High‐Energy Emission from the Prompt Gamma‐Ray Burst

Abstract

We study the synchrotron and synchrotron self-Compton (SSC) emission from internal shocks that are responsible for the prompt gamma-ray emission in gamma-ray bursts (GRBs) and consider the relation between these two components, taking into account the high-energy cutoff due to pair production and Thomson scattering. We find that in order for the peak energy of the synchrotron to be Ep ~ 300 keV with a variability time t_v >~ ms, a Lorentz factor of Gamma 1 ms, we need Gamma >~ 350, which emplies Ep ~ 1 GeV emission. For an electron power-law index p>2, the SSC component dominates the emission above ~100 MeV. For an electron power-law index p > 2, the SSC component dominates the emission above ~100 MeV. Future observations by GLAST may help determine the value of p and whether the high-energy emission is consistent with a single power law (implying that one component, the synchrotron, is dominant) or has a break where the nuFnu slope turns from negative to positive, which implies that the SSC component becomes dominant above ~100 MeV. The high-energy emission is expected to show similar variability and time structure to that of the soft gamma-ray emission. Finally, we find that in order to see delayed high-energy emission from the prompt GRB due to pair production with the cosmic IR background, extremely small intergalactic magnetic fields (<10^-22 G) are required.