The synchrotron self-Compton spectrum of relativistic blast waves at large Y

Abstract

Recent analyses of multiwavelength light curves of gamma-ray bursts afterglows point to values of the magnetic turbulence well below the canonical ~1 per cent of equipartition, in agreement with theoretical expectations of a microturbulence generated in the shock precursor, which then decays downstream of the shock front through collisionless damping. As a direct consequence, the Compton parameter Y can take large values in the blast. In the presence of decaying microturbulence and/or as a result of the Klein-Nishina suppression of inverse Compton cooling, the Y parameter carries a non-trivial dependence on the electron Lorentz factor, which modifies the spectral shape of the synchrotron and inverse Compton components. This paper provides detailed calculations of this synchrotron self-Compton spectrum in this large Y regime, accounting for the possibility of decaying microturbulence. It calculates the expected temporal and spectral indices α and β customarily defined by Fν ∝ tobs-α ν-β in various spectral domains. This paper also makes predictions for the very high energy photon flux; in particular, it shows that the large Y regime would imply a detection rate of gamma-ray bursts at > 10 GeV several times larger than currently anticipated.