High‐Energy Emission from the Prompt Gamma‐Ray Burst

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 E p ~ 300 keV with a variability time t v ≳ 1 ms, a Lorentz factor of Γ ≲ 200 is needed, implying no high-energy emission above ~30 MeV and the synchrotron component dominating at all energies. If we want both E p ~ 300 keV and prompt high-energy emission up to ~2 GeV, as detected by EGRET for GRB 940217, we need Γ ~ 600 and t v ~ 0.1 ms, which might be resolved by Super Agile. If such prompt high-energy emission is common in GRBs, as may be tested by the Gamma-Ray Large Area Space Telescope ( GLAST ), then for t v ≳ 1 ms, we need Γ ≳ 350, which implies E p ≲ 100 keV. Therefore, if X-ray flashes are GRBs with high values of t v and Γ, they should produce ≳1 GeV emission. 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 ν F ν 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.