening angle of about 0.1 rad, the viewing angle is about 0.05 rad, the circumburst density is about 10 cm-3, and the participation factor is about 0.1. the jet energy of the two bursts is required to be ~1051-1052 erg, which can be produced by a millisecond magnetar or a hyper-accreting black hole. | on the jet structures of grb 050820a and grb 070125 |
in a previous article, we argued that angular non-stationarities of gamma-ray burst (grb) jets can result in a statistical connection between the angle values deduced from jet break times and the variabilities of prompt light curves. the connection should be an anticorrelation if luminosity densities of jets follow a power-law or a uniform profile, and a correlation if they have a gaussian profile. in this follow-up paper, we search for the connection by measuring spearman's rank correlation coefficient in a sample of 19 long grbs observed by the swift satellite. using 16 of the grbs with well-defined angle measurements, we find $\rho = -0.38_{-0.1}^{+0.1}$ and $p = 0.15_{-0.09}^{+0.14}$. adding three more grbs to the sample, each with a pair of equally possible angle values, can strengthen the anticorrelation to $\rho =-0.46_{-0.08}^{+0.09}$ and $p=0.05_{-0.03}^{+0.07}$. we show that these results are incompatible with non-stationary jets having gaussian profiles, and that ≳100 grbs with observed afterglows would be needed to confirm the potential existence of the angle-variability anticorrelation with 3σ significance. if the connection is real, grb jet angles would be constrainable from prompt gamma light curves, without the need of afterglow observations. | statistical search for angular non-stationarities of long gamma-ray burst jets using swift data |
the temporal behavior of the very dim optical afterglow of grb 080503 is at odds with the regular forward shock afterglow model and a sole kilonova component responsible for optical emission has been speculated in some literature. here we analyze the optical afterglow data available in archive and construct time-resolved spectra. the significant detection by keck i in g/r bands at t ~ 3 days, which has not been reported before, as well as the simultaneous gemini-north r-band measurement, are in favor of a power-law spectrum that is well consistent with the optical to x-ray spectrum measured at t ~ 4.5 days. however, for t ≤ 2 days, the spectrum is thermal-like and a straightforward interpretation is a kilonova emission from a neutron star merger, making it, possibly, the first detection of a very early kilonova signal at t ~ 0.05 day. a nonthermal nature of optical emission at late times (t ~ 2 days), anyhow, cannot be ruled out because of the large uncertainty of the g-band data. we also propose to classify the neutron star merger induced optical transients, according to the temporal behaviors of the kilonova and the nonthermal afterglow emission, into four types. grb 080503 would then represent the first observation of a subgroup of neutron star merger driven optical transients (i.e., type iv) consisting of an early blue kilonova and an adjacent nonthermal afterglow radiation. | grb 080503: a very early blue kilonova and an adjacent nonthermal radiation component |
a large fraction of the baryons at low redshift are undetected and likely reside in the tenuous, hot intergalactic medium (igm). one way to probe the missing baryons is through their absorption of bright sources. the anomalous absorption excess in the x-ray afterglows of gamma-ray bursts (grbs) has been suggested to result from the missing baryons. in order to test this hypothesis, the present paper employs illustristng simulations to compute the x-ray absorption effect on cosmological distances. the simulation shows that ionization of h and he in the igm leaves the metals responsible for >60% of the x-ray opacity of high-z sources. the high-z asymptotic optical depth at 0.5 kev in the simulation reaches 0.15 ± 0.07, while the grb afterglow values tend to ≈0.4, implying that the missing baryons can account for a significant fraction of the observed opacity. the remaining discrepancy is ascribed mainly to the low average metallicity in the simulation, which drops from 0.06 solar at z = 0 to 0.01 at z = 3, and which is below previously measured values. | absorption of gamma-ray burst x-ray afterglows by the missing baryons: confronting observations with cosmological simulations |
we consider the question of entanglement conservation in the context of the er=epr correspondence equating quantum entanglement with wormholes. in quantum mechanics, the entanglement between a system and its complement is conserved under unitary operations that act independently on each; er=epr suggests that an analogous statement should hold for wormholes. we accordingly prove a new area theorem in general relativity: for a collection of dynamical wormholes and black holes in a spacetime satisfying the null curvature condition, the maximin area for a subset of the horizons (giving the largest area attained by the minimal cross section of the multi-wormhole throat separating the subset from its complement) is invariant under classical time evolution along the outermost apparent horizons. the evolution can be completely general, including horizon mergers and the addition of classical matter satisfying the null energy condition. this theorem is the gravitational dual of entanglement conservation and thus constitutes an explicit characterization of the er=epr duality in the classical limit. | entanglement conservation, er=epr, and a new classical area theorem for wormholes |
we connect topological changes that can occur in $3$-space via surgery, with black hole formation, the formation of wormholes and new generalizations of these phenomena, including relationships between quantum entanglement and wormhole formation. by considering the initial manifold as the $3$-dimensional spatial section of spacetime, we describe the changes of topology occurring in these processes by determining the resulting $3$-manifold and its fundamental group. as these global changes are induced by local processes, we use the local form of morse functions to provide an algebraic formulation of their temporal evolution and propose a potential energy function which, in some cases, could give rise to the local forces related to surgery. we further show how this topological perspective gives new insight for natural phenomena exhibiting surgery, in all dimensions, while emphasizing the $3$-dimensional case, which describes cosmic phenomena. this work makes new bridges between topology and natural sciences and creates a platform for exploring geometrical physics. | topological surgery in cosmic phenomena |
many of the most exciting open problems in high-energy physics are related to the behavior and ultimate nature of gravity and spacetime. in this dissertation, several categories of new results in quantum and classical gravity are presented, with applications to our understanding of both quantum field theory and cosmology.a fundamental open question in quantum field theory is related to ultraviolet completion: which low-energy effective field theories can be consistently combined with quantum gravity? a celebrated example of the swampland program—the investigation of this question—is the weak gravity conjecture, which mandates, for a u(1) gauge field coupled consistently to gravity, the existence of a state with charge-to-mass ratio greater than unity. in this thesis, we demonstrate the tension between the weak gravity conjecture and the naturalness principle in quantum field theory, generalize the weak gravity conjecture to multiple gauge fields, and exhibit a model in which the weak gravity conjecture solves the standard model hierarchy problem. next, we demonstrate that gravitational effective field theories can be constrained by infrared physics principles alone, namely, analyticity, unitarity, and causality. in particular, we derive bounds related to the weak gravity conjecture by placing such infrared constraints on higher-dimension operators in a photon-graviton effective theory. we furthermore place bounds on higher-curvature corrections to the einstein equations, first using analyticity of graviton scattering amplitudes and later using unitarity of an arbitrary tree-level completion, as well as constrain the couplings in models of massive gravity. completing our treatment of perturba- tive quantum gravity, outside of the swampland program, we also reformulate graviton perturbation theory itself, finding a field redefinition and gauge-fixing of the einstein-hilbert action that drastically simplifies the feynman diagram expansion. furthermore, our reformulation also exhibits a hidden symmetry of general relativity that corresponds to the double copy relations equating gravity amplitudes to sums of squares of gluon amplitudes in yang-mills theory, a surprising correspondence that yields insights into the structure of quantum field theories.moving beyond perturbation theory into nonperturbative questions in quantum gravity, we consider the deep relation between spacetime geometry and properties of the quantum state. in the context of holography and the anti-de sitter/conformal field theory correspondence, we test the proposed er=epr correspondence equating quantum entanglement with wormholes in spacetime. in particular, we demonstrate that the no-cloning theorem in quantum mechanics and the no-go theorem for topology change of spacetime are dual under the er=epr correspondence. furthermore, we prove that the presence of a wormhole is not an observable in quantum gravity, rescuing er=epr from potential violation of linearity of quantum mechanics. excitingly, we also prove a new area theorem within classical general relativity for arbitrary dynamics of two collections of wormholes and black holes; this area theorem is the er=epr analogue of entanglement conservation.we next turn our attention to the emergence of spacetime itself, placing consistency conditions on the proposed correspondence between anti-de sitter space and the multiscale entanglement renormalization ansatz, a special tensor network that constitutes a computational tool for finding the ground state of certain quantum systems. | defining gravity: effective field theory, entanglement, and cosmology |
the field test of distributed acoustic sensing (das) conducted at garner valley, california on september 11-12, 2013 provided a continuous overnight record of ambient noise. the das array recorded ground motions every one meter of optical cable that was arranged approximately in the shape of a rectangle with dimensions of 160 m by 80 m. the long dimension of the array was adjacent to a state highway. three hours of record were used to compute noise cross-correlation functions (ncfs) in one-minute windows. the trace from each sensor channel was pre-processed by downsampling to 200 hz, followed by normalization in the time-domain and bandpass filtering between 2 and 20 hz (bensen et al., 2007). the one-minute ncfs were then stacked using the time-frequency domain phase-weighted stacking method (schimmel & gallart, 2007). the ncfs between channels were asymmetrical reflecting the direction of traffic noise. the group velocities were found using the frequency-time analysis method. the energy was concentrated between 5 and 15 hz, which falls into the typical traffic noise frequency band. the resulting velocities were between 100 and 300 m/s for frequencies between 10 and 20 hz, which are in the same range as described in the results for surface-wave dispersion obtained using an active source for the same site (lancelle et al., 2015). the group velocity starts to decrease for frequencies greater than ~10 hz, which was expected on the basis of a previous shear-wave velocity model (steidl et al., 1996). then, the phase velocity was calculated using the multichannel analysis of surface wave technique (masw - park et al., 1999) with 114 ncfs spaced one meter apart. the resulting dispersion curve between 5 and 15 hz gave phase velocities that ranged from approximately 170 m/s at 15 hz to 250 m/s at 5 hz. these results are consistent with other results of active-source das and seismometer records obtained at the garner valley site (e.g., stokoe et al. 2004). this analysis is part of the porotomo project (poroelastic tomography by adjoint inverse modeling of data from seismology, geodesy, and hydrology, http://geoscience.wisc.edu/feigl/porotomo). | properties of noise cross correlation functions obtained from a distributed acoustic sensing (das) array at garner valley, california |
our paper introduces a new technique for diagnosis of various heart diseases without the need of highly experts to investigate the electrocardiogram (ecg). using the same electrodes of the ecg machine, it will be able to transmit directly the electrical activity inside the heart to a moving picture. our technique is based on artificial intelligence algorithm using artificial neural networks (ann). finding the trans-membrane potential (tmp) inside the heart from the body surface potential (bsp) is known as the inverse problem of ecg. to have a unique solution for the inverse problem the data used should be obtained from a forward model. a three dimensional (3-d) model of cellular activation whole heart embedded in torso is simulated and solved using comsol multiphysics software. in our previous paper, one ann succeeded in displaying the wave propagation on the surface of a normal heart. in this paper, we used a configuration of anns to display different cases of heart with myocardial infarction (mi). to check the system accuracy, eight mi cases with different sizes and locations in the heart are simulated in the forward model. this configuration proved to be highly accurate in displaying each mi case -size and location- presenting the infarction as an area with no electrical activity. | reconstruction of depolarization pattern in myocardial infarction cases using two cascaded stages of artificial neural networks |
69227 deaths, 374643 injured, 17923 people missing, direct economic losses 845.1 billion, and a large number houses collapse were caused by wenchuan ms8 earthquake in sichuan province on may 12, 2008, how to reproduce characteristics of its strong ground motion and predict its intensity distribution, which have important role to mitigate disaster of similar giant earthquake in the future. taking yunnan-sichuan province, wenchuan town, chengdu city, chengdu basin and its vicinity as the research area, on the basis of the available three-dimensional velocity structure model and newly topography data results from chinaarray of institute of geophysics, china earthquake administration, 2 type complex source rupture process models with the global and local source parameters are established, we simulated the seismic wave propagation of wenchuan ms8 earthquake throughout the whole three-dimensional region by the gms discrete grid finite-difference techniques with cerjan absorbing boundary conditions, and obtained the seismic intensity distribution in this region through analyzing 50×50 stations data (simulated ground motion output station). the simulated results indicated that: (1)simulated wenchuan earthquake ground motion (pga) response and the main characteristics of the response spectrum are very similar to those of the real wenchuan earthquake records. (2)wenchuan earthquake ground motion (pga) and the response spectra of the plain are much greater than that of the left mountain area because of the low velocity of the shallow surface media and the basin effect of the chengdu basin structure. simultaneously, (3) the source rupture process (inversion) with far-field p-wave, gps data and insar information and the longmenshan front fault (source rupture process) are taken into consideration in gms numerical simulation, significantly different waveform and frequency component of the ground motion are obtained, though the strong motion waveform is distinct asymmetric, which should be much more real. it indicated that the longmenshan front fault may be also involved in seismic activity during the long time(several minutes) wenchuan earthquake process. (4) simulated earthquake records in hanyuan region are indeed very strong, which reveals source mechanism is one reason of hanyuan intensity abnormaly. | numerical simulation analysis on wenchuan seismic strong motion in hanyuan region |
on 12 may, 2008, the sichuan province in china suffered the catastrophic wenchuan earthquake (ms 8). prior to the event, a large number of small to moderate earthquakes occurred in the area were recorded at stations of sichuan seismic network (scsn). the wave data were collected during the years 2006-2008, the fourier amplitude spectra of lg wave are used to determine attenuation and site responses. we analyze over 3300 seismograms for lg-wave propagation from 291 local and regional earthquakes recorded at distances from 100 to 700 km, the earthquakes varied in ml2.0 and 5.7.a joint inversion method estimating attenuation and site responses from seismic spectral ratios is implemented in the study; modeling errors are determined using a delete-j jackknife resampling technique.variations of the lg attenuation in a chronological order are studied. the event occurred on the longmen shan fault (lsf), the lsf constitutes boundary betweeb bayan har block and eastern. the data are divided into two subgroups based on the seismic ray paths which contained entirely within the sichuan basin or the bayan har block. the waveforms were processed in a frequency range of 1-7 hz with an interval of 0.2 hz. on the vertical component, lg attenuation in the bayan har block are fit by a frequency-dependent function q(f)=250.2±13.7f0.52±0.03,the sichuan basin is characterized by function q(f)=193±23f0.0.81±0.05. the obtained attenuation curves indicate that the spectral amplitudes decay faster in the sichuan basin than in the bayan har block. site responses from the 48 stations are estimated, the site responses vary among these stations by more than a factor of 10 within the frequency range of interest.the results from the regrouping of data in chronological order show that when the whenchuan earthquake is approaching, the changes in attenuation occur significantly, but the changes in site responses do not occur. | lg attenuation and site response in the sichuan basin and the bayan har block before the 2008 ms8.0 wenchuan earthquake |
full-waveform inversion (fwi) is an effective method for imaging subsurface properties using sparsely recorded data. it involves solving a wave propagation problem to estimate model parameters that accurately reproduce the data. recent trends in fwi have led to the development of extended methodologies, among which source extension methods leveraging reconstructed wavefields to solve penalty or augmented lagrangian (al) formulations have emerged as robust algorithms, even for inaccurate initial models. despite their demonstrated robustness, challenges remain, such as the lack of a clear physical interpretation, difficulty in comparison, and reliance on difficult-to-compute least squares (ls) wavefields. this paper is divided into two critical parts. in the first, a novel formulation of these methods is explored within a unified lagrangian framework. this novel perspective permits the introduction of alternative algorithms that employ ls multipliers instead of wavefields. these multiplier-oriented variants appear as regularizations of the standard fwi, are adaptable to the time domain, offer tangible physical interpretations, and foster enhanced convergence efficiency. the second part of the paper delves into understanding the underlying mechanisms of these techniques. this is achieved by solving the fwi equations using iterative linearization and inverse scattering methods. the paper provides insight into the role and significance of lagrange multipliers in enhancing the linearization of fwi equations. it explains how different methods estimate multipliers or make approximations to increase computing efficiency. additionally, it presents a new physical understanding of the lagrange multiplier used in the al method, highlighting how important it is for improving algorithm performance when compared to penalty methods. | full waveform inversion and lagrange multipliers |
earth introduces strong attenuation and dispersion to propagating waves. the time-fractional wave equation with very small fractional exponent, based on kjartansson's constant-q theory, is widely recognized in the field of geophysics as a reliable model for frequency-independent q anelastic behavior. nonetheless, the numerical resolution of this equation poses considerable challenges due to the requirement of storing a complete time history of wavefields. to address this computational challenge, we present a novel approach: a nearly optimal sum-of-exponentials (soe) approximation to the caputo fractional derivative with very small fractional exponent, utilizing the machinery of generalized gaussian quadrature. this method minimizes the number of memory variables needed to approximate the power attenuation law within a specified error tolerance. we establish a mathematical equivalence between this soe approximation and the continuous fractional stress-strain relationship, relating it to the generalized maxwell body model. furthermore, we prove an improved soe approximation error bound to thoroughly assess the ability of rheological models to replicate the power attenuation law. numerical simulations on constant-q viscoacoustic equation in 3d homogeneous media and variable-order p- and s- viscoelastic wave equations in 3d inhomogeneous media are performed. these simulations demonstrate that our proposed technique accurately captures changes in amplitude and phase resulting from material anelasticity. this advancement provides a significant step towards the practical usage of the time-fractional wave equation in seismic inversion. | compressing the memory variables in constant-q viscoelastic wave propagation via an improved sum-of-exponentials approximation |
solar metallicity — the fraction per unit mass that is composed of elements heavier than he — is a critical and fundamental quantity indicative of the history and future evolution of the sun. over the last decade spectroscopic observations of the solar photosphere using inversion techniques of increasing sophistication have led to a downward revision of the abundances of heavy elements, specifically c, n, and o, and thus of the solar metallicity. this in turn has led to a crisis of solar models, which became inconsistent with the results of helioseismology as a consequence of the missing opacity from these elements. we present recently released solar wind compositional data to determine the metallicity of the sun. we focus on a present-day solar sample available to us, which is the least fractionated solar wind from coronal holes near the poles of the sun. using these data, we derive a metallicity of z = 0.0196 ± 0.0014, which is signicantly larger than recent published values based on photospheric spectroscopy, but consistent with results from helioseismology. | solar metallicity derived from in-situ solar wind composition |
non-hermitian systems offer new platforms for unusual physical properties that can be flexibly manipulated by redistribution of the real and imaginary parts of refractive indices, whose presence breaks conventional wave propagation symmetries, leading to asymmetric reflection and symmetric transmission with respect to the wave propagation direction. here, we use supervised and unsupervised learning techniques for knowledge acquisition in non-hermitian systems which accelerate the inverse design process. in particular, we construct a deep learning model that relates the transmission and asymmetric reflection in non-conservative settings and propose sub-manifold learning to recognize non-hermitian features from transmission spectra. the developed deep learning framework determines the feasibility of a desired spectral response for a given structure and uncovers the role of effective gain-loss parameters to tailor the spectral response. these findings offer a route for intelligent inverse design and contribute to the understanding of physical mechanism in general non-hermitian systems. | machine learning for knowledge acquisition and accelerated inverse-design for non-hermitian systems |
in this paper, we are concerned with the 2d and 3d geometric shape generation by prescribing a set of characteristic values of a specific geometric body. one of the major motivations of our study is the 3d human body generation in various applications. we develop a novel method that can generate the desired body with customized characteristic values. the proposed method follows a machine-learning flavour that generates the inferred geometric body with the input characteristic parameters from a training dataset. one of the critical ingredients and novelties of our method is the borrowing of inverse scattering techniques in the theory of wave propagation to the body generation. this is done by establishing a delicate one-to-one correspondence between a geometric body and the far-field pattern of a source scattering problem governed by the helmholtz system. it in turn enables us to establish a one-to-one correspondence between the geometric body space and the function space defined by the far-field patterns. hence, the far-field patterns can act as the shape generators. the shape generation with prescribed characteristic parameters is achieved by first manipulating the shape generators and then reconstructing the corresponding geometric body from the obtained shape generator by a stable multiple-frequency fourier method. our method is easy to implement and produces more efficient and stable body generations. we provide both theoretical analysis and extensive numerical experiments for the proposed method. the study is the first attempt to introduce inverse scattering approaches in combination with machine learning to the geometric body generation and it opens up many opportunities for further developments. | an inverse scattering approach for geometric body generation: a machine learning perspective |
observational insights into dynamo processes are gleaned from the cyclic variations of solar differential rotation known as torsional oscillations. these oscillations are closely intertwined with the evolution of the magnetic field on the solar surface throughout the solar cycle, and they can be observed using helioseismology techniques across the entire convection zone. our analysis focuses on helioseismic data obtained from both ground-based such as gong and space-based instruments such as soho/mdi and sdo/hmi, covering a span of over two decades, from the beginning of solar cycle 23 to the onset of solar cycle 25. we have examined how solar rotation has changed over this period and to characterize the evolution of different spatial components of solar differential rotation throughout the solar cycle. understanding the characteristics of the tachocline, which is believed to be the source of solar activity, such as its width and position, is of paramount importance in comprehending solar dynamo processes. in order to achieve this understanding, we employ both inversion and forward modeling techniques. since the spatial resolution at the base of the convection zone is typically insufficient, resolving thin structures like the tachocline poses a significant challenge. thus, the combined utilization of inversion and forward modeling is essential for accurately identifying and characterizing this region. additionally, studying the evolution of torsional oscillations throughout the solar cycle is crucial. notably, a correlation exists between torsional oscillations and the solar magnetic cycle. furthermore, it is imperative to account for outliers in the data that may lead to spurious conclusions. therefore, we developed a technique that effectively eliminates outliers and takes their potential impact on our conclusions into consideration. | unveiling the evolution of solar differential rotation across two solar cycles |
microscopic traffic models describe how cars interact with their neighbors in an uninterrupted traffic flow and are frequently used for reference in advanced vehicle control design. in this paper, we propose a novel mechanical system inspired microscopic traffic model using a mass-spring-damper-clutch system. this model naturally captures the ego vehicle's resistance to large relative speed and deviation from a (driver and speed dependent) desired relative distance when following the lead vehicle. comparing to existing car following (cf) models, this model offers physically interpretable insights on the underlying cf dynamics, and is able to characterize the impact of the ego vehicle on the lead vehicle, which is neglected in existing cf models. thanks to the nonlinear wave propagation analysis techniques for mechanical systems, the proposed model therefore has great scalability so that multiple mass-spring-damper-clutch system can be chained to study the macroscopic traffic flow. we investigate the stability of the proposed model on the system parameters and the time delay using spectral element method. we also develop a parallel recursive least square with inverse qr decomposition (prls-iqr) algorithm to identify the model parameters online. these real-time estimated parameters can be used to predict the driving trajectory that can be incorporated in advanced vehicle longitudinal control systems for improved safety and fuel efficiency. the prls-iqr is computationally efficient and numerically stable so it is suitable for online implementation. the traffic model and the parameter identification algorithm are validated on both simulations and naturalistic driving data from multiple drivers. promising performance is demonstrated. | a new microscopic traffic model using a spring-mass-damper-clutch system |
quantitative photoacoustic tomography (qpat) is an imaging technique aimed at estimating chromophore concentrations inside tissues from photoacoustic images, which are formed by combining optical information and ultrasonic propagation. the application of qpat as a transcranial imaging modality is complicated by shear waves that can be produced when ultrasound waves travel from soft tissue to bone. because of this, the estimation of chromophores distributions near the skull can be problematic. in this paper, we take steps towards compensating for aberrations of the recorded photoacoustic signals caused by elastic wave propagation. with photoacoustic data simulated in a coupled elastic-acoustic domain, we conduct inversions in a purely acoustic domain. estimation of the posterior density of the initial pressure is achieved by inversion under the bayesian framework. we utilize the bayesian approximation error approach to compensate for the modelling errors arising from approximating a coupled elastic-acoustic domain with a purely fluid domain. the resulting reconstructions and corresponding uncertainty estimates are then used to evaluate the posterior density of the optical absorption parameter. in the sense of the posterior uncertainty, the results show that the bayesian approximation error approach yields a more feasible estimate for the posterior model of the initial pressure which, in turn, yields a more feasible estimate for the posterior model of the absorption coefficient. | coupled elastic-acoustic modelling for quantitative photoacoustic tomography |
the meridional circulation and other large scale flows are important features of the sun's internal structure and are closely connected to the transport of momentum and magnetic flux throughout the solar interior, and in effect to the solar magnetic cycle. therefore, reliable measurements of such flows is crucially important for improving our understanding of the dynamic nature of the sun. helioseismology can be used to detect such flows. in particular, by measuring the travel times of helioseismic waves observable on the solar surface, internal flows can be inferred using so-called sensitivity kernels. simple ray-approximation kernels and more realistic born-approximation kernels have been developed and used for this purpose. still, in particular in the deeper interior, different techniques don't fully agree and considerable uncertainties remain. we have recently developed realistic 3-d sensitivity kernels for horizontal flows as an alternative to the existing kernels using numerical simulations of helioseismic wave propagation through small test perturbations. by placing small, localized flow perturbations in the interior of our global-sun simulations and measuring their effect on the travel times of waves, we are able to solve for the sensitivity kernels. in this paper, we present the first results of inversions performed with these new kernels using numerically simulated wave fields in the presence of a prescribed meridional flow. north-south travel-time differences are measured from the simulation data and azimuthally averaged, analogous to actual solar observations, and are used with our new kernels to infer the meridional flow. these results can be compared directly to the flow prescribed in the simulation. | meridional-flow inversions using realistic 3-d sensitivity kernels |
observations of the sun in the visible spectral range belong to standard measurements obtained by instruments both on the ground and in the space. nowadays, both nearly continuous full-disc observations with medium resolution and dedicated campaigns of high spatial, spectral and/or temporal resolution constitute a holy grail for studies that can capture (both) the long- and short-term changes in the dynamics and energetics of the solar atmosphere. observations of photospheric spectral lines allow us to estimate not only the intensity at small regions, but also various derived data products, such as the doppler velocity and/or the components of the magnetic field vector. we show that these measurements contain not only direct information about the dynamics of solar plasmas at the surface of the sun but also imprints of regions below and above it. here, we discuss two examples: first, the local time-distance helioseismology as a tool for plasma dynamic diagnostics in the near subsurface and second, the determination of the solar atmosphere structure during flares. the methodology in both cases involves the technique of inverse modelling. | exploiting solar visible-range observations by inversion techniques: from flows in the solar subsurface to a flaring atmosphere |
normal-mode coupling is a helioseismic technique that uses measurements of mode eigenfunctions to infer the interior structure of the sun. this technique has led to insights into the evolution and structure of toroidal flows in the solar interior. here, we validate an inversion algorithm for normal-mode coupling by generating synthetic seismic measurements associated with input flows and comparing the input and inverted velocities. we study four different cases of input toroidal flows and compute synthetics that take into account the partial visibility of the sun. we invert the synthetics using subtractive optimally localized averages (sola) and also try to mitigate the systematics of mode leakage. we demonstrate that, ultimately, inversions are only as good as the model we assume for the correlation between flow velocities. | validating inversions for toroidal flows using normal-mode coupling |
tomography was a non-destructive method for investigating the internal structure of an object, usually used to find internal anomalies caused by differences in the physical parameters. the fresnel volume tomography method was an alternative method for reconstructing the image of an object using travel times, whereas this method did not use a ray path in its calculations. in calculating the fresnel zone, the finite difference method which was the solution to the equation of wave propagation was used. the frequency of the waves was also considered, here we used ultrasonic waves with a frequency of 1 mhz for the inverse modelling process. the reconstruction algorithm we used was modified simultaneous iterative reconstruction technique for fresnel volume. the application of the inversion of synthetic data resulted in an estimation of a velocity model that has good imaging quality and similarity with the synthetic model. | ultrasonic tomography of core plug using fresnel volume approach |
diffraction tomography is a noninvasive technique that estimates the refractive indices of unknown objects and involves an inverse-scattering problem governed by the wave equation. recent works have shown the benefit of nonlinear models of wave propagation that account for multiple scattering and reflections. in particular, the lippmann-schwinger~(lis) model defines an inverse problem to simulate the wave propagation. although accurate, this model is hard to solve when the samples are highly contrasted or have a large physical size. in this work, we introduce instead a helmholtz-based nonlinear model for inverse scattering. to solve the corresponding inverse problem, we propose a robust and efficient multigrid-based solver. moreover, we show that our method is a suitable alternative to the lis model, especially for strongly scattering objects. numerical experiments on simulated and real data demonstrate the effectiveness of the helmholtz model, as well as the efficiency of the proposed multigrid method. | diffraction tomography with helmholtz equation: efficient and robust multigrid-based solver |
digital holography is a 3d imaging technique by emitting a laser beam with a plane wavefront to an object and measuring the intensity of the diffracted waveform, called holograms. the object's 3d shape can be obtained by numerical analysis of the captured holograms and recovering the incurred phase. recently, deep learning (dl) methods have been used for more accurate holographic processing. however, most supervised methods require large datasets to train the model, which is rarely available in most dh applications due to the scarcity of samples or privacy concerns. a few one-shot dl-based recovery methods exist with no reliance on large datasets of paired images. still, most of these methods often neglect the underlying physics law that governs wave propagation. these methods offer a black-box operation, which is not explainable, generalizable, and transferrable to other samples and applications. in this work, we propose a new dl architecture based on generative adversarial networks that uses a discriminative network for realizing a semantic measure for reconstruction quality while using a generative network as a function approximator to model the inverse of hologram formation. we impose smoothness on the background part of the recovered image using a progressive masking module powered by simulated annealing to enhance the reconstruction quality. the proposed method is one of its kind that exhibits high transferability to similar samples, which facilitates its fast deployment in time-sensitive applications without the need for retraining the network. the results show a considerable improvement to competitor methods in reconstruction quality (about 5 db psnr gain) and robustness to noise (about 50% reduction in psnr vs noise increase rate). | dh-gan: a physics-driven untrained generative adversarial network for 3d microscopic imaging using digital holography |
wind speed retrieval at sea surface is of primary importance for scientific and operational applications. besides weather models, in-situ measurements and remote sensing technologies, especially satellite sensors, provide complementary means to monitor wind speed. as sea surface winds produce sounds that propagate underwater, underwater acoustics recordings can also deliver fine-grained wind-related information. whereas model-driven schemes, especially data assimilation approaches, are the state-of-the-art schemes to address inverse problems in geoscience, machine learning techniques become more and more appealing to fully exploit the potential of observation datasets. here, we introduce a deep learning approach for the retrieval of wind speed time series from underwater acoustics possibly complemented by other data sources such as weather model reanalyses. our approach bridges data assimilation and learning-based frameworks to benefit both from prior physical knowledge and computational efficiency. numerical experiments on real data demonstrate that we outperform the state-of-the-art data-driven methods with a relative gain up to 16% in terms of rmse. interestingly, these results support the relevance of the time dynamics of underwater acoustic data to better inform the time evolution of wind speed. they also show that multimodal data, here underwater acoustics data combined with ecmwf reanalysis data, may further improve the reconstruction performance, including the robustness with respect to missing underwater acoustics data. | learning-based temporal estimation of in-situ wind speed from underwater passive acoustics |
this paper identifies certain interesting mathematical problems of stochastic quantization type in the modeling of laser propagation through turbulent media. in some of the typical physical contexts the problem reduces to stochastic schrodinger equation with space-time white noise of gaussian, poisson and levy type. we identify their mathematical resolution via stochastic quantization. nonlinear phenomena such as kerr effect can be modeled by stochastic nonlinear schrodinger equation in the focusing case with space-time white noise. a treatment of stochastic transport equation, the korteweg-de vries equation as well as a number of other nonlinear wave equations with space-time white noise is also given. main technique is the s-transform (we will actually use closely related hermite transform) which converts the stochastic partial differential equation with space time white noise to a deterministic partial differential equation defined on the hida-kondratiev white noise distribution space. we then utlize the inverse s-transform/hermite transform known as the characterization theorem combined with the infinite dimensional implicit function theorem for analytic maps to establish local existence and uniqueness theorems for pathwise solutions of these class of problems. the particular focus of this paper on singular white noise distributions is motivated by practical situations where the refractive index fluctuations in propagation medium in space and time are intense due to turbulence, ionospheric plasma turbulence, marine-layer fluctuations, etc. since a large class of partial differential equations that arise in nonlinear wave propagation have polynomial type nonlinearities, white noise distribution theory is an effective tool in studying these problems subject to different types of white noises. | stochastic quantization of laser propagation models |
full waveform inversion is a high-resolution subsurface imaging technique, in which full seismic waveforms are used to infer subsurface physical properties. we present a novel, target-enclosing, full-waveform inversion framework based on an interferometric objective function. this objective function exploits the equivalence between the convolution and correlation representation formulas, using data from a closed boundary around the target area of interest. because such equivalence is violated when the knowledge of the enclosed medium is incorrect, we propose to minimize the mismatch between the wavefields independently reconstructed by the two representation formulas. the proposed method requires only kinematic knowledge of the subsurface model, specifically the overburden for redatuming and does not require prior knowledge of the model below the target area. in this sense it is truly local: sensitive only to the medium parameters within the chosen target, with no assumptions about the medium or scattering regime outside the target. we present the theoretical framework and derive the gradient of the new objective function via the adjoint-state method and apply it to a synthetic example with exactly redatumed wavefields. a comparison with fwi of surface data and target-oriented fwi based on the convolution representation theorem only shows the superiority of our method both in terms of the quality of target recovery and reduction in computational cost. | target-enclosing inversion using an interferometric objective function |
in this paper, we refine a previously developed acoustic source filter, improving its reliability and extending its capabilities. we demonstrate how to fine-tune the filter to meet observational constraints and focus on specific wave-front speeds. this refinement enables discrimination of acoustic source depths and tracking of local source wave fronts, thereby facilitating ultralocal helioseismology. by utilizing the photospheric doppler signal from a subsurface source in a muram simulation, we demonstrate that robust ultralocal three-dimensional helioseismic inversions for the granular flows and the local sound speed to depths of at least 80 km below the photosphere are possible. the capabilities of the national science foundation's new daniel k. inouye solar telescope will enable such measurements of the real sun. | identifying acoustic wave sources on the sun. ii. improved filter techniques for source wavefield seismology |
we present anseicca, an open-source package for forward and inverse modelling of seis- mic ambient noise cross-correlations at local scales, where the effects of earth's sphericity are negligible. the package implements a nonlinear finite-frequency inversion technique wherein measurements of cross-correlation energy are used to invert for the spatial distribution of am- bient noise sources, under the assumption of a fixed earth structure model. it is seamlessly integrated with other open-source python packages for seismic wave propagation modelling, in- cluding a c-based numerical solver for acoustic modelling in 2-d media. it is a unique package insofar as the inversion is based on finite-frequency sensitivity kernels, but executed without the adjoint method. instead, speed and computational efficiency are achieved by parallelising the code. moreover, the hessian-based optimization ensures convergence in a relatively small num- ber of iterations (≈ 10), compared to purely gradient-based methods. we introduce the structure of the package in detail, describing both the serial and parallel versions of the code. perfor- mance benchmarks show that anseicca affords compute times of the order of a few minutes per iteration of inversion, with typical local-scale seismic array geometries. | anseicca: a python package for seismic ambient noise source inversion by cross-correlation modelling |
data-centric inverse problems are a process of inferring physical attributes from indirect measurements. full-waveform inversion (fwi) is a non-linear inverse problem that attempts to obtain a quantitative physical model by comparing the wave equation solution with observed data, optimizing an objective function. however, the fwi is strenuously dependent on a robust objective function, especially for dealing with cycle-skipping issues and non-gaussian noises in the dataset. in this work, we present an objective function based on the kaniadakis κ-gaussian distribution and the optimal transport (ot) theory to mitigate non-gaussian noise effects and phase ambiguity concerns that cause cycle skipping. we construct the κ-objective function using the probabilistic maximum likelihood procedure and include it within a well-posed version of the original ot formulation, known as the kantorovich–rubinstein metric. we represent the data in the graph space to satisfy the probability axioms required by the kantorovich–rubinstein framework. we call our proposal the κ-graph-space optimal transport fwi (κ-gsot-fwi). the results suggest that the κ-gsot-fwi is an effective procedure to circumvent the effects of non-gaussian noise and cycle-skipping problems. they also show that the kaniadakis κ-statistics significantly improve the fwi objective function convergence, resulting in higher-resolution models than classical techniques, especially when κ=0.6. | a graph-space optimal transport approach based on kaniadakis κ-gaussian distribution for inverse problems related to wave propagation |
the kagome superconductors kv3sb5 , rbv3sb5 , and csv3sb5 are known to display charge density wave (cdw) order which impacts the topological characteristics of their electronic structure. details of their structural ground states and how they evolve with temperature are revealed here using single crystal x-ray crystallographic refinements as a function of temperature, carried out with synchrotron radiation. the compounds kv3sb5 and rbv3sb5 present 2 ×2 ×2 superstructures in the f m m m space group with a staggered trihexagonal deformation of vanadium layers. csv3sb5 displays more complex structural evolution, whose details have been unravelled by applying machine learning methods to the scattering data. upon cooling through the cdw transition, csv3sb5 displays a staged progression of ordering from a 2 ×2 ×1 supercell and a 2 ×2 ×2 supercell into a final 2 ×2 ×4 supercell that persists to t =11 k and exhibits an average structure where vanadium layers display both trihexagonal and star of david patterns of deformations. diffraction from csv3sb5 under pulsed magnetic fields up to μ0h =28 t suggest the real component of the cdw state is insensitive to external magnetic fields. | structural evolution of the kagome superconductors a v3sb5 (a = k, rb, and cs) through charge density wave order |
radial profiles of density fluctuations and the radial electric field, er, have been measured using doppler reflectometry during the post-pellet enhanced confinement phase achieved, under different heating power levels and magnetic configurations, during the 2018 w7-x experimental campaign. a pronounced er-well is measured with local values as high as -40 kv m-1 in the radial range ρ ∼ 0.7-0.8 during the post-pellet enhanced confinement phase. the maximum er intensity scales with both the plasma density and electron cyclotron heating power level, following a similar trend to the plasma energy content. a good agreement is found when the experimental er profiles are compared to simulations carried out using the neoclassical codes, the drift kinetic equation solver (dkes) and kinetic orbit-averaging solver for stellarators (knosos). the density fluctuation level decreases from the plasma edge toward the plasma core and the drop is more pronounced in the post-pellet enhanced confinement phase than in reference gas-fuelled plasmas. besides, in the post-pellet phase, the density fluctuation level is lower in the high iota magnetic configuration than in the standard one. to determine whether this difference is related to the differences in the plasma profiles or to the stability properties of the two configurations, gyrokinetic simulations have been carried out using the codes stella and euterpe. the simulation results point to the plasma profile evolution after the pellet injection and the stabilization effect of the radial electric field profile as the dominant players in the stabilization of the plasma turbulence. | radial electric field and density fluctuations measured by doppler reflectometry during the post-pellet enhanced confinement phase in w7-x |
we propose a general method to self-consistently study the quasistationary evolution of the magnetic field in the cores of neutron stars. the traditional approach to this problem is critically revised. our results are illustrated by calculation of the typical timescales for the magnetic field dissipation as functions of temperature and the magnetic field strength. | evolution of the magnetic field in neutron stars |
we analyze the fundamental coherence limit of a nano-object with an embedded spin in a stern-gerlach interferometer. this limit stems from the which-path information provided by the object's rotational degrees of freedom due to the evolution of their quantum uncertainty. we show that such interferometry is straightforward in a weak magnetic field and short duration. large wave packet separation is made possible with proper fine-tuning over long durations. this opens the door to fundamental tests of quantum theory and quantum gravity. the results and conclusions are extendable to any type of interferometry with complex objects. | quantum uncertainty limit for stern-gerlach interferometry with massive objects |
under normal conditions in a neutron-star (ns) crust, ions are locked in place in the crustal lattice and only electrons are mobile, and magnetic-field evolution is thus directly related to the electron velocity. the evolution, however, builds magnetic stresses that can become sufficiently large for the crust to exceed its elastic limit, and to flow plastically. we consider the nature of this plastic flow and the back-reaction on the crustal magnetic-field evolution. we formulate a plane-parallel model for the local failure, showing that surface motions are inevitable once the crust yields, in the absence of extra dissipative mechanisms. we perform numerical evolutions of the crustal magnetic field under the joint effect of hall drift and ohmic decay, tracking the build-up of magnetic stresses, and diagnosing crustal failure with the von mises criterion. beyond this point we solve for the coupled evolution of the plastic velocity and magnetic field. our results suggest that to have a coexistence of a magnetar corona with small-scale magnetic features, the viscosity of the plastic flow must be roughly 1036-1037 g cm-1s-1. we find significant motion at the surface at a rate of 10-100 cm yr-1, and that the localized magnetic field is weaker than in evolutions without plastic flow. we discuss astrophysical implications, and how our local simulations could be used to build a global model of field evolution in the ns crust. | magnetic-field evolution in a plastically failing neutron-star crust |
here we present details of an operator-split, implicit-explicit numerical scheme for the solution of the gyrokinetic-poisson system of equations in the local limit. this scheme has been implemented in a new code called stella, which is capable of evolving electrostatic fluctuations with full kinetic electron effects and an arbitrary number of ion species in general magnetic geometry. we demonstrate the advantages of this mixed approach over a fully explicit treatment and provide linear and nonlinear benchmark comparisons for both axisymmetric and non-axisymmetric magnetic equilibria. | stella: an operator-split, implicit-explicit δf-gyrokinetic code for general magnetic field configurations |
in the interior of neutron stars, the induction equation regulates the long-term evolution of the magnetic fields by means of resistivity, hall dynamics and ambipolar diffusion. despite the apparent simplicity and compactness of the equation, the dynamics it describes is not trivial and its understanding relies on accurate numerical simulations. while a few works in 2d have reached a mature stage and a consensus on the general dynamics at least for some simple initial data, only few attempts have been performed in 3d, due to the computational costs and the need for a proper numerical treatment of the intrinsic non-linearity of the equation. here, we carefully analyze the general induction equation, studying its characteristic structure, and we present a new cartesian 3d code, generated by the user-friendly, publicly available simflowny platform. the code uses high-order numerical schemes for the time and spatial discretization, and relies on the highly-scalable samrai architecture for the adaptive mesh refinement. we present the application of the code to several benchmark tests, showing the high order of convergence and accuracy achieved and the capabilities in terms of magnetic shock resolution and three-dimensionality. this paper paves the way for the applications to a realistic, 3d long-term evolution of neutron stars interior and, possibly, of other astrophysical sources. | a simflowny-based high-performance 3d code for the generalized induction equation |
in this paper, we study the thermal evolution of three types of quantum correlations under the homogeneous and inhomogeneous spin star hamiltonian. it is shown that quantum discord (qd) is more stable than the other measures in the thermal regime, but concurrence is more efficient when the hamiltonian parameters are employed. however, all quantum correlations can reach their maximum, if the inhomogeneous parameter raises. quantum correlations can be enhanced by a weak external magnetic field and strong coupling parameter. but they vanish in the case of a strong magnetic field, weak coupling parameter, and high temperatures. | thermal quantum correlations in a two-dimensional spin star model |
simulating the long-term evolution of temperature and magnetic fields in neutron stars is a major effort in astrophysics, having significant impact in several topics. a detailed evolutionary model requires, at the same time, the numerical solution of the heat diffusion equation, the use of appropriate numerical methods to control non-linear terms in the induction equation, and the local calculation of realistic microphysics coefficients. here we present the latest extension of the magneto-thermal 2d code in which we have coupled the crustal evolution to the core evolution, including ambipolar diffusion. it has also gained in modularity, accuracy, and efficiency. we revise the most suitable numerical methods to accurately simulate magnetar-like magnetic fields, reproducing the hall-driven magnetic discontinuities. from the point of view of computational performance, most of the load falls on the calculation of microphysics coefficients. to a lesser extent, the thermal evolution part is also computationally expensive because it requires large matrix inversions due to the use of an implicit method. we show two representative case studies: (i) a non-trivial multipolar configuration confined to the crust, displaying long-lived small-scale structures and discontinuities; and (ii) a preliminary study of ambipolar diffusion in normal matter. the latter acts on timescales that are too long to have relevant effects on the timescales of interest but sets the stage for future works where superfluid and superconductivity need to be included. | magneto-thermal evolution of neutron stars with coupled ohmic, hall and ambipolar effects via accurate finite-volume simulations |
fast radio bursts are millisecond-duration radio pulses of extragalactic origin. a recent statistical analysis has found that the burst energetics of the repeating source frb 121102 follow a power law, with an exponent that is curiously consistent with the gutenberg-richter law for earthquakes. this hints that repeat bursters may be compact objects undergoing violent tectonic activity. for young magnetars, possessing crustal magnetic fields which are both strong (b ≳ 1015 g) and highly multipolar, hall drift can instigate significant field rearrangements even on ≲ century long time-scales. this reconfiguration generates zones of magnetic stress throughout the outer layers of the star, potentially strong enough to facilitate frequent crustal failures. in this paper, assuming a quake scenario, we show how the crustal field evolution, which determines the resulting fracture geometries, can be tied to burst properties. highly anisotropic stresses are generated by the rapid evolution of multipolar fields, implying that small, localized cracks can occur sporadically throughout the crust during the hall evolution. each of these shallow fractures may release bursts of energy, consistent in magnitude with those seen in the repeating sources frb 121102 and frb 180814.j0422+73. | young magnetars with fracturing crusts as fast radio burst repeaters |
we present ultraviolet (uv), optical and infrared photometry and optical spectroscopy of the type ic superluminous supernova (slsn) gaia16apd (=sn 2016eay), covering its evolution from 26 d before the g-band peak to 234.1 d after the peak. gaia16apd was followed as a part of the not unbiased transient survey (nuts). it is one of the closest slsne known (z = 0.102 ± 0.001), with detailed optical and uv observations covering the peak. gaia16apd is a spectroscopically typical type ic slsn, exhibiting the characteristic blue early spectra with o ii absorption, and reaches a peak mg = -21.8 ± 0.1 mag. however, photometrically it exhibits an evolution intermediate between the fast and slowly declining type ic slsne, with an early evolution closer to the fast-declining events. together with lsq12dlf, another slsn with similar properties, it demonstrates a possible continuum between fast and slowly declining events. it is unusually uv-bright even for an slsn, reaching a non-k-corrected muvm2 ≃ -23.3 mag, the only other type ic slsn with similar uv brightness being sn 2010gx. assuming that gaia16apd was powered by magnetar spin-down, we derive a period of p = 1.9 ± 0.2 ms and a magnetic field of b = 1.9 ± 0.2 × 1014 g for the magnetar. the estimated ejecta mass is between 8 and 16 m⊙, and the kinetic energy between 1.3 and 2.5 × 1052 erg, depending on opacity and assuming that the entire ejecta is swept up into a thin shell. despite the early photometric differences, the spectra at late times are similar to slowly declining type ic slsne, implying that the two subclasses originate from similar progenitors. | gaia16apd - a link between fast and slowly declining type i superluminous supernovae |
magnetic reconnection is a process that contributes significantly to plasma dynamics and energy transfer in a wide range of plasma and magnetic field regimes, including inertial confinement fusion experiments, stellar coronae, and compact, highly magnetized objects like neutron stars. laboratory experiments in different regimes can help refine, expand, and test the applicability of theoretical models to describe reconnection. laser-plasma experiments exploring magnetic reconnection at a moderate intensity (il ∼ 1014 w cm-2) have been performed previously, where the biermann battery effect self-generates magnetic fields and the field dynamics studied using proton radiography. at high laser intensities ( ilλl2>1018 wcm-2μ m2 ), relativistic surface currents and the time-varying electric sheath fields generate the azimuthal magnetic fields. numerical modeling of these intensities has shown the conditions that within the magnetic field region can reach the threshold where the magnetic energy can exceed the rest mass energy such that σcold = b2/(μ0nemec2) > 1 [a. e. raymond et al., phys. rev. e 98, 043207 (2018)]. presented here is the analysis of the proton radiography of a high-intensity (∼1018 w cm-2) laser driven magnetic reconnection geometry. the path integrated magnetic fields are recovered using a "field-reconstruction algorithm" to quantify the field strengths, geometry, and evolution. | field reconstruction from proton radiography of intense laser driven magnetic reconnection |
the large-scale magnetic fields of stars can be obtained with the zeeman doppler imaging (zdi) technique, but their interpretation is still challenging as the contribution of the small-scale field or the reliability of the reconstructed field properties is still not fully understood. to quantify this, we use 3d non-potential magnetic field simulations for slowly rotating solar-like stars as inputs to test the capabilities of zdi. these simulations are based on a flux transport model connected to a non-potential coronal evolution model using the observed solar flux emergence pattern. we first compare four field prescriptions regarding their reconstruction capabilities and investigate the influence of the spatial resolution of the input maps on the corresponding circularly polarized profiles. we then generate circularly polarized spectra based on our high-resolution simulations of three stellar models with different activity levels, and reconstruct their large-scale magnetic fields using a non-potential zdi code assuming two different stellar inclination angles. our results show that the zdi technique reconstructs the main features of slowly rotating solar-like stars but with ∼ one order of magnitude less magnetic energy. the large-scale field morphologies are recovered up to harmonic modes ℓ ∼ 5, especially after averaging over several maps for each stellar model. while zdi is not able to reproduce the input magnetic energy distributions across individual harmonic modes, the fractional energies across the modes are generally within 20 per cent agreement. the fraction of axisymmetric and toroidal field tends to be overestimated for stars with solar flux emergence patterns for more pole-on inclination angles. | observing the simulations: applying zdi to 3d non-potential magnetic field simulations |
the long-term evolution of the internal, strong magnetic fields of neutron stars needs a specific numerical modelling. the diversity of the observed phenomenology of neutron stars indicates that their magnetic topology is rather complex and 3d simulations are required, for example, to explain the observed bursting mechanisms and the creation of surface hotspots. we present matins, a new 3d numerical code for magnetothermal evolution in neutron stars, based on a finite-volume scheme that employs the cubed-sphere system of coordinates. in this first work, we focus on the crustal magnetic evolution, with the inclusion of realistic calculations for the neutron star structure, composition, and electrical conductivity assuming a simple temperature evolution profile. matins follows the evolution of strong fields (1014 - 1015 gauss) with complex non-axisymmetric topologies and dominant hall-drift terms, and it is suitable for handling sharp current sheets. after introducing the technical description of our approach and some tests, we present long-term simulations of the non-linear field evolution in realistic neutron star crusts. the results show how the non-axisymmetric hall cascade redistributes the energy over different spatial scales. following the exploration of different initial topologies, we conclude that during a few tens of kyr, an equipartition of energy between the poloidal and toroidal components happens at small-scales. however, the magnetic field keeps a strong memory of the initial large scales, which are much harder to be restructured or created. this indicates that large-scale configuration attained during the neutron star formation is crucial to determine the field topology at any evolution stage. | 3d code for magneto-thermal evolution in isolated neutron stars, matins: the magnetic field formalism |
the tayler instability is an important but poorly studied magnetohydrodynamic (mhd) instability that likely operates in stellar interiors. the non-linear saturation of the tayler instability is poorly understood and has crucial consequences for dynamo action and angular momentum transport in radiative regions of stars. we perform three-dimensional mhd simulations of the tayler instability in a cylindrical geometry, including strong buoyancy and coriolis forces as appropriate for its operation in realistic rotating stars. the linear growth of the instability is characterized by a pre-dominantly m = 1 oscillation with growth rates roughly following analytical expectations. the non-linear saturation of the instability appears to be caused by secondary shear instabilities and is also accompanied by a morphological change in the flow. we argue, however, that non-linear saturation likely occurs via other mechanisms in real stars where the separation of scales is larger than those reached by our simulations. we also observe dynamo action via the amplification of the axisymmetric poloidal magnetic field, suggesting that tayler instability could be important for magnetic field generation and angular momentum transport in the radiative regions of evolving stars. | magnetohydrodynamic simulations of the tayler instability in rotating stellar interiors |
context. the magneto-rotational instability (mri) has been proposed as a mechanism to transport angular momentum (am) and chemical elements in theoretical stellar models.aims: using as a prototype a massive star of 15 m⊙ with solar metallicity, we explore the effects of the mri on the evolution of massive stars.methods: we used the geneva stellar evolution code to simulate the evolution of various models, up to the end of oxygen burning, including the mri through effective, one-dimensional, diffusion coefficients. we consider different trigger conditions (depending on the weighting of chemical gradients through an arbitrary but commonly used factor) and different treatments of meridional circulation as either advective or diffusive. we also compare the mri with the tayler-spruit (ts) dynamo in models that included both instabilities.results: the mri triggers throughout stellar evolution. its activation is highly sensitive to the treatment of meridional circulation and the existence of chemical gradients. the mri is very efficient at transporting both matter and am, leading to noticeable differences in rotation rates and chemical structure, which may be observable in young main sequence stars. while the ts dynamo is the dominant mechanism for transferring am, the mri remains relevant in models where both instabilities are included. extrapolation of our results suggests that models that include the mri tend to develop more compact cores, which likely produce failed explosions and black holes, than models where only the ts dynamo is included (where explosions and neutron stars may be more frequent).conclusions: the mri can be an important factor in massive star evolution but is very sensitive to the implementation of other processes in the model. the transport of am and chemical elements due to the mri alters the rotation rates and the chemical makeup of the star from the core to the surface and may change the explodability properties of massive stars. | the magneto-rotational instability in massive stars |
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