The Fermi Gamma-ray Space Telescope (FGST) has opened a new high-energy window in the study of Gamma-Ray Bursts (GRBs). Here we present a thorough analysis of GRB 080825C, which triggered the Fermi Gamma-ray Burst Monitor (GBM), and was the first firm detection of a GRB by the Fermi Large Area Telescope (LAT). We discuss the LAT event selections, background estimation, significance calculations, and localization for Fermi GRBs in general and GRB 080825C in particular. We show the results of temporal and time-resolved spectral analysis of the GBM and LAT data. We also present some theoretical interpretation of GRB 080825C observations as well as some common features observed in other LAT GRBs.
Gamma-ray bursts (GRB) are generally believed to be efficient particle accelerators. In the presence of energetic protons in a GRB jet, interactions between these protons and intense radiation field of the GRB are supposed to induce electromagnetic cascade. Electrons/positrons generated in the cascade will produce an additional spectrum of robust feature, which is in the form of a power-law distribution up to GeV regime with an index of $\lesssim 2$. We suggest that measurements of Fermi-LAT at GeV band can provide independent constraints on the key GRB model parameters such as the dissipation…
The Very Bright and Nearby GRB 130427A: The Extra Hard Spectral Component and Implications for Very High-energy Gamma-ray Observations of Gamma-ray Bursts
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 canonball model, which unifies cosmic ray bursts (CRBs) and gamma ray bursts (GRBs), is used to predict the maximum isotropic equivalent gamma ray energy release in a GRB. The predicted maximum is based on the observed knee around 1 TeV in the energy spectrum of Galactic cosmic ray electrons, and on the Amati correlation in GRBs. Both were predicted by the cannonball model of CRBs and GRBs before their empirical discoveries. The predicted maximum agrees well with that concluded from up to date GRB observations.
Detections of very-high-energy (VHE; > 100 GeV) photons from Gamma-ray bursts (GRBs) can provide fundamental information on the involved radiative processes, physical composition of the ejecta and acceleration processes. The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is the best gamma-ray instrument to study transient phenomena over a long period of time from 100 GeV to 100 TeV. Its large field of view and duty cycle (> 95%) allow it to search blindly for sources of GRBs and to follow up on external alerts from satellite instruments, such as Fermi and Swift, searching for their…
In light of the joint multimessenger detection of a binary neutron star merger as the gamma-ray burst GRB 170817A and in gravitational waves as GW170817, we reanalyze the Fermi Gamma-ray Burst Monitor data of one of the closest short gamma-ray bursts: GRB 150101B. We find this burst is composed of a short hard spike followed by a comparatively long soft tail. This apparent two-component nature is phenomenologically similar to that of GRB 170817A. While GRB 170817A was distinct from the previously known population of short gamma-ray bursts in terms of its prompt intrinsic energetics, GRB 150101…
Macrolensing of gamma-ray bursts (GRBs) is expected to manifest as a GRB recurring with the same light curve and spectrum as a previous one, but with a different flux and a slightly offset position. Identifying such lensed GRBs may give important information about the lenses, cosmology, as well as the GRBs themselves. Here we present a search for lensed GRBs among $\sim 2700$ GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM) during 11 years of operations. To identify lensed GRBs, we perform initial cuts on position, time-averaged spectral properties and relative duration. We then use th…
For the first time, a short gamma-ray burst (GRB) was unambiguously associated with a gravitational wave (GW) observation from a binary neutron star (NS) merger. This allows us to link the details of the central engine properties to GRB emission models. We find that photospheric models (both dissipative and non-dissipative variants) have difficulties accounting for the observations. Internal shocks give the most natural account of the observed peak energy, viewing angle and total energy. We also show that a simple external shock model can reproduce the observed GRB pulse with parameters consis…
We statistically examine the gamma-ray burst (GRB) photon indices obtained by the Fermi-GBM and Fermi-LAT observations and compare the LAT GRB photon indices to the GBM GRB photon indices. We apply the jitter radiation to explain the GRB spectral diversities in the high-energy bands. In our model, the jitter radiative spectral index is determined by the spectral index of the turbulence. We classify GRBs into three classes depending on the shape of the GRB high-energy spectrum when we compare the GBM and LAT detections: the GRB spectrum is concave (GRBs turn out to be softer and are labeled as …
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…
The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the …
A thermal component is suggested to be the physical composition of the ejecta of several bright gamma-ray bursts (GRBs). Such a thermal component is discovered in the time-integrated spectra of several short GRBs as well as long GRBs. In this work, we present a comprehensive analysis of ten very short GRBs detected by Fermi Gamma-Ray Burst Monitor to search for the thermal component. We found that both the resultant low-energy spectral index and the peak energy in each GRB imply a common hard spectral feature, which is in favor of the main classification of the short/hard versus long/soft dich…
Synchrotron radiation from a decelerating blastwave is a widely accepted model of radio to X-ray afterglow emission from gamma-ray bursts (GRBs). GeV gamma-ray emission detected by the Fermi Large Area Telescope (LAT) and the duration of which extends beyond the prompt gamma-ray emission phase, is also compatible with broad features of afterglow emission. We revisit the synchrotron self-Compton (SSC) emission model from a decelerating blastwave to fit multiwavelength data from three bright GRBs, namely GRB~190114C, GRB~130427A and GRB~090510. We constrain the afterglow model parameters using t…
