Fermi Gamma-ray Space Telescope

High-energy non-thermal and thermal emissions from GRB141207A detected by Fermi LAT

M. Arimoto
K. Asano, M. Ohno, N. Kawai, on behalf of the Fermi/LAT collaboration

Abstract:

Gamma-ray bursts (GRBs) are the brightest explosions in the universe and their emission mechanism is still enigmatic. Especially, an additional high-energy power-law component from a few keV to a few GeV which cannot be easily explained by synchrotron emission has been found in several GRBs. To reveal its origin, it is important to increase the sample of such GRBs and investigate the temporal and spectral behaviors of these bursts. A high-energy gamma-ray emission from GRB141207A was detected by two instruments aboard Fermi; the GBM and LAT detectors which cover energy ranges from 8 keV to 40 MeV and from 20 MeV to 300 GeV, respectively. Its duration is approximately 20 s, which classifies it as a long GRB. Its prominent features include the detection of a highest energy photon of 5.5 GeV and that the GeV emission is delayed with respect to the soft gamma-ray emission below the MeV range. Such a delay has been found in other GRBs detected by Fermi/LAT and the feature seems to be common in GRBs which have high-energy emission in the GeV range. Our detailed spectral analysis shows that an extra high-energy power-law component in addition to a synchrotron component expressed by the canonical Band function is required statistically. Furthermore, we found that the Band component itself has a large low-energy photon index (e.g., +0.2 ~ +0.3), which contradicts with the synchrotron shock scenario of the canonical GRB model. In this talk, we will present the obtained results and their scientific interpretation.