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Register a new userSearching for an additional high-energy component in Fermi-LAT GRB afterglows
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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 spectrum of GRB131231A shows an upturn above a break of 1.6+-0.8~GeV, supporting the BPL model. For GRB131231A, we performed a modeling of its X-ray and gamma-ray spectra, and found that the SSC model can explain the upturn with acceptable parameter values. In the cases of GRBs 190114C, 171210A, 150902A, 130907A, 130427A, and 090902B, the improvement of the BPL fit compared to the PL fit is tentative or marginal. There is no conclusive evidence that an additional higher energy component commonly exists in Fermi-LAT GRB afterglows, except for a group of Fermi-LAT GRBs mentioned above. Such an additional high-energy component may be explained by the synchrotron self-Compton mechanism. Current and future VHE observations will provide important constraints on the issue.
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The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time ($approx10^2$ s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with a SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band and the very energetic photons with SSC emission from the forward shock when the outflow carries a significant magnetic field ($R_{rm B} simeq 30$) and it decelerates in a uniform-density medium with a very low density ($n=4.554^{+1.128}_{-1.121}times 10^{-4},{rm cm^{-3}}$).
We present the light-curves and spectra of 24 afterglows that have been monitored by Fermi-LAT at 0.1-100 GeV over more than a decade in time. All light-curves (except 130427) are consistent with a single power-law starting from their peaks, which occurred, in most cases, before the burst end. The light-curves display a brightness-decay rate correlation, with all but one (130427) of the bright afterglows decaying faster than the dimmer afterglows. We attribute this dichotomy to a quick deposition of the relativistic ejecta energy in the external-shock for the brighter/faster-decaying afterglows and to an extended energy-injection in the afterglow shock for the dimmer/slower-decaying light-curves. The spectra of six afterglows (090328, 100414, 110721, 110731, 130427, 140619B) indicate the existence of a harder component above a spectral dip/ankle at energy 0.3-3 GeV, offering evidence for an inverse-Compton emission at higher energies, and suggesting that the harder power-law spectra of five other LAT afterglows (130327B, 131231, 150523, 150627, 160509) could also be inverse-Compton, while the remaining softer LAT afterglows should be synchrotron. Marginal evidence for a spectral break and softening at higher energies is found for two afterglows (090902B and 090926).
Since the launch of the Fermi Gamma-ray Space Telescope on June 11, 2008, 55 gamma-ray bursts (GRBs) have been observed at coordinates that fall within 66^circ of the Fermi Large Area Telescope (LAT) boresight with precise localizations provided by the NASA Swift mission or other satellites. Imposing selection cuts to exclude low Galactic latitudes and high zenith angles reduces the sample size to 41. Using matched filter techniques, the Fermi/LAT photon data for these fields have been examined for evidence of bursts that have so far evaded detection at energies above 100 MeV. Following comparisons with similar random background fields, two events, GRB 080905A and GRB 091208B, stand out as excellent candidates for such an identification. After excluding the six bright bursts previously reported by the LAT team, the remaining 35 events exhibit an excess of LAT diffuse photons with a statistical significance greater than 2 sigma, independent of the matched filter analysis. After accounting for the total number of photons in the well-localized fields and including estimates of detection efficiency, one concludes that somewhere in the range of 11% to 19% of all GRBs within the LAT field of view illuminate the detector with two or more energetic photons. These are the most stringent estimates of the high energy photon content of GRBs to date. The two new events associated with high energy photon emission have similar ratios of high to low energy fluences as observed previously. This separates them from bursts with similar low energy fluences by a factor of ten, suggesting a distinct class of events rather than a smooth continuum.
From the launch of the Fermi Gamma-ray Space Telescope to July 9, 2010, the Gamma-ray Burst Monitor (GBM) has detected 497 probable GRB events. Twenty-two of these satisfy the simultaneous requirements of an estimated burst direction within 52^circ of the Fermi Large Area Telescope (LAT) boresight and a low energy fluence exceeding 5 $mu$erg/cm^2. Using matched filter techniques, the spatially correlated Fermi/LAT photon data above 100 MeV have been examined for evidence of bursts that have so far evaded detection at these energies. High energy emission is detected with great confidence for one event, GRB 090228A. Since the LAT has significantly better angular resolution than the GBM, real-time application of these methods could open the door to optical identification and richer characterization of a larger fraction of the relatively rare GRBs that include high energy emission.
At least a fraction of Gravitational Wave (GW) progenitors are expected to emit an electromagnetic (EM) signal in the form of a short gamma-ray burst (sGRB). Discovering such a transient EM counterpart is challenging because the LIGO/VIRGO localization region is much larger (several hundreds of square degrees) than the field of view of X-ray, optical and radio telescopes. The Fermi Large Area Telescope (LAT) has a wide field of view ($sim 2.4$ sr), and detects $sim 2-3$ sGRBs per year above 100 MeV. It can detect them not only during the short prompt phase, but also during their long-lasting high-energy afterglow phase. If other wide-field high-energy instruments such as Fermi-GBM, Swift-BAT or INTEGRAL-ISGRI cannot detect or localize with enough precision an EM counterpart during the prompt phase, the LAT can potentially pinpoint it with $lesssim 10$ arcmin accuracy during the afterglow phase. This routinely happens with gamma-ray bursts. Moreover, the LAT will cover the entire localization region within hours of any triggers during normal operations, allowing the $gamma$-ray flux of any EM counterpart to be measured or constrained. We illustrate two new ad hoc methods to search for EM counterparts with the LAT and their application to the GW candidate LVT151012.
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