Constraints on the emission mechanisms of gamma-ray bursts

Abstract

ABSTRACT If the emission of gamma–ray bursts were due to the synchrotron process in the stan-dard internal shock scenario, then the typical observed spectrum should have a slopeF ν ∝ν −1/2 , which strongly conflicts with the much harder spectra observed. Thisdirectly follows from the cooling time being much shorter than the dynamical time.Particle re–acceleration, deviations from equipartition, fastly changing magnetic fieldsand adiabatic losses are found to be inadequate to account for this discrepancy. Wealso find that in the internal shock scenario the relativistic inverse Compton scatteringis always as important as the synchrotron process, and faces the same problems. Thisindicates that the burst emission is not produced by relativistic electrons emittingsynchrotron and inverse Compton radiation.Key words: gamma rays: bursts — X–rays: general — radiation mechanisms: non–thermal 1 INTRODUCTIONSince the observational breakthrough by BeppoSAX (Costaet al. 1997; van Paradijs et al. 1997) the physics of gamma–ray bursts (GRB) has started to be disclosed. The huge en-ergy and power releases required by their cosmological dis-tances support the fireball scenario (Cavallo & Rees 1978;Rees & M´esz´aros 1992; M´esz´aros & Rees 1993), whose evo-lution and behavior is (unfortunately) largely independentof their origin.We do not know yet in any detail how the GRB event isrelated to the afterglow emission, but in the most acceptedpicture of formation of and emission from internal/externalshocks (Rees & M´esz´aros 1992; Rees & M´esz´aros 1994; Sari& Piran 1997), the former is due to collisions of pairs ofrelativistic shells (internal shocks), while the latter is gen-erated by the collisionless shocks produced by shells inter-acting with the interstellar medium (external shocks). Theshort spikes (t