Optical and GeV-TeV Flashes from Gamma-Ray Bursts

The synchrotron optical flash caught in GRB 990123 overlaps with the MeVradiation front, and the optical-emitting electrons must also produce GeV-TeVemission by inverse Compton scattering of MeV photons. The ultra-high-energyflash can be much stronger than its optical counterpart. We also note thatCompton cooling by MeV photons immediately terminates the optical emissionunless the fireball Lorentz factor exceeds 10^3. Severe Compton losses mayexplain the non-detections of optical flashes in several long GRBs. Such failedoptical flashes should be especially efficient GeV producers and likely todevelop e+- cascades. This probably happened in GRB 941017, and its mysterioushigh-energy component is well explained by Compton upscattering of GRB photonsat the fireball deceleration radius. The proposed mechanism of GeV emissionshould not work for short gamma-ray bursts that early decouple from thefireball and avoid interaction with the electrons in the deceleration flash.Observations by Swift and GLAST will provide an opportunity to test theseexpectations. The existing data for GRB 990123 already impose interestingconstraints on the explosion.Comment: accepted to ApJ Letter