Insights into the physics of GRBs from the high-energy extension of their prompt emission spectra

Abstract

The study of the high-energy (MeV-GeV) part of GRBs spectrum can play a crucial role in investigating the physics of the prompt emission, but it is often hampered by low statistic and the paucity of GeV observations. In this work, we analyze the prompt emission spectra of the 22 brightest GRBs which have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning 6 orders of magnitude in energy. The high-energy photon spectra can be modelled with a power-law $N(E)\propto E^{-\beta }$ possibly featuring an exponential cutoff. We find that, with the inclusion of the LAT data, the spectral index $\beta$ is softer than what typically inferred from the analysis of Fermi/GBM data alone. Under the assumption that the emission is synchrotron, we derive the index $p\sim 2.79$ of the power-law energy distribution of accelerated particles ($N(\gamma )\propto {\gamma }^{-p}$). In 9 out of 22 GRB spectra, we find a significant presence of an exponential cut-off at high-energy, ranging between 14 and 298 MeV. By interpreting the observed cut-off as a sign of pair-production opacity, we estimate the jet bulk Lorentz factor $\Gamma$, finding values in the range 130-330. These values are consistent with those inferred from the afterglow light curve onset time. Finally, by combining the information from the high-energy prompt emission spectrum with the afterglow lightcurve, we provide a method to derive the distance R from the central engine where the prompt emission occurs. These results highlight the importance of including high-energy data, when available, in the study of prompt spectra and their role in addressing the current challenges of the GRB standard model.

17 pages, 17 figures. Accepted for publication in A&A

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