Optical flashes and GeV-TeV flashes from gamma-ray bursts

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  • 3 October 2004
Abstract
The optical flash caught by ROTSE in GRB 990123 overlaps with the MeV radiation front. This overlapping has two implications: (1) If the flash has a synchrotron origin, the electrons emitting optical radiation must also create a GeV-TeV flash by inverse Compton scattering of the MeV photons. The high-energy flash can be much stronger than its optical counterpart and comparable with the main MeV burst. (2) Compton cooling by MeV photons immediately terminates the synchrotron optical flash unless the fireball has a Lorentz factor >~ 10^3. This rather strong condition may be rarely satisfied and can explain the non-detections of bright flashes in several long GRBs. The failed optical flashes are especially efficient producers of GeV-TeV radiation and likely to develop e+- cascades. The high-energy component of GRB 941017 can be explained in this way. The observed component trails the prompt GRB and is consistent with Compton upscattering of 0.1 MeV photons at the fireball deceleration radius. MeV bursts of short durations can early decouple from the fireball and avoid interaction with the flash electrons; then the proposed mechanism of high-energy flashes should not work. 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 flash mechanism and the burst environment.

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