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the x-ray afterglow of grb 130831a shows an “internal plateau” with a decay slope of ∼0.8, followed by a steep drop at around 105 s with a slope of ∼6. after the drop, the x-ray afterglow continues with a much shallower decay. the optical afterglow exhibits two segments of plateaus separated by a luminous optical flare, followed by a normal decay with a slope basically consistent with that of the late-time x-ray afterglow. the decay of the internal x-ray plateau is much steeper than what we expect in the simplest magnetar model. we propose a scenario in which the magnetar undergoes gravitational-wave-driven r-mode instability, and the spin-down is dominated by gravitational wave losses up to the end of the steep plateau, so that such a relatively steep plateau can be interpreted as the internal emission of the magnetar wind and the sharp drop can be produced when the magnetar collapses into a black hole. this scenario also predicts an initial x-ray plateau lasting for hundreds of seconds with an approximately constant flux which is compatible with observation. assuming that the magnetar wind has a negligible contribution in the optical band, we interpret the optical afterglow as the forward shock emission by invoking the energy injection from a continuously refreshed shock following the prompt emission phase. it is shown that our model can basically describe the temporal evolution of the multi-band afterglow of grb 130831a. | modeling the multi-band afterglow of grb 130831a: evidence for a spinning-down magnetar dominated by gravitational wave losses? |
aims: we investigate the long gamma-ray burst (grb) 140629a through multiwavelength observations to derive the properties of the dominant jet and its host galaxy.methods: the afterglow and host galaxy observations were taken in the optical (swift/uvot and various facilities worldwide), infrared (spitzer), and x-rays (swift/xrt) between 40 s and 3 yr after the burst trigger.results: polarisation observations by the master telescope indicate that this burst is weakly polarised. the optical spectrum contains absorption features, from which we confirm the redshift of the grb as originating at z = 2.276 ± 0.001. we performed spectral fitting of the x-rays to optical afterglow data and find there is no strong spectral evolution. we determine the hydrogen column density nh to be 7.2 × 1021 cm-2 along the line of sight. the afterglow in this burst can be explained by a blast wave jet with a long-lasting central engine expanding into a uniform medium in the slow cooling regime. at the end of energy injection, a normal decay phase is observed in both the optical and x-ray bands. an achromatic jet break is also found in the afterglow light curves ∼0.4 d after trigger. we fit the multiwavelength data simultaneously with a model based on a numerical simulation and find that the observations can be explained by a narrow uniform jet in a dense environment with an opening angle of 6.7° viewed 3.8° off-axis, which released a total energy of 1.4 × 1054 erg. using the redshift and opening angle, we find grb 140629a follows both the ghirlanda and amati relations. from the peak time of the light curve, identified as the onset of the forward shock (181s after trigger), the initial lorentz factor (γ0) is constrained in the range 82-118. fitting the host galaxy photometry, we find the host to be a low mass, star-forming galaxy with a star formation rate of log (sfr) 1.1+0.9-0.4 m⊙ yr-1. we obtain a value of the neutral hydrogen density by fitting the optical spectrum, log nhi = 21.0 ± 0.3, classifying this host as a damped lyman-alpha. high ionisation lines (n v, si iv) are also detected in the spectrum. research supported by the china scholarship council. | multiwavelength observations of grb 140629a. a long burst with an achromatic jet break in the optical and x-ray afterglow |
the short hard gamma ray burst (shb) 170817a that followed gw170817, the first neutron stars merger (nsm) detected in gravitational waves (gws), has shown beyond doubt that nsms produce shbs. its low luminosity and other properties that differ from those of ordinary shbs were predicted by the cannonball model for relatively nearby shbs, which almost entirely are low luminosity shbs viewed from far off-axis. such far off-axis low luminosity shbs are mostly the smoking guns of nearby nsms, like those detectable by ligo-virgo. the smoking guns of much more distant nsms are pwn afterglows powered by the spin-down of the nascent n*s, preceded by a visible shb, or by an invisible shb, which is beamed away from earth. | the smoking guns of neutron stars mergers |
we systematically analyze the properties of the x-ray shallow decay phase in swift grbs observed from 2004 december to 2021 march. these events have at least the three obvious emission phases: the steep decay, shallow decay and the following decay. we find that the distributions of the characteristic properties of the shallow decay phase are basically lognormal or normal. we also find that the luminosities at the beginning time and the end time of the x-ray shallow decay phase are correlated with the peak luminosity and isotropic energy of the prompt gamma-ray phase, and are also correlated with the corresponding beginning/end time and the total energy of the x-ray shallow decay phase. there is a tighter correlation between the total energy of the shallow decay phase and the isotropic energy of the prompt phase, i.e, eγ ,iso∝ex,iso 0.89 ±0.06. in addition, we also find that the three-parameter l −t −e and l −t −l correlations still exist for our sample. at the same time, there are similar three-parameter correlations for the parameters at the beginning time of the shallow decay phase, i.e., lx ,b∝tb,z −1.58 ±0.11eγ,iso 0.83 ±0.05 and lx ,b∝tb,z −1.69 ±0.16lp0.49 ±0.06. the subsamples of short grbs, grbs with the internal plateau and grbs with the early tail emission all follow the same correlations with other events. our results show that the emission of the x-ray shallow decay phase is highly correlated with the prompt emission, supporting the re-activity of the long-lived central engine. | statistical properties of the x-ray afterglow shallow decay phase and their relationships with the prompt gamma-ray emission of gamma-ray bursts |
we present a detailed analysis of grb 151006a, the first gamma-ray burst (grb) detected by astrosat cadmium-zinc-telluride imager (czti). we study the long-term spectral evolution by exploiting the capabilities of fermi and swift satellites at different phases, which is complemented by the polarization measurement with the czti. while the light curve of the grb in different energy bands shows a simple pulse profile, the spectrum shows an unusual evolution. the first phase exhibits a hard-to-soft evolution until ∼16-20 s, followed by a sudden increase in the spectral peak reaching a few mev. such a dramatic change in the spectral evolution in the case of a single pulse burst is reported for the first time. this is captured by all models we used namely, band function, blackbody+band and two blackbodies+power law. interestingly, the fermi large area telescope also detects its first photon (>100 mev) during this time. this new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself that, however, is not seen in the otherwise smooth pulse profile. by constructing bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. the swift data at late epochs (>t90 of the grb) also show a significant spectral evolution consistent with the early second phase. the czti data (100-350 kev), though having low significance (1σ), show high values of polarization in the two epochs (77-94 per cent), in agreement with our interpretation. | surprise in simplicity: an unusual spectral evolution of a single pulse grb 151006a |
violations of both the weak equivalence principle (wep) and lorentz invariance can produce vacuum birefringence, which leads to an energy-dependent rotation of the polarization vector of linearly polarized emission from a given astrophysical source. however, the search for the birefringent effect has been hindered by our ignorance concerning the intrinsic polarization angle in different energy bands. considering the contributions to the observed linear polarization angle from both the intrinsic polarization angle and the rotation angles induced by violations of the wep and lorentz invariance, and assuming the intrinsic polarization angle is an unknown constant, we simultaneously obtain robust bounds on possible deviations from the wep and lorentz invariance, by directly fitting the multiwavelength polarimetric data of the optical afterglows of gamma-ray burst (grb) 020813 and grb 021004. here, we show that at the 3 σ confidence level, the difference of the parameterized post-newtonian parameter γ values characterizing the departure from the wep is constrained to be δ γ =(-4 .5-16.0+10.0)×10-24 and the birefringent parameter η quantifying the broken degree of lorentz invariance is limited to be η =(6 .5-14.0+15.0)×10-7 . these are the first simultaneous verifications of the wep and lorentz invariance in the photon sector. more stringent limits can be expected as the analysis presented here is applied to future multiwavelength polarization observations in the prompt gamma-ray emission of grbs. | testing the weak equivalence principle and lorentz invariance with multiwavelength polarization observations of grb optical afterglows |
strong downstream magnetic fields of the order of ∼1 g, with large correlation lengths, are believed to cause the large synchrotron emission at the afterglow phase of gamma-ray bursts (grbs). despite the recent theoretical efforts, models have failed to fully explain the amplification of the magnetic field, particularly in a matter-dominated scenario. we revisit the problem by considering the synchrotron emission to occur at the expanding shock front of a weakly magnetized relativistic jet over a magnetized surrounding medium. analytical estimates and a number of high-resolution 2d relativistic magnetohydrodynamical (rmhd) simulations are provided. jet opening angles of θ = 0°-20°, and ambient to jet density ratios of 10-4-102 were considered. we found that most of the amplification is due to compression of the ambient magnetic field at the contact discontinuity between the reverse and forward shocks at the jet head, with substantial pile-up of the magnetic field lines as the jet propagates sweeping the ambient field lines. the pile-up is maximum for θ → 0, decreasing with θ, but larger than in the spherical blast problem. values obtained for certain models are able to explain the observed intensities. the maximum correlation lengths found for such strong fields is of lcorr ≤ 1014 cm, 2-6 orders of magnitude larger than the found in previous works. | ambient magnetic field amplification in shock fronts of relativistic jets: an application to grb afterglows |
considered here is the acceleration and heating of relativistic outflow by local magnetic energy dissipation process in poynting-flux dominated outflow. adopting the standard assumption that the reconnection rate scales with the alfvén speed, i show here that the fraction of energy dissipated as thermal photons cannot exceed {(13\hat{γ }-14)}-1=30 % (for adiabatic index \hat{γ }=4/3) of the kinetic energy at the photosphere. even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. these results imply that calorimetry of the kinetic energy that can be done during the afterglow phase could be used to constrain the magnetization of gamma-ray bursts (grb) outflows. i discuss the recent observational status and its implications on constraining the magnetization in grb outflows. | constraining magnetization of gamma-ray bursts outflows using prompt emission fluence |
multiwavelength observation of the gamma-ray burst, grb 190114c, opens a new window for studying the emission mechanism of grb afterglows. its very high energy (vhe; ≳ 100 gev) detection has motivated an inverse compton interpretation for the emission, but this has not been tested. here, we revisit the early afterglow emission from 68 to 180 s and perform the modelling likelihood analysis with the kev to tev data sets. we compute for the first time the statistical preference in the combined synchrotron (syn) and synchrotron self-compton (ssc) model over the syn-only model. in agreement with earlier analyses, between 68 and 110 s an unstable preference for the ssc model can be found, which can also be explained by systematic cross-calibration effect between the included instruments. we conclude that there is no stable statistical preference for one of the two models. | probing the multiwavelength emission scenario of grb 190114c |
gamma-ray bursts (grbs) are known to be the most violent explosions in the universe, and a variety of correlations between observable grb properties have been proposed in the literature, but none of these correlations are valid for both long and short grbs. in this paper we report the discovery of a universal correlation that is suitable for both long and short grbs using three prompt emission properties of grbs, i.e., the isotropic peak luminosity liso, the peak energy of the time-integrated prompt emission spectrum epeak, and the "high signal" timescale t0.45, ${l}_{\mathrm{iso}}\propto {e}_{\mathrm{peak}}^{1.94}{t}_{0.45}^{0.37}$ . this universal correlation involves properties of grb prompt emission and does not require knowledge of the afterglow phase, which can be used as a relatively unbiased redshift estimator. here we use this correlation to estimate the pseudoredshifts for short grbs and then use the lynden-bell method to obtain a nonparametric estimate of their luminosity function and formation rate. the luminosity function is $\psi ({l}_{0})\propto {l}_{0}^{-0.63\pm 0.07}$ for dim sgrbs and $\psi ({l}_{0})\propto {l}_{0}^{-1.96\pm 0.28}$ for bright sgrbs, with the break point ${6.95}_{-0.76}^{+0.84}\times {10}^{50}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ . the local formation rate of sgrbs is about 15 events gpc-3 yr-1 . this universal correlation may have important implications for grb physics, implying that the long and short grbs should share similar radiation processes. | discovery of a universal correlation for long and short grbs and its application for the study of luminosity function and formation rate |
spectropolarimetric measurements of gamma-ray burst (grb) optical afterglows contain polarization information for both continuum and absorption lines. based on the zeeman effect, an absorption line in a strong magnetic field is polarized and split into a triplet. in this paper, we solve the polarization radiative transfer equations of the absorption lines, and obtain the degree of linear polarization of the absorption lines as a function of the optical depth. in order to effectively measure the degree of linear polarization for the absorption lines, a magnetic field strength of at least 103 g is required. the metal elements that produce the polarized absorption lines should be sufficiently abundant and have large oscillation strengths or einstein absorption coefficients. we encourage both polarization measurements and high-dispersion observations of the absorption lines in order to detect the triplet structure in early grb optical afterglows. | on the polarized absorption lines in gamma-ray burst optical afterglows |
sub-relativistic materials launched during the merger of binary compact objects and the core collapse of massive stars acquire velocity structures when expanding in a stratified environment. the remnant (either a spinning magnetized neutron star (ns) or a central black hole) from the compact object or core collapse could additionally inject energy into the afterglow via spin-down luminosity or/and by accreting fallback material, producing a refreshed shock, modifying the dynamics, and leading to rich radiation signatures at distinct timescales and energy bands with contrasting intensities. we derive the synchrotron light curves evolving in a stratified environment when a power-law velocity distribution parameterizes the energy of the shock, and the remnant continuously injects energy into the blast wave. as the most relevant case, we describe the latest multiwavelength afterglow observations (≳900 days) of the gw170817/grb 170817a event via a synchrotron afterglow model with energy injection of a sub-relativistic material. the features of the remnant and the synchrotron emission of the sub-relativistic material are consistent with a spinning magnetized ns and the faster blue kilonova afterglow, respectively. using the multiband observations of some short bursts with evidence of kilonovae, we provide constraints on the expected afterglow emission. | grb afterglow of the sub-relativistic materials with energy injection |
synchrotron and inverse compton emission successfully explain the observed spectra of gamma-ray burst (grb) afterglows. it is thought that most grbs are products of extremely relativistic outflows and the afterglow marks the interaction of that ejecta with the surrounding matter. a faster decay of afterglow light curves at late times is indicative of nonspherical geometries, and is usually interpreted as evidence for jet geometry. recent numerical simulations have shown that ring-like geometries are also permissible for relativistic outflows. we therefore extend the standard theory of afterglow evolution to ring geometries. an analytic prescription for the light curves and spectra produced by relativistic toroidal blast waves is presented. we compare these to their spherical and jet-like counterparts, and show that ring afterglows decay faster than spherical outflows but not as fast as jets. | on the theory of ring afterglows |
gamma-ray bursts (grbs) are the most luminous objects known: they outshine their host galaxies, making them ideal candidates for probing large-scale structure. our aim is to determine the spatial two-point correlation function of the grbs with spectroscopic redshifts. we used all observations till 4 june 2021, and we also split the data according to the origin of the redshift (afterglow or host galaxy). we analyzed the possible correlation between the grb redshift and sky position. there is only one region, called the faraway grb patch, where nine distant grbs shows deviation from the randomness with an ≈1% significance, showing that the sky and the radial component of the grb distribution could be factorized and both can be determined independently. the grbs' cumulative monthly distribution was used to show that it is impossible to synthesize the sky exposure function, even from the perfect observational logs. we estimated the sky exposure function using the gaussian kernel, and with the radial distribution, it was used for the estimation of the spatial two-point correlation function. monte carlo simulations were used to determine the poissonian errors. our result shows that the current spatial two-point correlation function estimations are consistent with zero for the full and the afterglow/host galaxy datasets. | the spatial distribution of gamma-ray bursts with measured redshifts from 24 years of observation |
using deep and high-cadence gamma-ray burst (grb) afterglow data from the reionization and transients infra-red/optical camera, we observe a sharp and achromatic light curve break 12.6 days after the grb, accompanied by an approximately achromatic bump. fitting of the optical, near-infrared, and x-ray data suggest a very narrow (2°) jet that remains collimated at late time. we argue that the sharp light curve bump suggests an edge-brightened jet, perhaps emitting only during a brief period of lateral jet expansion. the light curve also exhibits a gradual spectral evolution lasting >10 days. the evolution of the flux can be modeled as {flux}∼ {≤ft(\tfrac{t}{[20{days}]}\right)}α {≤ft(\tfrac{λ }{[800{nm}]}\right)}β , with a temporal slope α = -0.956 ± 0.003 and a gradually time-varying spectral slope β =(0.60 +/- 0.07)+(0.26+/- 0.06){log}≤ft(\tfrac{t}{20 {days}}\right). | evidence for a bright-edged jet in the optical/near-infrared afterglow of grb 160625b |
the potential existence of two separate classes of long-duration gamma-ray bursts (lgrbs) with and without radio afterglow emission, corresponding to radio-bright/loud and radio-dark/quiet populations, has been recently argued and favoured in the grb literature. the radio-quiet lgrbs have been found to have, on average, lower total isotropic gamma-ray emissions (eiso) and shorter intrinsic prompt gamma-ray durations (e.g. t90z). in addition, a redshift -t90z anticorrelation has been discovered among the radio-loud lgrbs, which is reportedly missing in the radio-quiet class. here, we discuss the significance of the differences between the energetics and temporal properties of the two proposed classes of radio-loud and radio-quiet lgrbs. we show that much of the proposed evidence in support of the two distinct radio populations of lgrbs can be explained away in terms of selection effects and sample incompleteness. our arguments are based on the recent discovery of the relatively strong, highly significant positive correlation between the total isotropic emission (eiso) and the intrinsic prompt duration (t90z) that is present in both populations of short-hard and long-soft grbs, predicted, quantified, and reported for the first time by shahmoradi (2013a;2013b) and shahmoradi & nemiroff (2015). | are there radio-loud and radio-quiet gamma-ray bursts? |
we describe the implementation of a remotely operated telescope on the skynet robotic telescope network, a system developed and run by the university of north carolina-chapel hill. our telescope, operated by appalachian state university at its dark sky observatory, runs robotically on this queue-scheduled system, automatically taking calibration images and acquiring program images, and responding to internet commands to image the afterglow of accessible gamma-ray burst events. we describe the process of implementing a skynet-run telescope from our client-side view, and offer advice for others who might consider putting telescopes on skynet. the implementation has proven very successful, obtaining over a hundred thousand images over the past six years, of various targets for research and educational purposes, and has responded to several grb observation requests with several afterglow detections. | implementation and operation of a robotic telescope on skynet |
aims: the aim of the study is to constrain the physics of gamma-ray bursts (grbs) by analysing the multi-wavelength afterglow data set of grb 121024a that covers the full range from radio to x-rays.methods: using multi-epoch broad-band observations of the grb 121024a afterglow, we measured the three characteristic break frequencies of the synchrotron spectrum. we used six epochs of combined xrt and grond data to constrain the temporal slopes, the dust extinction, the x-ray absorption, and the spectral slope with high accuracy. two more epochs of combined data from xrt, grond, apex, carma, and evla were used to set constraints on the break frequencies and therefore on the micro-physical and dynamical parameters.results: the xrt and grond light curves show a simultaneous and achromatic break at around 49 ks. as a result, the crossing of the synchrotron cooling break is no suitable explanation for the break in the light curve. the multi-wavelength data allow us to test two plausible scenarios explaining the break: a jet break, and the end of energy injection. the jet-break scenario requires a hard electron spectrum, a very low cooling break frequency, and a non-spreading jet. the energy injection avoids these problems, but requires ɛe > 1 (k = 2), spherical outflow, and ɛb < 10-9.conclusions: in light of the extreme microphysical parameters required by the energy-injection model, we favour a jet-break scenario where νm < νsa to explain the observations. this scenario gives physically meaningful microphysical parameters, and it also naturally explains the reported detection of linear and circular polarisation. | microphysics and dynamics of the gamma-ray burst 121024a |
utilizing more than 100 long gamma-ray bursts (lgrbs) in the swift-ryan-2012 sample that includes the observed redshifts and jet angles, le & mehta performed a timely study of the rate density of lgrbs with an assumed broken power-law grb spectrum and obtained a grb-burst-rate functional form that gives acceptable fits to the pre-swift and swift redshift, and jet angle distributions. the results indicated an excess of lgrbs at redshift below z ∼ 2 in the swift sample. in this work, we are investigating if the excess is caused by the cosmological hubble constant h0, the gamma-ray energy released e_{*γ }, the low- and high-energy indices (α, β) of the band function, the minimum and maximum jet angles θj, min and θj, max, or that the excess is due to a bias in the swift-ryan-2012 sample. our analyses indicate that none of the above physical parameters resolved the excess problem, but suggesting that the swift-ryan-2012 sample is biased with possible afterglow selection effect. the following model physical parameter values provide the best fit to the swift-ryan-2012 and pre-swift samples: the hubble constant h_0 = 72 {km s^{-1} mpc^{-1}}, the energy released e_{*γ }∼ 4.47 × 10^{51} erg, the energy indices α ∼ 0.9 and β ∼ -2.13, the jet angles of θj, max ∼ 0.8 rad, and θj, min ∼ 0.065 and ∼0.04 rad for pre-swift and swift, respectively, s ∼ -1.55 the jet angle power-law index, and a grb formation rate that is similar to the hopkins & beacom observed star formation history and as extended by li. using the swift gamma-ray burst host galaxy legacy survey (shoals) swift-perley lgrb sample and applying the same physical parameter values as above, however, our model provides consistent results with this data set and indicating no excess of lgrbs at any redshift. | resolving the excess of long grb's at low redshift in the swift era |
a variety of scientific results have been achieved over the last 10 years with the grond simultaneous 7-channel imager at the 2.2 m telescope of the max-planck society at eso/la silla. while designed primarily for rapid observations of gamma-ray burst afterglows, the combination of simultaneous imaging in the sloan g‧r‧i‧z‧ and near-infrared jhksbands at a medium-sized (2.2 m) telescope and the very flexible scheduling possibility has resulted in an extensive use for many other astrophysical research topics, from exoplanets and accreting binaries to galaxies and quasars. based on observations made with eso telescopes at the la silla paranal observatory under programme id 292.d-5029(a). | the benefit of simultaneous seven-filter imaging: 10 years of grond observations |
we extend the standard model of forward-reverse shock (fs-rs) for gamma-ray burst (grb) afterglow to more general cases. on one hand, we derive the analytical solution to the hydrodynamics of the shocks in two limiting cases, i.e. an ultrarelativistic reverse shock case and a newtonian reverse shock case. based on the asymptotic solutions in these two limiting cases, we constitute a semi-analytical solution for the hydrodynamics of the shocks in the generic case, covering the mildly relativistic reverse shock case. on the other hand, we derive the evolution of the system taking into account the condition of energy conservation which is not satisfied in the standard fs-rs model. a generic solution of semi-analytical expressions is also given. in both the extended standard fs-rs model (satisfying pressure balance condition) and the model satisfying energy conservation, we find that the results in the ultrarelativistic reverse shock case and in the early stage of the newtonian reverse shock case are different from those in the standard fs-rs model by only a factor that close to one while the same initial conditions adopted. however, the asymptotic solutions in the limiting cases are not good approximations to those in the intermediate case. our semi-analytical results agree well with the numerical results for a large range of model parameters, and hence can be easily employed to diagnose the physical quantities of the grb shell and circumburst environment. | a semi-analytical solution to the forward-reverse shock hydrodynamics of the gamma-ray burst afterglow |
in recent years, more and more gamma-ray bursts (grbs) with late rebrightenings in their multi-band afterglows have revealed the late-time activity of their central engines. grb 100814a is a special case among the well-sampled events, with complex temporal and spectral evolution. the single power-law shallow decay index of the optical light curve observed by grond between 640 s and 10 ks is {{α }opt}=0.57+/- 0.02, which apparently conflicts with expectations from the simple external shock model. in particular, there is remarkable rebrightening in the optical to near-infrared bands at late times, challenging the external shock model with synchrotron emission coming from the interaction of the blast wave with the surrounding interstellar medium. in this paper, we invoke a magnetar with spin evolution to explain the complex multi-band afterglow emission of grb 100814a. the initial shallow decay phase in the optical bands and the plateau in the x-ray can be explained as being due to energy injection from a spin-down magnetar. at late times, with materials from the fall-back disk falling onto the central object of the burster, the angular momentum of the accreted materials is transferred to the magnetar, which leads to a spin up process. as a result, the magnetic dipole radiation luminosity will increase, resulting in significant rebrightening of the optical afterglow. we show that the model can well reproduce the observed multi-band afterglow emission. | signature of a spin-up magnetar from multi-band afterglow rebrightening of grb 100814a |
a planned rapid submillimeter (submm) gamma ray burst (grbs) follow-up observations conducted using the greenland telescope (glt) is presented. the glt is a 12-m submm telescope to be located at the top of the greenland ice sheet, where the high-altitude and dry weather porvides excellent conditions for observations at submm wavelengths. with its combination of wavelength window and rapid responding system, the glt will explore new insights on grbs. summarizing the current achievements of submm grb follow-ups, we identify the following three scientific goals regarding grbs: (1) systematic detection of bright submm emissions originating from reverse shock (rs) in the early afterglow phase, (2) characterization of forward shock and rs emissions by capturing their peak flux and frequencies and performing continuous monitoring, and (3) detections of grbs as a result of the explosion of first-generation stars result of grbs at a high redshift through systematic rapid follow ups. the light curves and spectra calculated by available theoretical models clearly show that the glt could play a crucial role in these studies. | a new era of submillimeter grb afterglow follow-ups with the greenland telescope |
at2018cow is a recently detected transient (smartt et al. atel #11727), associated with a galaxy cgcg 137-068 at z = 0.014. its physical nature is debatable, with possible scenarios including the type ic supernova (e.g., xu et al. atel#11740, izzo et al. atel#11753), an off-axis gamma-ray burst afterglow (e.g., rivera sandoval & maccarone atel#11737), a tidal disruption of a white dwarf by an intermediate mass black hole, or a magnetar (e.g., kuin et al. 2018 ; perley et al. 2018 ; prentice et al. 2018 ). to enable the distinction between these models, we conducted the very long baseline interferometry (vlbi) observations of at2018cow using the european vlbi network (evn). | the evn detection of at2018cow at 1.67ghz |
the long gamma-ray burst grb 161017a was detected by fermi and swift, and its afterglow was observed by the mitsume 50 cm optical telescope promptly, about 50 s after the burst. early optical observations revealed that the optical light curve exhibits a plateau and rebrightening in the early afterglow phase about 500 and 5000 s after the trigger, respectively. by investigating the behavior of the spectral and temporal flux variation, it was found that the plateau and rebrightening cannot be explained in the context of the simple standard afterglow model. these observational features can be explained with two independent refreshed shocks, which indicate the long-acting central engine. we evaluated the physical parameters of the subsequent shells, and we then determined the kinetic energy ratios of the two colliding shells to the leading shell to be roughly 1 and 8, respectively. in addition, two prominent x-ray flares about 200 s after the trigger may be signatures of delayed ejections of the energetic jets responsible for the refreshed shocks. such late activity of the central engine and x-ray flares play a crucial role in understanding the mechanisms for jet formation and photon emission. | late engine activity of grb 161017a revealed by early optical observations |
we present the micro-channel x-ray telescope (mxt), a new narrow-field (about 1°) telescope that will be flying on the sino-french svom mission dedicated to gamma-ray burst science, scheduled for launch in 2021. mxt is based on square micro pore optics (mpos), coupled with a low noise ccd. the optics are based on a "lobster eye" design, while the ccd is a focal plane detector similar to the type developed for the seven erosita telescopes. mxt is a compact and light (<35 kg) telescope with a 1 m focal length, and it will provide an effective area of about 45 cmsq on axis at 1 kev. the mxt psf is expected to be better than 4.2 arc min (fwhm) ensuring a localization accuracy of the afterglows of the svom grbs to better than 1 arc min (90\% c.l. with no systematics) provided mxt data are collected within 5 minutes after the trigger. the mxt sensitivity will be adequate to detect the afterglows for almost all the svom grbs as well as to perform observations of non-grb astrophysical objects. these performances are fully adapted to the svom science goals, and prove that small and light telescopes can be used for future small x-ray missions. | the microchannel x-ray telescope on board the svom satellite |
recent observations have confirmed that gamma-ray burst (grb) afterglows produce very high-energy radiation (vhe, e>100gev). this highly anticipated discovery opens new scenarios in the interpretation of grbs and in their role as probes of extragalactic background light (ebl) and lorentz invariance violation (liv). however, some fundamental questions about the actual nature of vhe emission in grbs and its evolution during the burst are still unsolved. these questions will be difficult to address, even with future imaging cherenkov telescopes, such as the cherenkov telescope array (cta). here we investigate the prospects of gamma-ray sky monitoring with extensive air showers arrays (eas) to address these problems. we discuss the theoretical aspects connected with vhe radiation emission and the implications that its temporal evolution properties have on the interpretation of grbs. by revisiting the high-energy properties of some fermi-lat detected grbs, we estimate the typical fluxes expected in the vhe band and compare them with a range of foreseeable instrument performances, based on the southern wide field-of-view gamma-ray observatory concept (swgo). we focus our analysis on how different instrument capabilities affect the chances to explore the burst onset and early evolution in vhe, providing invaluable complementary information with respect to cherenkov telescope observations. we show that under the assumption of conditions already observed in historical events, the next-generation ground monitoring detectors can actually contribute to answer several key questions. | probing gamma-ray burst vhe emission with the southern wide-field-of-view gamma-ray observatory |
we present the results of a spectroscopic search for narrow emission and absorption features in the x-ray spectra of long gamma-ray burst (grb) afterglows. using xmm-newton data, both epic and rgs spectra, of six bright (fluence > 10-7 erg cm-2) and relatively nearby (z = 0.54-1.41) grbs, we performed a blind search for emission or absorption lines that could be related to a high cloud density or metal-rich gas in the environ close to the grbs. we detected five emission features in four of the six grbs with an overall statistical significance, assessed through monte carlo simulations, of ≲ 3.0σ. most of the lines are detected around the observed energy of the oxygen edge at ~ 0.5 kev, suggesting that they are not related to the grb environment but are most likely of galactic origin. no significant absorption features were detected. a spectral fitting with a free galactic column density (nh) testing different models for the galactic absorption confirms this origin because we found an indication of an excess of galactic nh in these four grbs with respect to the tabulated values. | searching for narrow absorption and emission lines in xmm-newton spectra of gamma-ray bursts |
current models suggest gamma-ray bursts could be used as a way of probing population-iii stars - the first stars in the early universe. in this paper, we use numerical simulations to demonstrate that late-time radio observations of gamma-ray burst afterglows could provide a means of identifying bursts that originate from population-iii stars, if these were highly massive, independently from their redshift. we then present the results from a pilot study using the australia telescope compact array at 17 ghz, designed to test the hypothesis that there may be population-iii gamma-ray bursts amongst the current sample of known events. we observed three candidates plus a control gamma-ray burst, and make no detections with upper limits of 20-40 μjy at 500-1300 d post-explosion. | gamma-ray bursts from massive population-iii stars: clues from the radio band |
we examine the emission from optically bright gamma-ray burst (grb) afterglows, as the ultraviolet and optical telescope (uvot) on the neil gehrels swift observatory first begins observing, following the slew to target the grb, while the pointing of the swift satellite is still settling. we verify the photometric quality of the uvot settling data using bright stars in the field of view. in the majority of cases, we find no problems with the settling exposure photometry, but in one case, we excise the first second of the exposure to mitigate a spacecraft attitude reconstruction issue, and in a second case, we exclude the first second of the exposure in which the uvot photocathode voltage appears to be ramping up. of a sample of 23 afterglows which have peak v magnitudes <16, we find that all are detected in the settling exposures, when swift arrives on target. for nine of the grbs, the uvot settling exposure took place before the conclusion of the prompt gamma-ray emission. five of these grbs have well-defined optical peaks after the settling exposures, with rises of >0.5 mag in their optical light curves, and there is a marginal trend for these grbs to have long t90. such a trend is expected for thick-shell afterglows, but the temporal indices of the optical rises and the timing of the optical peaks appear to rule out thick shells. | a study of gamma-ray burst afterglows as they first come into view of the swiftultraviolet and optical telescope |
aims: we present a comprehensive temporal and spectral analysis of the long swift grb 120327a afterglow data to investigate possible causes of the observed early-time colour variations.methods: we collected data from various instruments and telescopes in x-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. we studied the overall temporal behaviour and the spectral energy distributions from early to late times.results: the ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 104 s after the burst event. the x-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. at early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. the observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction. table 2 is only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/607/a29 | colour variations in the grb 120327a afterglow |
we present the high-energy emission properties of grb 160509a, from its prompt mission to late afterglow phase. grb 160509a contains two emission episodes: 0-40 s and 280-420 s after the burst onset ({t}0). the relatively high fluence of grb 160509a allows us to establish an evolving spectrum above 100 mev. during the first emission episode, the >100 mev spectrum is soft with γ ≥ 3.0, which can be smoothly connected to kev energies with a band function with or without a high-energy cutoff. the >100 mev spectrum rapidly changes to a hard spectrum with γ ≤ 1.5 after {t}0+40 s. the existence of very energetic photons, e.g., a 52 gev that arrives at {t}0+77 s and a 29 gev that arrives at {t}0+70 ks, is hard to reconcile by the synchrotron emission from forward-shock electrons, but is likely due to an inverse-compton (ic) mechanism (e.g., synchrotron self-compton emission). a soft spectrum (γ ∼ 2) between 300 and 1000 s after the burst onset is also found at a significance of about 2 standard deviations, which suggests a different emission mechanism at work for this short period of time. grb 160509a represents the latest example where ic emission has to be taken into account in explaining the afterglow gev emission, which had been suggested long before the launch of the fermi large area telescope. | an evolving gev spectrum from prompt to afterglow: the case of grb 160509a |
a significant number of the parameters of a gamma-ray burst (grb) and its host galaxy are calculated from the afterglow. there are various methods obtaining extinction values for the necessary correction for galactic foreground. these are: galaxy counts, from hi 21 cm surveys, from spectroscopic measurements and colors of nearby galactic stars, or using extinction maps calculated from infrared surveys towards the grb. we demonstrate that akari far-infrared surveyor sky surface brightness maps are useful uncovering the fine structure of the galactic foreground of grbs. galactic cirrus structures of a number of grbs are calculated with a 2 arcminute resolution, and the results are compared to that of other methods. | foreground of gamma-ray bursts (grbs) from akari fis data |
different forms of long gamma-ray burst (grb) luminosity functions are considered on the basis of an explicit physical model. the inferred flux distributions are compared with the observed ones from two samples of grbs, swift and fermi gbm. the best-fit parameters of the luminosity functions are found, and the physical interpretations are discussed. the results are consistent with the observation of a comparable number of flat-phase afterglows and monotonically decreasing ones. | differential source count for gamma-ray bursts |
a number of gamma-ray bursts (grbs) exhibit the simultaneous bumps in their optical and x-ray afterglows around the jet break. these bumps are similar to the afterglows of grb 170817a, except preceded by a long shallow decay. its origin is unclear. we suggest that these late simultaneous bumps may sound a transition of circumburst environment from a free-wind medium to a constant density medium, e.g., the shocked-wind medium. in this paper, we study the emission of an external-forward shock propagating in a free-to-shocked wind environment at different viewing angles. the late simultaneous bumps/plateaux followed by a steep decay are found in the optical and x-ray afterglows for high-viewing-angle observers. in addition, these theoretical bumps are preceded by a long plateau or shallow decay, which is formed during the external-forward shock propagating in the free-wind environment. for low-viewing-angle observers, the above bumps also appear but only in the situation where the structured jet has a low characteristic angle and the deceleration radius of the in-core jet flow is at around or beyond the free-wind boundary. we search grbs for afterglows with the late simultaneous optical and x-ray bumps followed by a steep decay. grbs 120326a, 100901a, 100814a, and 120404a are obtained. we find that an off-core (in-core) observed external-forward shock in a free-to-shocked wind environment can well explain the optical and x-ray afterglows in grbs 120326a, 100901a, and 100814a (grb 120404a). | late afterglow bump/plateau around the jet break: signature of a free-to-shocked wind environment in gamma-ray burst |
two types of emission can be observed from gamma-ray bursts (grbs): the prompt emission from the central engine which can be observed in gamma or x-ray (as a low energy tail) and the afterglow from the environment in x-ray and at shorter frequencies. we examined the swift xrt spectra with the xspec software. the correct estimation of the galactic interstellar medium is very important because we observe the host emission together with the galactic hydrogen absorption. we found that the estimated intrinsic hydrogen column density and the x-ray flux depend heavily on the redshift and the galactic foreground hydrogen. we also found that the initial parameters of the iteration and the cosmological parameters did not have much effect on the fitting result. | galactic and extragalactic hydrogen in the x-ray spectra of gamma ray bursts |
photospheric emission may originate from relativistic outflows in two qualitatively different regimes: last scattering of photons inside the outflow at the photospheric radius or radiative diffusion to the boundary of the outflow. in this work, the measurement of temperature and flux of the thermal component in the early afterglows of several gamma-ray bursts along with the total flux in the prompt phase is used to determine initial radii of the outflow as well as its lorentz factors. results indicate that in some cases the outflow has relatively low lorentz factors (γ < 10), favouring cocoon interpretation, while in other cases lorentz factors are larger (γ > 10), indicating diffusive photospheric origin of the thermal component, associated with an ultrarelativistic outflow. | diffusive photospheres in gamma-ray bursts |
we here present the spectroscopic follow-up observations with vlt/x-shooter of the swift long-duration gamma-ray burst grb 160804a at z = 0.737. typically, grbs are found in low-mass, metal-poor galaxies that constitute the sub-luminous population of star-forming galaxies. for the host galaxy of the grb presented here, we derive a stellar mass of log (m*/ m⊙) = 9.80 ± 0.07, a roughly solar metallicity (12 + log (o/h) = 8.74 ± 0.12) based on emission line diagnostics, and an infrared luminosity of m3.6/(1 + z) = -21.94 mag, but find it to be dust-poor (e(b - v) < 0.05 mag). this establishes the galaxy hosting grb 160804a as one of the most luminous, massive and metal-rich grb hosts at z < 1.5. furthermore, the gas-phase metallicity is found to be representative of the physical conditions of the gas close to the explosion site of the burst. the high metallicity of the host galaxy is also observed in absorption, where we detect several strong fe ii transitions as well as mg ii and mg i. although host galaxy absorption features are common in grb afterglow spectra, we detect absorption from strong metal lines directly in the host continuum (at a time when the afterglow was contributing to < 15 per cent). finally, we discuss the possibility that the geometry and state of the absorbing and emitting gas are indicative of a galactic scale outflow expelled at the final stage of two merging galaxies. | the luminous, massive and solar metallicity galaxy hosting the swift γ-ray burst grb 160804a at z = 0.737 |
we discuss the inhomogeneous behavior of gamma-ray burst afterglow light curves in optic. we use well-sampled light curves based on mostly our own observations to find and identify deviations (inhomogeneities) from broken power law. by the inhomogeneous behavior we mean flashes, bumps, slow deviations from power law (wiggles) in a light curve. in particular we report parameters of broken power law, describe phenomenology, compare optical light curves with x-ray ones and classify the inhomogeneities. we show that the duration of the inhomogeneities correlates with their peak time relative to gamma-ray burst (grb) trigger and the correlation is the same for all types of inhomogeneities. | inhomogeneities in the light curves of gamma-ray bursts afterglow |
the x-ray plateaus observed in the afterglows of some short gamma-ray bursts (grbs) are usually interpreted as the contribution from newborn magnetars with magnetic dipole radiation. however, fan et al. re-examined that interpretation and found that a rapidly rotating magnetar might lose most of the angular momentum to gravitational waves because the total energy released within the x-ray plateau phases in some short grbs was much smaller than that expected in the magnetar model, which indicates that the signal of gravitational-wave radiation (gwr) might have already existed in current electromagnetic data of short grbs. in addition, theoretical analysis also showed that the rotational energy of newborn magnetars was carried away via gwr, which was more efficient than magnetic dipole radiation and the shape of the light curve of grb can be adjusted from the magnetar model merely by considering magnetic dipole radiation. with this motivation, we investigate the x-ray light curve of grb 090510 in detail and find that the x-ray light curve can be well fit with a magnetar model supposing that the spin evolution of the magnetar is governed by both gravitational radiation generated by unstable r-mode oscillations at early-time and magnetic dipole radiation dominating late-time spin-down evolution. this result suggests that the r-mode instability is likely to play an important role in the evolution of the post-merger remnant. we also discuss the detectability of the r-mode gwr emitted by the long-lived remnant of grb 090510. | signature of r-mode gravitational-wave emission in the x-ray afterglow of short grb 090510 |
we obtained well-sampled optical photometry of grb 110213a, including swift/uvot and xrt. combining our data from those of other ground-based telescopes, we present 15 optical multicolor light curves showing similar shapes with two peaks. in contrast, in the x-ray band, only a single peak is observed between the two optical peaks. temporal and spectral analysis of grb 110213a shows that the x-rays differ from the optical for phases i-iii (before the second peak of the optical band at ~5.6 ks). moreover, they have the same spectral behavior at late times (phases iv-vi). these data indicate that the optical and x-ray emission are dominated by different components. the synchrotron-supported pair cascade emission is included in the standard external forward-shock model, which is dominated by synchrotron radiation and synchrotron self-compton (ssc). we find that the optical bands of grb 110213a are dominated by the cascade emission from synchrotron radiation + ssc at the early stage, while the primary synchrotron + ssc radiation dominates the x-ray band. at late stages, both the x-ray and optical bands are dominated by emission from primary synchrotron + ssc radiation. the cascade component can reasonably explain the first optical peak. in contrast, the primary synchrotron + ssc emission mainly contributes to the second peak. | grb 110213a: a study of afterglow electromagnetic cascade radiation |
we present a time-resolved spectral analysis of bright x-ray flares observed by swift x-ray telescope from 2004 december to 2021 december. we selected 58 bright flares from 56 gamma-ray burst x-ray afterglows to perform time-resolved spectral analysis. we find that there is a strong spectral evolution in x-ray flares. spectral evolution exists in all the x-ray flares. we have not found flares without spectral evolutions in our samples. the spectrum of an x-ray afterglow is dominated by the flare when it exists. four spectral evolution patterns were found, i.e., hard-to-soft, intensity tracking, soft-to-hard, and anti-intensity tracking. most of the flares (46 flares, 79.3%) show a spectral evolution from hard to soft. five (8.6%) flares are intensity tracking. two (3.5%) flares show soft-to-hard. five (8.6%) flares show anti-intensity tracking. therefore, hard-to-soft spectral evolution patterns dominate the spectral evolutions of x-ray flares. in other words, the hard-to-soft spectral evolution pattern is the main evolution pattern of x-ray flares. hard-to-soft, intensity tracking, and soft-to-hard spectral evolution patterns are the same as prompt emission spectral evolutions, indicating that the origin of flares should be the same as prompt emissions and the flares are the lower energy band emissions of the prompt emissions. the spectral evolution intensities are independent of the peak time and fwhm of the flares. in other words, the spectral evolution intensity is irrelevant to the flare occurrence time and its duration but related to the peak fluxes of flares. this means that the more luminous the flares, the stronger the spectral evolutions. | the spectral evolution patterns and implications of gamma-ray burst x-ray flares |
short gamma-ray bursts (sgrbs) are widely believed to be from mergers of binary compact objects involving at least one neutron star and hence have a broad range of spatial offsets from their host galaxies. in this work, we search for possible correlations between the emission properties of 18 sgrbs and their offsets from the host galaxies. the sgrbs with and without extended emission do not show significant differences between their offset distributions, in agreement with some previous works. there are, however, possible correlations between the optical and x-ray afterglow emission and the offsets. the underlying physical origins are examined. | possible correlations between the emission properties of sgrbs and their offsets from the host galaxies |
two-thirds of long duration gamma-ray bursts (grbs) show soft x-ray absorption in excess of the milky way. the column densities of metals inferred from uv and optical spectra differ from those derived from soft x-ray spectra, at times by an order of magnitude, with the latter being higher. the origin of the soft x-ray absorption excess observed in grb x-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the grb host galaxy on our x-ray afterglow observations. nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at z> 2), or they have required fairly extreme conditions to be present within the absorbing material. in this paper, we investigate the absorption of the grb afterglow by a collisionally ionised and turbulent interstellar medium (ism). we find that a dense (3 cm-3) collisionally ionised ism could produce uv/optical and soft x-ray absorbing column densities that differ by a factor of 10. however the uv/optical and soft x-ray absorbing column densities for such sightlines are 2-3 orders of magnitude lower in comparison to the grb afterglow spectra. for those grbs with a larger soft x-ray excess by up to an order of magnitude, the contribution in absorption from a turbulent ism as considered here would ease the required conditions of additional absorbing components, such as the grb circumburst medium and intergalactic medium. | soft x-ray absorption excess in gamma-ray burst afterglow spectra: absorption by turbulent ism |
the recent discovery of gravitational waves from gw170817, associated with a short gamma-ray burst (grb) at a distance of 40 mpc, has demonstrated that short grbs can occur locally and at a reasonable rate. furthermore, gravitational waves enable us to detect close-by grbs, even when we are observing at latitudes far from the jet's axis. we consider here compton echoes, the scattered light from the prompt and afterglow emission. compton echoes, an as yet undetected counterpart of grbs, peak in x-rays and maintain a roughly constant flux for hundreds to thousands of years after the burst. though too faint to be detected in typical cosmological grbs, a fraction of close-by bursts with a sufficiently large energy output in x-rays, and for which the surrounding medium is sufficiently dense, may indeed be observed in this way. the detection of a compton echo could provide unique insight into the burst properties and the environment's density structure. in particular, it could potentially determine whether or not there was a successful jet that broke through the compact binary merger ejecta. we discuss here the properties and expectations from compton echoes and suggest methods for detectability. | compton echoes from nearby gamma-ray bursts |
the statistics of x-ray flares in the afterglow of gamma-ray bursts (grbs) have been studied extensively without considering the possible different origins of each flare. by satisfying six observational criteria, we find a sample composed of 16 long grbs observed by \textit{swift} satellite may share a same origin. by applying the markov chain monte carlo iteration and the machine learning algorithms (locally weighted regression and gaussian process regression), impressively, the flares in these grbs show strong correlations with the energy released in the prompt emission. these correlations were never discovered in previous papers, and they could not be well explained by previous models. these correlations imply that the prompt emission and the x-ray flare are not independent, they may be originated following a same sequence. the new thesus satellite will provide us a larger sample and more detailed spectra to refine the results we obtained in this article. | revisiting the statistics of x-ray flares in gamma-ray bursts. |
the infra-red telescope (irt) on board theseus will play a key role in identifying and characterizing moderate to high redshift gamma-ray bursts afterglows. the irt enables the on board grb autonomous redshift determination, thus allowing the ground based telescopes to be triggered on a redshift pre-selected sample and finally fulfilling the cosmological goals of the mission. the irt will be composed by a primary mirror of 0.7 m of diameter coupled to a single camera in a cassegrain design. it will work in the 0.7-1.8 mu m wavelength range, and will provide a 10×10 arc min imaging field of view with sub-arc second localization capabilities, and, at the same time, a 5×5 arc min field of view with moderate (r up to ∼500) spectroscopic capabilities. its sensitivity, mainly limited by the satellite jitter, is adapted to detect all the grbs, localized by the theseus sxi/xgis instruments, and to acquire spectra for the majority of them. | the infra-red telescope on board the theseus mission |
we show that the weibel or currente filamentation instability can lead to the emission of circularly polarized radiation. using particle-in-cell (pic) simulations and a radiation post-processing numerical algorithm, we demonstrate that the level of circular polarization increases with the initial plasma magnetization, saturating at ~13% when the magnetization, given by the ratio of magnetic energy density to the electron kinetic energy density, is larger than 0.05. furthermore, we show that this effect requires an ion-electron mass ratio greater than unity. these findings, which could also be tested in currently available laboratory conditions, show that the recent observation of circular polarization in gamma ray burst afterglows could be attributed to the presence of magnetized current filaments driven by the weibel or the current filamentation instability. | magnetized current filaments as a source of circularly polarized light |
we present the statistical analysis of the properties of gamma-ray bursts with measured host galaxy redshifts and peaked optical light curves in proper frames of reference. the optical transients are classified by comparing the time lag of the optical peak relative to the grb trigger with the duration of the gamma-ray emission itself. the results of the correlation analysis of all possible pairs of energy, spectral, and temporal characteristics of both gamma-ray and optical emissions are given. we specify the pairs of the parameters with correlation coefficients greater than 50% at significance levels better than 1%. the following empirical relations, obtained for the first time, are specifically discussed: a correlation between the peak optical afterglow r band luminosity and redshift lr∝ (z +1)5.39±0.74 and a correlation between the peak luminosity of the prompt optical emissions and the time of the peak lr∝ t peak -3.85±1.22 . we also analyze the similarity of the relationships between the peak optical luminosity and the isotropic equivalent of the total energy of gamma-ray bursts for afterglows ( lr∝ e iso 1.06±0.22 ) and for prompt optical emissions ( lr∝ e iso 1.59±0.21 ). | statistical analysis of the parameters of gamma-ray bursts with known redshifts and peaked optical light curves |
the x-ray afterglows of almost one-half of gamma-ray bursts have been discovered by the swift satellite to have a shallow decay phase of which the origin remains mysterious. two main models have been proposed to explain this phase: relativistic wind bubbles (rwbs) and structured ejecta, which could originate from millisecond magnetars and rapidly rotating black holes, respectively. based on these models, we investigate polarization evolution in the shallow decay phase of x-ray and optical afterglows. we find that in the rwb model, a significant bump of the polarization degree evolution curve appears during the shallow decay phase of both optical and x-ray afterglows, while the polarization position angle abruptly changes its direction by 90°. in the structured ejecta model, however, the polarization degree does not evolve significantly during the shallow decay phase of afterglows whether the magnetic field configuration in the ejecta is random or globally large-scale. therefore, we conclude that these two models for the shallow decay phase and relevant central engines would be testable with future polarization observations. | testing models for the shallow decay phase of gamma-ray burst afterglows with polarization observations |
we perform a comprehensive stacking analysis of data collected by the fermi large area telescope (lat) of γ-ray bursts (grbs) localized by the swift spacecraft, which were not detected by the lat but which fell within the instrument’s field of view at the time of trigger. we examine a total of 79 grbs by comparing the observed counts over a range of time intervals to that expected from designated background orbits, as well as by using a joint likelihood technique to model the expected distribution of stacked counts. we find strong evidence for subthreshold emission at mev to gev energies using both techniques. this observed excess is detected during intervals that include and exceed the durations typically characterizing the prompt emission observed at kev energies and lasts at least 2700 s after the co-aligned burst trigger. by utilizing a novel cumulative likelihood analysis, we find that although a burst’s prompt γ-ray and afterglow x-ray flux both correlate with the strength of the subthreshold emission, the x-ray afterglow flux measured by swift’s x-ray telescope at 11 hr post trigger correlates far more significantly. overall, the extended nature of the subthreshold emission and its connection to the burst’s afterglow brightness lend further support to the external forward shock origin of the late-time emission detected by the lat. these results suggest that the extended high-energy emission observed by the lat may be a relatively common feature but remains undetected in a majority of bursts owing to instrumental threshold effects. | fermi lat stacking analysis of swift localized grbs |
rapid spectroscopy of grb afterglows is an important and hard task. based on the archival xs-grb spectral database, we here perform a pilot study on the afterglow's spectroscopy in the forthcoming svom era in two aspects. first, our simulation indicates that the color acquired from the svom/vt blue and red channels is effective in discriminating between low-z (z ≲ 3) and distant grb candidates until z ∼ 6. second, by doubling the sample size, we find that the previously proposed global photoionization response of the circumburst gas to the prompt emission (i.e., the civ/cii- ${l}_{{\rm{iso}}}/{e}_{{\rm{peak}}}^{2}$ relationship) is roughly confirmed, although the confirmation is dissatisfactory at the large ${l}_{{\rm{iso}}}/{e}_{{\rm{peak}}}^{2}$ end. we believe that this issue can be further addressed in the svom era by a larger spectroscopy sample, thanks to its capability of rapid identification of optical candidates of afterglow and its anti-solar pointing strategy. | a pilot study of catching high-z grbs and exploring circumburst environment in the forthcoming svom era |
aims: we present the updated census and statistics of lyman-α emitting long gamma-ray burst host galaxies (lae-lgrbs). we investigate the properties of a subsample of lae-lgrbs and test the shell model that is commonly used to fit lyman-α (lyα) emission line spectra.methods: we considered all lgrb host galaxies with relevant publicly available information. we defined a golden sample of four lae-lgrbs (grbs 011211, 021004, 060926, and 070110) with afterglow and host galaxy observations allowing us to constrain the shell modeling of the lyα line.results: the census results in 29 detected lae-lgrbs. we present 5 new lyα emission detections in host-galaxy spectra and the corresponding unpublished vlt/x-shooter data (grbs 060926, 070110, 081121, 081222, and 100424a). from the comparison of the statistics and properties of lae-lgrbs to those of lae samples in the literature, we find evidence of lyα suppression in dusty systems. the fraction of lae-lgrbs in the overall lgrb hosts is lower than that found for lyman-break galaxy (lbg) samples at similar redshift range. this result can arise because the selection criteria of the parent samples are different and the spectral observations of lgrb samples are shallower than those of lbg. however, we find that lae-lgrbs are representative of lyα emission from the bulk of uv-selected galaxies at z ∼ 2. we find that the golden sample of lae-lgrbs we studied consists of complex systems characterized by multiple emission blobs and by signs of possible galaxy interactions. the fitting procedure recovers the hi column densities (nhi) measured from the afterglow spectra and the other properties described by the shell-model parameters in the two low-nhi cases, but it fails to do this in the other two cases with high nhi. the afterglows of most lgrbs and lae-lgrbs show log(nhi/cm−2) > 20.3, implying that statistically, the bulk of lyα photons that is expected to be produced by massive stars in the star-forming region hosting the grb will be surrounded by these opaque lines of sight. we therefore interpret our results in the context of more sophisticated models and of different dominant lyα-emitting regions. we also compare lae-lgrbs to lae lyman continuum (lyc) leakers in the literature in terms of the properties that are identified as possible indirect indicators of lyc leakage. we find that only one lgrb (grb 021004) would likely be a strong lyc leaker and discuss the validity of these indicators at high redshift. while our work shows that lgrbs are useful tools for probing laes and radiative transfer models, larger statistics are required to strengthen our findings. the reduced spectra presented in table 3 are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/653/a83 | gamma-ray bursts as probes of high-redshift lyman-α emitters and radiative transfer models |
early shallow-decaying x-ray afterglows of gamma-ray bursts (grbs) may be attributed to the dipole radiations of newly born magnetars. assuming that the grb jets powered by magnetars are quasi-universal, we find that the jet structure can be parameterized as a uniform jet with a luminosity of $\mathrm{log}{l}_{{\rm{j}}}/\mathrm{erg}\ {{\rm{s}}}^{-1}={52.68}_{-0.33}^{+0.76}$ (1σ) and an opening angle ${\theta }_{{\rm{j}}}={2.10}_{-1.28}^{+1.90}$ (50% confidence level), surrounding by a power-law decay component with an index of $-{4.00}_{-0.37}^{+0.27}$ (1σ). the inferred local grb rate is ρ = 9.6 gpc-3 yr-1 by including both the typical grbs and ll-grbs as the same population. the typical viewing angle is 3.3°, and may be 20° ∼ 30° for ll-grbs. the x-ray luminosity function of the dipole radiation wind can be empirically described by a broken power-law function with indices ${\beta }_{1}={0.78}_{-0.15}^{+0.16}$ and ${\beta }_{2}\gt 1.6$ broken at $\mathrm{log}{l}_{{\rm{b}},{\rm{w}}}/\mathrm{erg}\ {{\rm{s}}}^{-1}={48.51}_{-0.65}^{+0.53}$ . in the case where the wind outflow is collimated and coaxial with the grb jet, we find that the wind structure is similar to the grb jet, i.e., $\mathrm{log}{l}_{{\rm{c}},{\rm{w}}}/\mathrm{erg}\,{{\rm{s}}}^{-1}={48.38}_{-0.48}^{+0.30}$ , ${\theta }_{{\rm{c}},{\rm{w}}}={{2.65}^{o}}_{-{1.19}^{o}}^{+1.{73}^{o}}$ , and ${k}_{{\rm{w}}}={4.57}_{-0.75}^{+1.21}$ . the observed correlation between the prompt gamma-ray luminosity and x-ray luminosity of the wind may result from the viewing angle effect in such a jet-wind system. a discussion on surveys using the x-ray instruments on board the einstein probe mission in the soft x-ray band for the jet and wind emission is also presented. | magnetar as central engine of gamma-ray bursts: quasi-universal jet, event rate, and x-ray luminosity function of dipole radiations |
gamma-ray bursts, associated with the collapse of massive stars or the collisions of compact objects, are the most luminous events in our universe. however, there is still much to learn about the nature of the relativistic jets launched from the central engines of these objects. we examine how jet structure-that is, the energy and velocity distribution as a function of angle-affects observed grb afterglow light curves. using the package afterglowpy, we compute light curves arising from an array of possible jet structures, and present the suite of models that can fit the coincident electromagnetic observations of gw190814 (which is likely due to a background agn). our work emphasizes not only the need for broadband spectral and timing data to distinguish among jet structure models, but also the necessity for high resolution radio follow-up to help resolve background sources that may mimic a grb afterglow. | understanding gamma-ray burst jet structures from afterglow light curves |
gamma-ray bursts (grbs) are among the most luminous sources in the universe. the nature of their emission at tev energies is one of the most relevant open issues related to these events. the temporal and spectral features inferred from the early and late emissions usually known as prompt and afterglow, respectively, can be interpreted within the context of the fireball model. the synchrotron self-compton process is expected during the afterglow phase. we explain how the theoretical ssc light curves can be compared with hypothetical upper limit located at z=0.3. we show the allowed parameter space of the microphysical parameters and density of the circumburst medium. the most restrictive results are obtained when the ssc process lies in the fast cooling regime | constraints on the very high energy gamma-ray emission with hawc |
we study the gamma-ray burst (grb) afterglow light curves produced by an off-axis jet in a stratified circumburst medium and summarize the temporal indices of the coasting phase, the deceleration phase, the newtonian phase, and the deep newtonian phase for various viewing angles and power-law indices of medium density. generally, the afterglow light curves of off-axis grbs in the homogeneous interstellar medium have a steep rise arising due to jet deceleration. in the stratified medium, the flux rises is more shallow but peaks earlier for the same viewing angle due to faster deceleration of the jet running into the denser stratified medium, compared with the case of the interstellar medium (ism). observations of off-axis bursts will possibly increase over the coming years due to the arrival of the multi-messenger era and the forthcoming surveys in multiple bands. the temporal indices of off-axis afterglows derived in the paper will provide a reference for comparison with the observations and can diagnose the circumburst environment. the numerical code calculating the afterglow light curve also can be used to fit the multi-wavelength light curves. | afterglow light curves from off-axis grb jets in stratified circumburst medium |
giant flares are the brightest and rarest outbursts from magnetars, with isotropic energies of 1044-1046 erg. grb 200415a is suggested as a magnetar giant flare from ngc 253. fermi large area telescope detected the gev afterglow emission from the flare, which is the first time that the gev emission is detected from a giant flare. in this paper, we study the multiwavelength afterglow radiation of electrons accelerated by the forward and reverse shocks, produced via interactions between the ejecta and the circumburst medium in the afterglow phase of grb 200415a. we found that in the gev band, the forward shock emission is usually higher than the reverse shock emission, and can explain observations well, while in the optical and infrared bands, contributions from the forward and reverse shocks can be comparable for reasonable parameter sets, and the brightness of the forward-shock emission can reach 20 mag (ab). we predict that future telescopes such as wide field survey telescope, chinese space station telescope, the large synoptic survey telescope, and james webb space telescope can detect the optical and infrared afterglow emission from giant flares similar to grb 200415a. in addition, we consider two cases where the ejecta are dominated by protons or electron-positron pairs. we find that the reverse shock emission is comparable in these two cases for a hard electron spectrum, while for a soft electron spectrum, the reverse shock emission is much weaker in the pair-dominating case.. | the multiwavelength afterglow emission of magnetar giant flare-like event grb 200415a |
at 23:59:25 ut, the swift burst alert telescope (bat) triggered and located grb 210619b (trigger=1056757). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 319.713, +33.860 which is ra(j2000) = 21h 18m 51s dec(j2000) = +33d 51' 35" 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 100 sec. the peak count rate was ~100,000 counts/sec (15-350 kev), at ~0 sec after the trigger. the xrt began observing the field at 00:04:53.4 ut, 328.1 seconds after the bat trigger. xrt found a bright, uncatalogued x-ray source located at ra, dec 319.7161, 33.8495 which is equivalent to: ra(j2000) = 21h 18m 51.86s dec(j2000) = +33d 50' 58.2" with an uncertainty of 11.1 arcseconds (radius, 90% containment). this location is 38 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 4.27e-09 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 135 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 21:18:52.39 = 319.71831 dec(j2000) = +33:51:01.6 = 33.85044 with a 90%-confidence error radius of about 0.61 arc sec. this position is 7.5 arc sec. from the center of the xrt error circle. the estimated magnitude is 14.71 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.173. burst advocate for this burst is p. d'avanzo (paolo.davanzo at inaf.it). 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 210619b: swift detection of a bright burst and optical counterpart |
we used the 1m jacobus kapteyn telescope (jkt) at la palma, remotely operated by the sara consortium, to obtain bvri photometry of the afterglow of grb 221009a (swift j1913.1+1946; atel #15650) on october 10 ut. in image series from oct. 10.83-10.88, 30 hours after the initial burst detection, we find b=22.3, v=19.6, r=18.5, i=17.6. | bvri photometry of the grb 220019a afterglow |
observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (grbs). we report our optical observations of grb 111228a from 95 s to about 50 hr after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data from the swift and fermi missions. our joint optical and x-ray spectral fits to the afterglow data show that the ambient medium features a low dust-to-gas ratio. incorporating the energy injection effect, our best fit to the afterglow light curves with the standard afterglow model via the markov chain monte carlo technique shows that {ɛ }e=(6.9+/- 0.3)× {10}-2, {ɛ }b=(7.73+/- 0.62)× {10}-6,{e}k=(6.32+/- 0.86)× {10}53 {erg}, n=0.100+/- 0.014 cm-3. the low medium density likely implies that the afterglow jet may be in a halo or in a hot ism. a chromatic shallow decay segment observed in the optical and x-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. the ep of its time-integrated prompt gamma-ray spectrum is ∼26 kev. using the initial lorentz factor ({{{γ }}}0={476}-237+225) derived from our afterglow model fit, it is found that grb 111228a satisfies the {l}{{iso}}-{e}p,z-{{{γ }}}0 relation and bridges the typical grbs and low luminosity grbs in this relation. | multi-wavelength observations of grb 111228a and implications for the fireball and its environment |
at 19:51:27 ut, the swift burst alert telescope (bat) triggered and located grb 210610b (trigger=1054681). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 243.939, +14.391 which is ra(j2000) = 16h 15m 45s dec(j2000) = +14d 23' 29" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat light curve showed a comlex structure with a duration of about 100 sec. the peak count rate was ~11000 counts/sec (15-350 kev), at ~8 sec after the trigger. the xrt began observing the field at 19:52:51.4 ut, 83.9 seconds after the bat trigger. xrt found a bright, uncatalogued x-ray source located at ra, dec 243.9180, 14.3977 which is equivalent to: ra(j2000) = 16h 15m 40.32s dec(j2000) = +14d 23' 51.7" with an uncertainty of 6.7 arcseconds (radius, 90% containment). this location is 77 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 0.1 s image was 1.07e-08 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 91 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 16:15:40.41 = 243.91836 dec(j2000) = +14:23:56.7 = 14.39909 with a 90%-confidence error radius of about 0.61 arc sec. this position is 5.2 arc sec. from the center of the xrt error circle. the estimated magnitude is 13.70 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.044. burst advocate for this burst is k. l. page (klp5 at leicester.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 210610b: swift detection of a burst with bright optical afterglow |
aims: we characterize a sample of gamma-ray bursts with low luminosity x-ray afterglows (lla grbs), and study their properties. method: we select a sample consisting of the 12\% faintest x-ray afterglows from the total population of long grbs (lgrbs) with known redshift. we study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy and peak energy) to assess whether they belong to the same population than the brighter afterglow events. results: we present strong evidences that these events belong to a population of nearby events, different from that of the general population of lgrbs. these events are faint during their prompt phase, and include the few possible outliers of the amati relation. out of 14 grb-sn associations, 9 are in lla grb sample, prompting for caution when using sn templates in observational and theoretical models for the general lgrbs population. | properties of low luminosity afterglow gamma-ray bursts |
we report an optical spectrum on the afterglow of the extremely strong, long gamma-ray burst grb221009a/swift j1913.1+1946 (atel #15653, #15655, #15660, #15663, #15671, #15685) taken with the lrs2-r spectrograph (wavelength range: 6450-10500 angstroms, resolving power: 1800) of the 10m hobby-eberly telescope at mcdonald observatory on 2022-10-21.1 ut. the spectrum shows a redward slope due to high extinction of milky way dust. | detection of the emerging supernova spectrum from the afterglow of grb221009a |
the immensely bright and intrinsically simple afterglow spectra of gamma-ray bursts (grbs) have proven to be highly effective probes of the interstellar dust and gas in distant, star forming galaxies. despite significant progress, many aspects of the host galaxy attenuating material are still poorly understood. there is considerable discrepancy between the amount of x-ray and optical afterglow absorption, with the former typically an order of magnitude higher than that expected from the optical line absorption of neutral element species. similar inconsistencies exist between the abundance of interstellar dust derived from spectroscopic and photometric data, and the relation between the line of sight and integrated host galaxy interstellar medium (ism) remains unclear. in this paper we review our current understanding on the attenuation properties of the ism within grb host galaxies, and present some recent analysis that tries to consolidate multi-wavelength photometric and spectroscopic observations. | gamma-ray burst afterglows as probes of the ism |
x-ray flares are the most common phenomena in the afterglow phase of gamma-ray bursts (grbs) in the swift era, and are known as a canonical component in x-ray afterglows. in this work, we constrain the lorentz factor of x-ray flares with an updated sample. we extensively search for x-ray light curves showing flare and jet break simultaneously. a smooth broken power law function is used to fit the jet breaks in 11 grbs. we also use a smooth broken power law function to fit the profile of x-ray flares, and the total number of the flares is 20. we obtain the lower and upper limits of lorentz factor ({{{γ }}}{{x}}) with the timescale, half-opening angle and mean luminosity of the x-ray flares for interstellar medium (ism) and wind cases. the lower limits on {{{γ }}}{{x}} range from tens to a few hundred, and the upper limits are mainly about a few hundred. we also apply the limited lorentz factor to test correlations of {{{γ }}}0-{e}γ,{iso} and {{{γ }}}0-{l}γ,{iso} for grbs, and find x-ray flares in the ism case are much more consistent with those of prompt emission than the wind case in a statistical sense for both correlations. x-ray flares are almost consistent with the trend in the correlations of {{{γ }}}0-{e}γ,{iso}({l}γ,{iso}) for prompt grbs, indicating x-ray flares and prompt bursts may have the same physical origin. | the updated bulk lorentz factors of gamma-ray burst x-ray flares |
burst alert robotic telescope (bart) is an autonomous robotic observatory located at the ondřejov observatory in the czech republic. it was constructed in 1996 in order to follow up satellite triggers in hope to provide early observations of gamma‑ray burst optical afterglows. we present the new telescope of the observatory "small binocular telescope" (sbt), which has recently obtained its first light and is being prepared for routine observations. the updated system now counts with a dual 0.2‑m wide‑field optical telescope capable of continuous coverage of an error box of size up to 3.56 × 3.56 degrees. | small binocular telescope: the new epoch of burst alert robotic telescope |
we use joint observations by the neil gehrels swift x-ray telescope (xrt) and the fermi large area telescope (lat) of gamma-ray burst (grb) afterglows to investigate the nature of the long-lived high-energy emission observed by fermi lat. joint broadband spectral modeling of xrt and lat data reveal that lat nondetections of bright x-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the lat and/or xrt energy ranges. such a break is sufficient to suppress the high-energy emission so as to be below the lat detection threshold. by contrast, lat-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the xrt and lat energy ranges. we speculate that the primary difference between grbs with lat afterglow detections and the non-detected population may be in the type of circumstellar environment in which these bursts occur, with late-time lat detections preferentially selecting grbs that occur in low wind-like circumburst density profiles. furthermore, we find no evidence of high-energy emission in the lat-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed x-ray emission. the lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-compton (ssc) component in the lat energy range. alternatively, the peak of the ssc emission could be beyond the 0.1-100 gev energy range considered for this analysis. | investigating the nature of late-time high-energy grb emission through joint fermi\\swift observations |
a simplified one dimensional grid is used to model the evolution of magnetized plasma flow. we implement diffusion laws similar to those so-far used to model magnetic reconnection with cellular automata. as a novelty, we also explicitly superimpose a background flow. the aim is to numerically investigate the possibility that self-organized criticality appears in a one dimensional magnetized flow. the cellular automaton's cells store information about the parameter relevant to the evolution of the system being modelled. under the assumption that this parameter stands for the magnetic field, the magnetic energy released by one grid cell during one individual relaxation event is also computed. our results show that indeed in this system self-organized criticality is established. the possible applications of this model to the study of the x-ray afterglows of grbs is also briefly considered. | self organized criticality in an one dimensional magnetized grid. application to grb x-ray afterglows |
recent observations by the swift satellite have revealed long-lasting ($\sim 10^2-10^5\,\mathrm{s}$), "plateau-like" x-ray afterglows in the vast majority of short gamma-ray bursts events. this has put forward the idea of a long-lived millisecond magnetar central engine being generated in a binary neutron star (bns) merger and being responsible for the sustained energy injection over these timescales ("magnetar model"). we elaborate here on recent simulations that investigate the early evolution of such a merger remnant in general-relativistic magnetohydrodynamics. these simulations reveal very different conditions than those usually assumed for dipole spin-down emission in the magnetar model. in particular, the surrounding of the newly formed ns is polluted by baryons due to a dense, highly magnetized and isotropic wind from the stellar surface that is induced by magnetic field amplification in the interior of the star. the timescales and luminosities of this wind are compatible with early x-ray afterglows, such as the "extended emission". these isotropic winds are a generic feature of bns merger remnants and thus represent an attractive alternative to current models of early x-ray afterglows. further implications to bns mergers and short gamma-ray bursts are discussed. | magnetically-induced outflows from binary neutron star merger remnants |
molecular species, most frequently h2, are present in a small, but growing, number of gamma-ray burst (grb) afterglow spectra at redshifts z ∼ 2-3, detected through their rest-frame uv absorption lines. in rare cases, lines of vibrationally excited states of h2 can be detected in the same spectra. the connection between afterglow line-of-sight absorption properties of molecular (and atomic) gas, and the observed behaviour in emission of similar sources at low redshift, is an important test of the suitability of grb afterglows as general probes of conditions in star formation regions at high redshift. recently, emission lines of carbon monoxide have been detected in a small sample of grb host galaxies, at sub-mm wavelengths, but no searches for h2 in emission have been reported yet. in this paper we perform an exploratory search for rest-frame k band rotation-vibrational transitions of h2 in emission, observable only in the lowest redshift grb hosts (z ≲ 0.22). searching the data of four host galaxies, we detect a single significant rotation-vibrational h2 line candidate, in the host of grb 031203. re-analysis of spitzer mid-infrared spectra of the same grb host gives a single low significance rotational line candidate. the (limits on) line flux ratios are consistent with those of blue compact dwarf galaxies in the literature. new instrumentation, in particular on the jwst and the elt, can facilitate a major increase in our understanding of the h2 properties of nearby grb hosts, and the relation to h2 absorption in grbs at higher redshift. | infrared molecular hydrogen lines in grb host galaxies |
depletion studies provide a way to understand the chemical composition of interstellar dust grains. we here examine 23 gamma-ray bursts (grbs) optical afterglow spectra (spanning 0.6 ≤ z ≤ 5.0) and compare their silicon and iron dust-phase column densities with different extinction curve parameters to study the composition of the interstellar dust grains in these high-redshift grb host galaxies. the majority of our sample (87 per cent) show featureless extinction curves and only vary in shape. we observe strong correlations (with > 96 per cent significance) between the total-to-selective extinction, rv, and the dust-phase column densities of si and fe. since a large fraction of interstellar iron is locked in silicate grains, this indicates that high si and fe depletion leads to an increase in the fraction of large silicate grains and vice versa. this suggests that silicates play a vital role to induce the entire extinction at any wavelength. on the other hand, the far-ultraviolet (uv) extinction is usually attributed to the presence of small silicates. however, we find no trend between the far-uv parameter of the extinction curve, c4, and the abundance of si and fe in the dust phase. we, therefore, propose that the far-uv extinction could be a combined effect of small (probably nanoparticles) dust grains from various species. | silicon and iron dust in gamma-ray burst host galaxy absorbers |
we propose a method to detect possible non-stationarities of gamma-ray burst jets. assuming that the dominant source of variability in the prompt gamma light curve is the non-stationarity of the jet, we show that there should be a connection between the variability measure and the characteristic angle of the jet derived from the jet break time of the afterglow. we carried out monte carlo simulations of long gamma-ray burst observations assuming three radial luminosity density profiles for jets and randomizing all burst parameters, and created samples of gamma light curves by simulating jets undergoing brownian motions with linear restoring forces. we were able to demonstrate that the connection between the variability and the characteristic angle is an anticorrelation in case of uniform and power-law jet profiles, and a correlation in case of a gaussian profile. we have found that as low as 50 (144) gamma-ray burst observations with jet angle measurements can be sufficient for a 3σ (5σ) detection of the connection. the number of observations required for the detection depends on the underlying jet beam profile, ranging from 50 (144) to 237 (659) for the four specific profile models we tested. | a statistical method to detect non-stationarities of gamma-ray burst jets |
we report on master optical observations of an afterglow-like optical and x-ray transient at2021lfa/ztf21aayokph. we detected the initial steady brightening of the transient at 7σ confidence level. this allowed us to use smooth optical self-similar emission of grbs model to constrain the explosion time to better than 14 min as well as to estimate its initial lorentz factor γ0 = 20 ± 10. taking into consideration the low γ0 and non-detection in gamma-rays, we classify this transient as the first failed grb afterglow. | orphan optical flare as soss emission afterglow, localization in time |
context. among the large variety of astrophysical sources that we can observe, gamma-ray bursts (grbs) are the most energetic of the whole universe. their emission peaks in the γ-ray band, with a duration from a fraction of a second to a few hundred seconds, and is followed by an afterglow covering the whole electromagnetic spectrum. the definition of a general picture describing the physics behind grbs has always been a compelling task, but the results obtained so far from observations have revealed a puzzling landscape. the lack of a clear, unique paradigm calls for further observations and additional, independent techniques for this purpose. polarimetry constitutes a very useful example as it allows us to investigate some features of the source such as the geometry of the emitting region and the magnetic field configuration.aims: to date, only a handful of bursts detected by space telescopes have been accompanied by ground-based spectro-polarimetric follow-up, and therefore such an analysis of more grbs is of crucial importance in order to increase the sample of bursts with multi-epoch polarisation analysis. in this work, we present the analysis of the grb 080928 optical afterglow, with observations performed with the eso-vlt fors1 instrument.methods: starting from raw data taken in the imaging polarimetry (ipol) and spectro-polarimetry (pmos) modes, we performed data reduction, followed by the photometric analysis of ipol data, taken ~14 and ~40 h after the burst detection, and spectroscopy of pmos data (t ~ 14.95 h). after computing the reduced stokes parameters q/i and u/i, which describe the linear polarisation of the emitted radiation, we obtained the polarisation degree for the three observing epochs.results: we find that the grb optical afterglow was not significantly polarised on the first observing night. the polarisation degree (p) grew on the following night to a level of p ~ 4.5%, giving evidence of polarised radiation at a 4σ confidence level. the grb 080928 light curve is not fully consistent with standard afterglow models, making any comparison with polarimetric models partly inconclusive. the most conservative interpretation is that the grb emission was characterised by a homogeneous jet and was observed at an angle of 0.6 <θobs/θjet < 0.8. moreover, the non-zero polarisation degree on the second night suggests the presence of a dominant locally ordered magnetic field in the emitting region. | grb 080928 afterglow imaging and spectro-polarimetry |
high-precision measurement of the isotropic diffuse γ-ray background (igrb) has been extrapolated to the tev energy region using the fermi-lat experiment. various kinds of astrophysical sources are candidates for its origin. however, a consensus on the dominant source has been difficult to reach. recent observations of the γ-ray emission of γ-ray bursts (grbs), denoted as the synchrotron self-compton (ssc) component in the afterglow phase, in the sub-tev energy region by magic and hess experiments shed new light on this topic. in this work, we revisit the contribution from the ssc component of grbs to the igrb. first, a sample set of grb events is obtained and examined using observations from fermi-lat. second, the ssc component, described by the energy ratio rext and spectral index βext, is assigned to every grb event. we can obtain the total spectrum contribution based on this grb sample. we find that when rext and βext reach ∼20% and -1.6, respectively, the contribution from grb emission dominates in the energy region of hundreds of gev. we hope that the lhaaso and cta experiments under construction can observe a large number of grbs to fix those parameters in coming years. a surviving tail is expected, which can serve to check our calculations based on future satellite experiments such as herd and gamma400. | contribution of high-energy grb emissions to the spectrum of the isotropic diffuse γ-ray background |
at 11:55:21 ut, the swift burst alert telescope (bat) triggered and located grb 220826a (trigger=1121751). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 206.431, -44.044 which is ra(j2000) = 13h 45m 44s dec(j2000) = -44d 02' 39" 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 20 sec. the peak count rate was ~3500 counts/sec (15-350 kev), at ~7 sec after the trigger. the xrt began observing the field at 11:56:42.9 ut, 81.2 seconds after the bat trigger. using promptly downlinked data we find a bright, uncatalogued x-ray source located at ra, dec 206.41169, -44.03885 which is equivalent to: ra(j2000) = 13h 45m 38.81s dec(j2000) = -44d 02' 19.9" with an uncertainty of 3.5 arcseconds (radius, 90% containment). this location is 53 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 in excess of the galactic value (9.05 x 10^20 cm^-2, willingale et al. 2013), with an excess column of 2.8 (+2.60/-2.30) x 10^21 cm^-2 (90% confidence). uvot took a finding chart exposure of 150 seconds with the white filter starting 90 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 13:45:38.52 = 206.41049 dec(j2000) = -44:02:19.0 = -44.03862 with a 90%-confidence error radius of about 0.62 arc sec. this position is 3.3 arc sec. from the center of the xrt error circle. the estimated magnitude is 17.89 with a 1-sigma error of about 0.15. no correction has been made for the expected extinction corresponding to e(b-v) of 0.110. burst advocate for this burst is r. caputo (regina.caputo at 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 220826a: swift detection of a burst with an optical counterpart |
x-ray flares are frequently detected in the x-ray afterglow light curves and are highly correlated with the prompt emission of gamma-ray bursts (grbs). we compile a comprehensive sample of x-ray flares up to 2021 april, comprising 697 flares. we classify the total sample into four types: early flares (tp≤103 s), late flares (tp>103 s), long gamma-ray burst (lgrb) flares and short gamma-ray burst (sgrb) flares, and analyze the distributions and relationships of the flare parameters. it is found that the early flares have a higher frequency, shorter duration, and more asymmetrical structure. in addition, the distributions of the morphological parameters of the sgrb flares are similar to those of the lgrb flares. we also find that the durations and rising (decay) times of the early flares are positively correlated with the peak times, but the late flares follow the different dependent relations. there is a strong anti-correlation between the peak luminosities (lx,p) and the peak times of the flares, e.g., lx,p∝tp,z‑1.45 for the lgrb flares, and lx,p∝tp,z‑1.27 for the sgrb flares, respectively. furthermore, the peak luminosity is highly dependent on the isotropic energy (ex,iso) for the early lgrb flares, the best fit is lx,p∝ex,iso1.06(r=0.89). we also find a tight three-parameter correlation, lx,p∝tp,z‑1.03ex,iso0.92(r=0.96). all the late flares fall into the 3σ confidence region defined by the early flares. in terms of the point of kinematic arguments, both the sgrb and lgrb flares support a common scheme of internal origin. the sgrb flares have similar properties to the lgrb flares, suggesting that both of them share a similar physical mechanism from the late-time activity of central engine. | statistical properties of x-ray flares in gamma-ray bursts |
context. a thermal x-ray component is observed in the early afterglow of some gamma-ray bursts. possible explanations include shockwave breakout, relativistic photosphere, or emission from cocoon. the difficulties of these models are discussed.aims: we propose an alternative model that attributes such a thermal component to the interaction of the gamma-ray burst outflow with a baryonic material near the gamma-ray burst source.methods: the analytic model is based on relativistic energy-momentum conservation and a diffusion model for photons. the kinematic and observational properties of the supernova ejecta after the interaction are derived. in particular, the temperature and the duration of the thermal emission are obtained.results: the model is applied to a prototypical grb 090618 and other gamma-ray bursts associated with supernovae having thermal emission in the early afterglow. the mass of the baryonic material is found to be a few 10-4m⊙, indicating that this material can be a small fraction of the supernova ejecta. | thermal emission in the early afterglow of gamma-ray bursts from their interaction with supernova ejecta |
we study the electromagnetic radiation from a newborn magnetar whose magnetic tilt angle decreases rapidly. we calculate the evolution of the angular spin frequency, the perpendicular component of the surface magnetic field strength, and the energy loss rate through magnetic dipole radiation. we show that the spin-down of the magnetar experiences two stages characterized by two different timescales. the apparent magnetic field decreases with the decrease of the tilt angle. we further show that the energy loss rate of the magnetar is very different from that in the case of a fixed tilt angle. the evolution of the energy loss rate is consistent with the overall light curves of gamma-ray bursts which show a plateau structure in their afterglow stage. our model supports the idea that some gamma-ray bursts with a plateau phase in their afterglow stage may originate from newborn millisecond magnetars. supported by the national natural science foundation of china. | effect of rapid evolution of magnetic tilt angle on a newborn magnetar's dipole radiation |
we show that observed properties of the low luminosity grb171205a and its afterglow, like those of most other low-luminosity (ll) gamma ray bursts (grbs) associate with a supernova (sn), indicate that it is an ordinary sn-grb, which was produced by inverse compton scattering of glory light by a highly relativistic narrowly collimated jet ejected in a supernova explosion and viewed from a far off-axis angle. as such, vla/vlbi follow-up radio observations of a superluminal displacement of its bright radio afterglow from its parent supernova, will be able to test clearly whether it is an ordinary sn-grb viewed from far off-axis or it belongs to a distinct class of grbs, which are different from ordinary grbs, and cannot be explained by standard fireball models of grbs as ordinary grbs | origin of the far off-axis grb171205a |
the aim of the space mission concept theseus is to continue to collect and study the grb events like swift. it will allow us to study the early universe. moreover, it will offer us to study with unprecedented sensitivity grb emission and to measure the redshift for the bursts with z>5. in this work, we investigate the advantages of a optical and near-infrared telescope mounted on the same satellite that is triggered by the grb like theseus/irt. afterwards, we investigate the possible future developments in the grb science, first for the prompt phase and the for afterglow phase. we find that more than half of the sources detected by theseus, and will never be visible from a a ground-based telescope. moreover, only ∼50% of all observable sources are visible within one hour, i.e. <30% of all theseus transient sources. a higher number of observable sources can only be achieved with a network of telescopes. theseus will permit to detect the nir prompt phase of the longest grbs, increasing the number of events studied from gamma-rays to the near-infrared from a handful of events studied up to now to ≳10 grbs per year. | grb follow-up and science with theseus/irt . |
the number of long gamma-ray bursts (grbs) known to have occurred in the distant universe (z > 5) is small (∼15); however, these events provide a powerful way of probing star formation at the onset of galaxy evolution. in this paper, we present the case for grb 100205a being a largely overlooked high-redshift event. while initially noted as a high-z candidate, this event and its host galaxy have not been explored in detail. by combining optical and near-infrared gemini afterglow imaging (at t < 1.3 d since burst) with deep late-time limits on host emission from the hubble space telescope, we show that the most likely scenario is that grb 100205a arose in the range 4 < z < 8. grb 100205a is an example of a burst whose afterglow, even at ∼1 h post burst, could only be identified by 8-m class ir observations, and suggests that such observations of all optically dark bursts may be necessary to significantly enhance the number of high-redshift grbs known. | the case for a high-redshift origin of grb 100205a |
swift, chandra, and xmm have found a weak but nearly constant x-ray component from swift j1644+57 that appeared at ∼500 days and was visible at least until ∼1400 days after the stellar capture, which cannot be explained by standard tidal disruption theories. we suggest that this x-ray afterglow component may result from the thomson scattering between the primary x-rays and its surrounding plasma, i.e., a compton echo effect. similar phenomena have also been observed from molecular clouds in our galactic center, which were caused by the past activity of srg a*. if this interpretation of swift j1644+57 afterglow is correct, this is the first compton echo effect observed in the cosmological distances. | x-ray afterglow of swift j1644+57: a compton echo? |
grb 120308a, a long duration γ-ray burst (grb) detected by swift, was distinguished by a highly polarized early optical afterglow emission that strongly suggests an ordered magnetic field component in the emitting region. in this work, we model the optical and x-ray emission in the reverse and forward shock scenario and show that the strength of the magnetic field in the reverse-shock region is ~10 times stronger than that in the forward shock region. consequently, the outflow powering the highly polarized reverse-shock optical emission was mildly magnetized at a degree of σ ~ a few percent. considering the plausible magnetic energy dissipation in both the acceleration and prompt emission phases of the grb outflow, the afterglow data of grb 120308a provides us with compelling evidence that, at least for some grbs, a nonignorable fraction of the energy was released in the form of poynting flux, confirming the finding first made in the reverse-forward shock emission modeling of the optical afterglow of grb 990123 by fan et al. in 2002 and zhang et al. in 2003. | the magnetization degree of the outflow powering the highly polarized reverse-shock emission of grb 120308a |
the detection of optical re-brightenings and x-ray plateaus in the afterglows of gamma-ray bursts (grbs) challenges the generic external shock model. recently, we have developed a numerical method to calculate the dynamic of the system consisting of a forward shock and a reverse shock. here, we briefly review the applications of this method in the afterglow theory. by relating these diverse features to the central engines of grbs, we find that the steep optical re-brightenings would be caused by the fall-back accretion of black holes, while the shallow optical re-brightenings are the consequence of the injection of the electron-positron-pair wind from the central magnetar. these studies provide useful ways to probe the characteristics of grb central engines. | diverse features of the multiwavelength afterglows of gamma-ray bursts: natural or special? |
gamma-ray bursts (grbs) are the most violent explosions in the universe, releasing a huge amount of energy in few seconds. while our understanding of the prompt and the afterglow phases has increased with swift and fermi, we have very few information about their high energy (he, e <~ 100) emission components. this requires a ground-based experiment able to perform fast follow-up with enough sensitivity above ~ 50 gev. the magic (major atmospheric gamma-ray imaging cherenkov) telescopes have been designed to perform fast follow-up on grbs thanks to fast slewing movement and low energy threshold (~ 50 gev). since the beginning of the operations, magic followed-up 89 grbs in good observational conditions. in this contribution the magic grbs follow-up campaign and the results which could be obtained by detecting he and very high energy (vhe, e >~ 100 gev) γ-rays from grbs will be reviewed. | search for high energy emission from grbs with magic |
we demonstrate the use of the akari fis all-sky survey maps in the study of extragalactic objects. a quick but reliable estimate of the galactic foreground is essential for extragalactic research in general. we explored the galactic foreground and calculated hydrogen column densities using akari fis and other recent all-sky survey data, and compared our results to former estimates. our akari-fis-based foreground values were then used toward gamma-ray burst (grb) sources as input for x-ray afterglow spectrum fitting. from those fits the intrinsic column densities at the grb sources were derived. the high-angular-resolution akari-fis-based galactic foreground hydrogen column densities are statistically very similar, but for most of the tested directions somewhat lower than previous estimates based on low-resolution data. this is due to the low filling factor of high-density enhancements in all galactic latitudes. accordingly, our akari-fis-based new intrinsic hydrogen column densities are usually higher or similar compared to the values calculated based, e.g., on the low-resolution leiden/argentine/bonn survey data and listed in the leicester database. the variation, however, is typically smaller than the error of the estimate from the fits of the x-ray afterglow spectra. there are a number of directions where the improvement of the foreground estimates resulted in an overestimate of magnitude or higher increment of the derived intrinsic hydrogen column densities. we concluded that most of the grbs with formerly extremely low intrinsic hydrogen column densities are in fact normal, but we confirmed that grb050233 is indeed a non-enveloped long grb. | galactic foreground of gamma-ray bursts from akari far-infrared surveyor |
the afterglow data of gamma ray bursts (grbs) can be used to constrain the physical properties of the fireball (e.g., the jet structure and opening angle) and the circumburst medium. with the peak time of the early afterglow light curve being taken as the deceleration time, one can estimate the initial lorentz factor of the fireball. in this work, we perform a comprehensive analysis on the prompt emission and the afterglow data of grb 181110a, where a clear peak is detected by swift uvot and xrt in optical to x-ray bands. prompt emission spectral analysis shows that the spectrum of grb 181110a is soft, and both hard-to-soft and intensity-tracking spectral evolution are found. by fitting the afterglow light curve and building spectral energy distribution, we find that the standard external forward shock model with a constant circumburst medium is favored, and the jet structure of grb 181110a tends to be uniform rather than structured. with the peak time of early afterglow emission, we estimate the initial fireball lorentz factor of grb 181110a to be γ0=169‑40+92. we also compare grb 181110a with other typical long grbs in a statistical context. | grb 181110a: constraining the jet structure, circumburst medium and the initial lorentz factor |
at 20:39:13 ut, the swift burst alert telescope (bat) triggered and located grb 191221b (trigger=945521). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 154.832, -38.141 which is ra(j2000) = 10h 19m 20s dec(j2000) = -38d 08' 28" 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 15 sec. the peak count rate was ~17000 counts/sec (15-350 kev), at ~5 sec after the trigger. the xrt began observing the field at 20:40:39.3 ut, 86.2 seconds after the bat trigger. xrt found a bright, uncatalogued x-ray source located at ra, dec 154.8286, -38.1579 which is equivalent to: ra(j2000) = 10h 19m 18.86s dec(j2000) = -38d 09' 28.4" with an uncertainty of 6.8 arcseconds (radius, 90% containment). this location is 61 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 91 seconds after the bat trigger. a very bright afterglow candidate has been found at the xrt position in the initial data products. because of coincidence loss we cannot calculate the magnitude accurately at this time. no correction has been made for the expected extinction corresponding to e(b-v) of 0.08. burst advocate for this burst is s laha (sib.laha at gmail.com). 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 191221b: swift detection of a burst and a very bright optical candidate |
we present simulations of the detection probability for absorption lines from ions in the warm and hot ionized medium (whim) with athena in the spectra of gamma-ray burst afterglows. the simulations are based on swift xrt lightcurves of these afterglows and are performed using the end-to-end simulation framework sixte. we simulate both the case of single and multiple absorption lines, as well as results for line searches in absorption structures from a more complex medium. we show that the athena x-ifu can detect whim lines with strong ovii lines (equivalent widths larger than 0.14 ev) in spectra containing 3 x 106 counts. | observing the whim with athena |
the afterglows of gamma-ray bursts (grbs) are believed to originate from the synchrotron emission of shock-accelerated electrons produced by the interaction between the outflow and the external medium. the accelerated electrons are usually assumed to follow a power-law energy distribution with an index of p. observationally, although most grb afterglows have a p larger than 2, there are still a few grbs suggestive of a hard (p\lt 2) electron spectrum. the well-sampled broad-band afterglow data for grb 091127 show evidence of a hard electron spectrum and strong spectral evolution, with a spectral break moving from high to lower energies. the spectral break evolves very fast and cannot be explained by the cooling break in the standard afterglow model, unless evolving microphysical parameters are assumed. in addition, the multi-band afterglow light curves show an achromatic break at around 33 ks. based on the model of a hard electron spectrum with an injection break, we interpret the observed spectral break as the synchrotron frequency corresponding to the injection break, and the achromatic break as a jet break caused by the jet-edge effect. it is shown that the spectral evolution and the multi-band afterglow light curves of grb 091127 can be well reproduced by this model. | modeling the multi-band afterglow of grb 091127: evidence of a hard electron energy spectrum with an injection break |
the swift satellite has observed more than a thousand grbs with x-ray data. almost a third of them have redshift measurement, too. here we start to investigate the x-ray spectral fitting of the data considering the low energy part where the n(h) absorption happens. based on the available more accurate input data we examined the robustness of previous fittings and tested how sensitive the changes of the starting parameters are. we studied the change of the intrinsic hydrogen column density during the outburst for a few events. no significant variability of n(h) column density was identified. | studying the variability of the x-ray spectral parameters of high-redshift grbs' afterglows |
at 05:09:55 ut, the swift burst alert telescope (bat) triggered and located grb 220101a (trigger=1091101). swift slewed immediately to the burst. the bat on-board calculated location is ra, dec 1.380, +31.753 which is ra(j2000) = 00h 05m 31s dec(j2000) = +31d 45' 09" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). the bat lightcurve shows a complex lightcurve extending to at least t+170 with a peak of 7000 counts/s (15-350 kev), at ~89 sec after the trigger. the xrt began observing the field at 05:11:32.2 ut, 80.8 seconds after the bat trigger. xrt found a bright, uncatalogued x-ray source located at ra, dec 1.3561, 31.7702 which is equivalent to: ra(j2000) = 00h 05m 25.46s dec(j2000) = +31d 46' 12.7" with an uncertainty of 4.7 arcseconds (radius, 90% containment). this location is 95 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 1.30e-08 erg cm^-2 s^-1 (0.2-10 kev). uvot took a finding chart exposure of 150 seconds with the white filter starting 89 seconds after the bat trigger. there is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at ra(j2000) = 00:05:24.82 = 1.35340 dec(j2000) = +31:46:08.5 = 31.76903 with a 90%-confidence error radius of about 0.61 arc sec. this position is 9.3 arc sec. from the center of the xrt error circle. the estimated magnitude is 14.60 with a 1-sigma error of about 0.14. no correction has been made for the expected extinction corresponding to e(b-v) of 0.056. burst advocate for this burst is a. tohuvavohu (aaron.tohu at gmail.com). 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 220101a: swift detection of a burst with a bright optical counterpart |
we present our optical photometric and spectroscopical observations of grb 140629a. a redshift of z = 2.275 ±0.043 is measured through the metal absorption lines in our spectroscopic data. using our photometric data and multiple observational data from other telescopes, we show that its optical light curve is well interpreted with the standard forward shock models in the thin shell case. its optical-x-ray afterglow spectrum is jointly fitted with a single power-law function, yielding a photon index of -1.90 ± 0.05. the optical extinction and neutral hydrogen absorption of the gamma-ray burst (grb) host galaxy are negligible. the fit to the light curve with the standard models shows that the ambient density is 60 ± 9 cm-3 and the grb radiating efficiency is as low as ∼0.24%, likely indicating a baryonic-dominated ejecta of this grb. this burst agrees well with the {l}{{p},{iso}}{--}{e}p{\prime }{--}{{{γ }}}0 relation, but confidently violates those empirical relations involving geometric corrections (or jet break time). this gives rise to an issue of the possible selection effect on these relations since the jet opening angle of this grb is extremely narrow (0.04 rad). | photometric and spectroscopic observations of grb 140629a |
the first gamma-ray burst (grb) catalog presented by the fermi-large area telescope (lat) collaboration includes 28 grbs, detected above 100 mev over the first three years since the launch of the fermi mission. however, more than 100 grbs are expected to be found over a period of six years of data collection thanks to a new detection algorithm and to the development of a new lat event reconstruction, the so-called "pass 8." our aim is to provide revised prospects for grb alerts in the cta era in light of these new lat discoveries. we focus initially on the possibility of grb detection with the large size telescopes (lsts). moreover, we investigate the contribution of the middle size telescopes (msts), which are crucial for the search of larger areas on short post trigger timescales. the study of different spectral components in the prompt and afterglow phase, and the limits on the extragalactic background light are highlighted. different strategies to repoint part of - or the entire array - are studied in detail. | prospects for gamma-ray bursts detection by the cherenkov telescope array |
context. the very high-energy (vhe; ≥100 gev) component from at least two gamma-ray bursts (grbs), that is, grb 180720b and grb 190114c, has been detected in the afterglow phase. it is widely discussed that the gev to tev emission originated from a synchrotron self-compton (ssc) process. the vhe component may cause an upturn at the high-energy spectral ends in the fermi-large area telescope (fermi-lat) observing band.aims: we aim to find out whether an additional high-energy component commonly exists in the afterglows of fermi-lat grbs. this study will help us to better understand how common it is for a grb afterglow detected by fermi-lat to involve a vhe component.methods: first, we selected the grbs that emit ≥10 gev photons. the ≥10 gev photons can be considered as a plausible proxy for a vhe component. we systematically analyzed 199 grbs detected by fermi-lat from 2008-2019. if an additional high-energy component exists in the afterglows of fermi-lat grbs, the best-fit spectral model could be a broken power law (bpl) model with an upturn above a break energy. we compared the afterglow spectra using power-law (pl) and bpl representations.results: out of the 30 grbs with ≥10 gev photons that arrived after t90 (the time duration when 90% of the prompt emission was detected), 25 grbs are tentatively or significantly detected at 0.1-200 gev after 2 × t90. the spectrum of grb 131231a shows an upturn above an energy break of 1.6 ± 0.8 gev, supporting the bpl model. for grb 131231a, we performed a modeling of its x-ray and γ-ray spectra and found that the ssc model can explain the upturn with acceptable parameter values. in the cases of grb 190114c, grb 171210a, grb 150902a, grb 130907a, grb 130427a, and grb 090902b, the improvement of the bpl fit compared to the pl fit is tentative or marginal.conclusions: there is no conclusive evidence that an additional higher energy component commonly exists in fermi-lat grb afterglows, except for the group of fermi-lat grbs mentioned above. such an additional high-energy component may be explained by the ssc mechanism. current and future vhe observations will provide important constraints on the issue. | searching for an additional high-energy component in fermi-lat grb afterglows |
we present the discovery and the subsequent follow up of radio emission from sdss j130402.36+293840.6 (j1304+2938), the candidate host galaxy of the gamma-ray burst (grb) grb 200716c. the galaxy is detected in the racs (0.89 ghz), the nvss, the apertif imaging survey, and the first (1.4 ghz), the vlass (3 ghz), and in public lofar (130-170 mhz), wise (3.4-22 μm), and sdss (z, i, r, g, u filters) data. the luminosity inferred at 1.4 ghz is (5.1 ± 0.2) × 1030 erg s−1 hz−1. to characterise the emission and distinguish between different components within the galaxy, we performed dedicated, high-sensitivity and high-resolution observations with the european vlbi network (evn) + e-merlin at 1.6 and 5 ghz. we did not detect any emission from a compact core, suggesting that the presence of a radio-loud active galactic nucleus (agn) is unlikely, and therefore we ascribe the emission observed in the public surveys to star-forming regions within the galaxy. we confirm and refine the redshift estimate, z = 0.341 ± 0.004, with a dedicated telescopio nazionale galileo (tng) spectroscopic observation. finally, we compiled a list of all the known hosts of grb afterglows detected in radio and computed the corresponding radio luminosity: if grb 200716c belongs to j1304+2938, this is the third most radio-luminous host of a grb, implying one of the highest star-formation rates (sfrs) currently known, namely sfr ∼ 324±61 m⊙ yr−1. on the other hand, through the analysis of the prompt emission light curve, recent works suggest that grb 200716c might be a short-duration grb located beyond j1304+2938 and gravitationally lensed by an intermediate-mass black hole (imbh) hosted by the galaxy. neither the public data nor our very long baseline interferometry (vlbi) observations can confirm or rule out the presence of an imbh acting as a (milli-)lens hosted by the galaxy, a scenario still compatible with the set of radio observations presented in this work. | multi-scale vlbi observations of the candidate host galaxy of grb 200716c |
we present centimeter-band and millimeter-band afterglow observations of five long-duration γ-ray bursts (grbs; grb 130131a, 130420b, 130609a, 131229a, 140713a) with dust-obscured optical afterglow emission, known as "dark" grbs. we detect the radio afterglow of two of the dark grbs (grb 130131a and 140713a), along with a tentative detection of a third (grb 131229a) with the karl g. jansky very large array (vla). supplemented by three additional vla-detected dark grbs from the literature, we present uniform modeling of their broadband afterglows. we derive high line-of-sight dust extinctions of av,grb ≈ 2.2- ≳ 10.6 mag. additionally, we model the host galaxies of the six bursts in our sample, and derive host galaxy dust extinctions of av,host ≈ 0.3-4.7 mag. across all tested γ-ray (fluence and duration) and afterglow properties (energy scales, geometries, and circumburst densities), we find dark grbs to be representative of more typical unobscured long grbs, except in fluence, for which observational biases and inconsistent classification may influence the dark grb distribution. additionally, we find that av,grb is not related to a uniform distribution of dust throughout the host, nor to the extremely local environment of the burst, indicating that a larger-scale patchy dust distribution is the cause of the high line-of-sight extinction. since radio observations are invaluable to revealing heavily dust-obscured grbs, we make predictions for the detection of radio emission from host star formation with the next-generation vla. | a radio-selected population of dark, long gamma-ray bursts: comparison to the long gamma-ray burst population and implications for host dust distributions |
it has been widely recognized that gamma-ray burst (grb) afterglows arise from interactions between the grb outflow and circumburst medium, while their evolution follows the behaviors of relativistic shock waves. assuming the distribution of circumburst medium follows a general power-law form, that is, n = a * r -k , where r denotes the distance from the burst, it is obvious that the value of the density-distribution index k can affect the behaviors of the afterglow. in this paper, we analyze the temporal and spectral behaviors of grb radio afterglows with arbitrary k values. in the radio band, a standard grb afterglow produced by a forward shock exhibits a late-time flux peak, and the relative peak fluxes, as well as peak times at different frequencies, show dependencies on k. thus, with multiband radio-peak observations, one can determine the density profile of the circumburst medium by comparing the relations between peak flux/time and frequency at each observing band. also, the effects of transrelativistic shock waves, as well as jets in afterglows, are discussed. by analyzing 31 long and 1 short grb with multiband data of radio afterglows, we find that nearly half of them can be explained with a uniform interstellar medium (k = 0), ~1/5 can be constrained to exhibiting a stellar-wind environment (k = 2), while less than ~1/3 of the samples show 0 < k < 2. | diagnosing the circumburst environment with multiband gamma-ray burst radio afterglows |
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