One of the most impressive recent discovery of SWIFT is the evidence that X-ray flares occurring during the GRB afterglows are quite common, being observed in roughly 50% of the afterglows. These X-ray flares range fluences comparable with the GRB prompt emission and could be also repetitive. Several pictures have been proposed on their origin and among them the most accepted regards the internal shock scenario, interpreting the X-ray flares as late time activity of the GRB central engine. We propose to describe the spectral shape of the X-ray flares adopting the same physical model recently used to interpret the GRB prompt emission: the log-parabolic function. In particular, we show that their spectral energy distribution (SED) is remarkably curved, while no significant curvature appears in the underlying X-ray afterglow emission. In addition, the log-parabolic function is statistically favored with respect to other proposed spectral models. By using a time resolved spectral analysis, we show the evolution of the peak energy and the curvature parameters of the SED during the X-ray flares in two of the brightest GRBs afterglows observed by SWIFT. We found that in the X-ray flares there is an anti-correlation between the peak energy and the curvature, as expected in a stochastic acceleration scenario.
It has been reported that some X-ray spectra of gamma-ray burst (GRB) afterglows cannot be fitted by a simple power law. A blackbody component is added to precisely fit the thermal feature in these spectra. Alternatively, we propose that bremsstrahlung radiation can also be one possible mechanism to explain the thermal component of the GRB X-ray afterglow. In particular, we examine the X-ray afterglow of the ultra-long GRB 130925A in this paper. By our calculation, we find that the X-ray thermal component observed by both Swift-XRT and NuSTAR can be well explained by the bremsstrahlung radiati…
A new model using a general expression of the radiation energy and explaining the dynamics of the afterglows is proposed. It is shown that this model is suitable for the ultra-relativistic and non-relativistic phases as well as the study of radiative and adiabatic fireballs.
The late-time high-resolution X-ray and radio observations of GRB980425/SN1998bw, the closest known gamma ray burst (GRB) associated with a supernova (SN) explosion, may have actually resolved the hyperluminal source that produced the GRB and its afterglow. Its hyperluminal speed ~350c is consistent with that expected in the cannonball (CB) model of GRBs. The observed superluminal expansion of the late-time radio image of GRB030329/SN2003dh, the GRB with the brightest and longest followed up radio afterglow to date, is also consistent with that expected in the CB model of GRBs and extrapolates…
GRB 140903A, a short duration $γ-$ray burst (SGRB) detected by {\it Swift}, is characterized by its long-lasting radio emission among SGRBs. In addition to the $\sim 10^{6}$ s radio afterglow emission, the afterglow of GRB 140903A displays a plateau from $10^3$ s to $7 \times 10^3 $ s in the X-rays. In this work, we attribute the X-ray plateau to the energy injection into the decelerating blast wave and then model the later radio/optical/X-ray afterglow emission within the standard fireball afterglow model. The afterglow emission has been well reproduced with reasonable physical parameters, in…
We analyzed 199 GRBs detected by Fermi-LAT during the years 2008-2019. We found 67 photons at energies >=10 GeV, which come from 34 GRBs. Out of these 34 GRBs, Fermi-LAT detects significant (TS>=4) afterglow 0.1-200 GeV photons from 25 GRBs. We present time-integrated 0.1-200 GeV spectra of these 25 GRBs. The spectra of a significant fraction (9/25) of these GRBs revealed a harder component above an energy break of 0.3-2 GeV. While shock synchrotron may account for the photons at the lower energy end, high energy photons above the break is naturally explained by synchrotron self-Compton (SSC) …
The afterglow of GRBs is 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 ($p<2$) electron spectrum. GRB 091127, with well-sampled broad-band afterglow data, shows evidence of a hard electron spectrum and strong spectral evolution, with a spectral break moving fr…
Electrons accelerated in relativistic collisionless shocks are usually assumed to follow a power-law energy distribution with an index of $p$. Observationally, although most gamma-ray bursts (GRBs) have afterglows that are consistent with $p>2$, there are still a few GRBs suggestive of a hard ($p<2$) electron energy spectrum. Our previous work showed that GRB 091127 gave strong evidence for a double power-law hard electron energy (DPLH) spectrum with $1<p_1<2$, $p_2>2$ and an "injection break" assumed as $γ_{\rm b}\propto γ^q$ in the highly relativistic regime, where $γ$ is the bulk Lorentz fa…
Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from extragalactic sources followed by fading afterglow emission, are associated with stellar core collapse events. We report the detection of very-high-energy (VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The low luminosity and redshift of GRB 190829A reduce both internal and external absorption, allowing determination of its intrinsic energy spectrum. Between energies of 0.18 and 3.3 tera-electron volts, this spectrum is describ…
The VHE component from at least two GRBs, i.e., GRB180720B and GRB190114C, has been detected in the afterglow phase. We systematically analyzed 199 GRBs detected by Fermi-LAT during 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 compare the afterglow spectra using PL and BPL representations. Out of the 30 GRBs with >10GeV photons that arrived after T90, 25 GRBs are tentatively or significantly detected at 0.1-200 GeV after 2*T90. The spectru…
GRB 090926A and Bright Late-time Fermi LAT GRB Afterglows
GRB 090926A是由费米伽马射线太空望远镜上的GBM和LAT仪器探测到的。在初始触发后约13小时开始迅速的后续观察。光学余辉在触发后近23天被检测到，将其置于长寿命类别。余辉特别令人感兴趣，因为它在后期的亮度，以及在T0+10^5 s及以后的光学耀斑的存在，这可能表明后期的中央发动机活动。LAT总共探测到了16个伽玛暴，其中9个伽玛暴，包括GRB 090926A，也被雨燕观测到。在雨燕观测到的9个LAT爆发中，有6个被UVOT探测到，其中5个爆发具有明亮、长寿命的光学余辉。相比之下，雨燕已经运行了5年，探测到了近500个爆发，但只有约30%的爆发的光学余辉寿命超过10^5秒。我们已经计算了预测的伽马射线通量，正如BAT在Swift上所看到的那样，LAT爆发，以确定与BAT触发的爆发相比，这种高比例的长寿命光学余辉是否是独特的。我们发现，除了短爆发GRB 090510A，预测的BAT注量表明LAT爆发比所有雨燕爆发的88%更有活力，也有比平均更明亮的X射线和光学余辉。
Short-duration GRBs with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also rare (about 30%) due to their smaller luminosity. We present a multi-band data set of the short burst with extended emission GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8hrs. The analysis of this unique GRB afterglow might shed light on the understandi…
We present photometric and polarimetric measurements of gamma-ray burst (GRB) optical afterglows observed by the RINGO3 imaging polarimeter over its $\sim$7 year lifetime mounted on the Liverpool Telescope. During this time, RINGO3 responded to 67 GRB alerts. Of these, 28 had optical afterglows and a further ten were sufficiently bright for photometric and polarimetric analysis ($R\lessapprox{17}$). We present high quality multicolour light curves of ten sources: GRB 130606A, GRB 130610A, GRB 130612A, GRB 140430A, GRB 141220A, GRB 151215A, GRB 180325A, GRB 180618A, GRB 190114C, and GRB 191016A…
We study GRB 221009A, the brightest gamma-ray burst in the history of observations, using Fermi data. To calibrate them for large inclination angles, we use the Vela X gamma-ray source. Light curves in different spectral ranges demonstrate a 300 s overlap of afterglow and delayed episodes of soft prompt emission. We demonstrate that a relatively weak burst precursor that occurs 3 minutes before the main episode has its own afterglow, i.e., presumably, its own external shock. The main afterglow is the brightest one, includes a photon with an energy of 400 GeV 9 hours after the burst, and is vis…
Gamma-ray bursts (GRBs) are ultra-relativistic collimated outflows, which emit synchrotron radiation throughout the entire electromagnetic spectrum when they interact with their environment. This afterglow emission enables us to probe the dynamics of relativistic blast waves, the microphysics of shock acceleration, and environments of GRBs. We perform Bayesian inference on a sample of GRB afterglow data sets consisting of 22 long GRBs and 4 short GRBs, using the afterglow model "scalefit", which is based on 2D relativistic hydrodynamic simulations. We make use of Gaussian processes to account …
Context. Gamma-ray burst (GRB) afterglows originate from the interaction between the relativistic ejecta and the surrounding medium. Consequently, their properties depend on several aspects: radiation mechanisms, relativistic shock micro-physics, circumburst environment, and the structure and geometry of the relativistic jet. While the standard afterglow model accounts for the overall spectral and temporal evolution for a number of GRBs, its validity limits emerge when the data set is particularly rich and constraining, especially in the radio band. Aims. We aimed to model the afterglow of the…
We present the result of a search for orphan Gamma-Ray Burst (GRB) afterglows in the Pan-STARRS1 (PS1) COSMOS survey. There is extensive theoretical and observational evidence suggesting that GRBs are collimated jets; the direct observation of orphan GRB afterglows would further support this model. An optimal survey strategy is designed by coupling the PS1 survey with the Subaru/Hyper-Suprime-Cam (HSC) survey. The PS1 COSMOS survey, one of the survey fields in the PS1 Medium Deep Survey (PS1/MDS), searches a field of 7 deg$^2$ from December 2011 to January 2014, reaching a limiting magnitude R…
We examine a sample of 21 gamma-ray burst (GRB) afterglow light curves at radio frequencies, and compare them to the X-ray and/or optical properties of the afterglows and to the predictions of the standard jet/fireball model. Our sample includes every \textit{Swift} GRB with an X-ray light curve indicating a jet break and with a published radio light curve, as well as several other targets with observed X-ray or and/optical jet breaks. We examine the late-time decline of each burst, and attempt to fit an analytical model based on the standard GRB afterglow equations to each data set. We show t…
Afterglows of gamma-ray bursts often show flares, plateaus, and sudden intensity drops: these temporal features are difficult to explain as coming from the forward shock. We calculate radiative properties of early GRB afterglows with the dominant contribution from the reverse shock (RS) propagating in an ultra-relativistic (pulsar-like) wind produced by the long-lasting central engine. RS emission occurs in the fast cooling regime -- this ensures high radiative efficiency and allows fast intensity variations. We demonstrate that: (i) mild wind power, of the order of $\sim 10^{46}$ erg s$^{-1}$…
Gamma-ray burst (GRB) afterglows are likely produced in the shock that is driven as the GRB jet interacts with the external medium. Long-duration GRBs are also associated with powerful supernovae (SNe). We consider the optical and radio afterglows of long GRBs for both blasts viewed along the jet axis ("on-axis" afterglows) and misaligned observes ("off-axis" afterglows). Comparing the optical emission from the afterglow with that of the accompanying SN, using SN 1998bw as an archetype, we find that only a few percent of afterglows viewed off-axis are brighter than the SN. For observable optic…
We present the results of our radio observational campaign on GRB 100418a, for which we used the Australia Telescope Compact Array (ATCA), Very Large Array (VLA) and the Very Long Baseline Array (VLBA). GRB 100418a was a peculiar GRB with unusual X-ray and optical afterglow profiles featuring a plateau phase with a very shallow rise. This observed plateau phase was believed to be due to a continued energy injection mechanism which powered the forward shock, giving rise to an unusual and long-lasting afterglow. The radio afterglow of GRB 100418a was detectable several weeks after the prompt emi…
GW170817, the first neutron star merger event detected by advanced LIGO/Virgo detectors, was associated with an underluminous short duration GRB 170817A. In this work we compare the forward shock afterglow emission of GW170817/GRB 170817A to other luminous short GRBs (sGRBs) with both a known redshift and an afterglow emission lasting at least one day after the burst. In the rapid decay phase, the afterglow emission of the bright sGRBs and GW170817/GRB 170817A form a natural and continuous sequence, though separated by an observation time gap. If viewed on-axis, the forward shock afterglow emi…
Time Evolution of Optical Darkness in GRB Afterglow: The Case of GRB 240825A
Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；University of Chinese Academy of Sciences（中国科学院大学）；Center for Astronomical Mega-Science, Chinese Academy of Sciences（中国科学院天文大科学研究中心）；Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences（中国科学院天体结构与演化重点实验室）；South-Western Institute for Astronomy Research, Yunnan University（云南大学西南天文研究所）
Polarization measurements of gamma-ray burst (GRB) afterglows are a promising means of probing the structure, geometry, and magnetic composition of relativistic GRB jets. However, a precise treatment of instrumental calibration is vital for a robust physical interpretation of polarization data, requiring tests of and validations against potential instrumental systematics. We illustrate this with ALMA Band 3 (97.5 GHz) observations of GRB 171205A taken $\approx5.19$ days after the burst, where a detection of linear polarization was recently claimed. We describe a series of tests for evaluating …
Short duration Gamma Ray Bursts(SGRB) and their afterglows are among the most promising electro-magnetic (EM) counterparts of Neutron Star (NS) mergers. The afterglow emission is broadband, visible across the entire electro-magnetic window from $γ$-ray to radio frequencies. The flux evolution in these frequencies is sensitive to the multi-dimensional afterglow physical parameter space. Observations of gravitational wave (GW) from BNS mergers in spatial and temporal coincidence with SGRB and associated afterglows can provide valuable constraints on afterglow physics. We run simulations of GW-de…
The detection of GRB 180722B and GRB 190114C in sub-TeV gamma-rays has opened up a new window to study gamma-ray bursts in high energy gamma-rays. Recently it is shown that the synchrotron and inverse Compton processes are responsible for the production of these high energy gamma-rays during the afterglow. Here, for the first time we demonstrate that, the photohadronic scenario which is successful in explaining the multi-TeV flaring in high energy blazars is also applicable for gamma-ray bursts. We show that the sub-TeV spectra of GRB 190114C and GRB 180720B are due to the interaction of high …
