Optical and GeV-TeV Flashes from Gamma-Ray Bursts

Abstract

The synchrotron optical flash caught in GRB 990123 overlaps with the MeV radiation front, and the optical-emitting electrons must also produce GeV-TeV emission by inverse Compton scattering of MeV photons. The ultra-high-energy flash can be much stronger than its optical counterpart. We also note that Compton cooling by MeV photons immediately terminates the optical emission unless the fireball Lorentz factor exceeds 10^3. Severe Compton losses may explain the non-detections of optical flashes in several long GRBs. Such failed optical flashes should be especially efficient GeV producers and likely to develop e+- cascades. This probably happened in GRB 941017, and its mysterious high-energy component is well explained by Compton upscattering of GRB photons at the fireball deceleration radius. The proposed mechanism of GeV emission should not work for short gamma-ray bursts that early decouple from the fireball and avoid interaction with the electrons in the deceleration flash. Observations by Swift and GLAST will provide an opportunity to test these expectations. The existing data for GRB 990123 already impose interesting constraints on the explosion.