The 2009 Fermi Symposium

The Origin of the Prompt GRB Spectrum

Frédéric Daigne, Institut d'Astrophysique de Paris - France

Abstract:

The low energy gamma-ray spectrum observed in GRBs is well fitted by the "Band function". A well known problem is the discrepancy between the predicted value of the low-energy photon slope alpha in the synchrotron fast cooling spectrum (alpha=-1.5) and the observations, which show alpha ∼ -1 with a large dispersion. On the other hand, alternative radiative processes to synchrotron radiation, such as synchrotron self-Compton, usually predict additional bright components that are not observed in most GRBs, and are probably incompatible with the GRB detection rate by the LAT on board Fermi. We discuss how to reconcile the synchrotron process with the observed GRB spectrum by including the modification of the spectrum due to inverse Compton scatterings in Klein-Nishina regime. We show that the physical conditions necessary for this regime can be reached in internal shocks within the GRB relativistic outflow. We also compute the high-energy emission expected in this scenario (Fermi-LAT range), and especially the predicted spectral evolution for a GRB pulse. We show that these results offer a promising explanation for several features observed in recent bursts detected by GBM and LAT an board Fermi.