Nonthermal gamma-ray and X-ray flashes from shock breakout in gamma-ray bursts/supernovae

Abstract

Thermal X-ray emission which is simultaneous with the prompt gamma-rays has been detected for the first time from a supernova connected with a gamma-ray burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from the breakout of a mildly relativistic, radiation-dominated shock from a dense stellar wind surrounding the progenitor star. There is also evidence for the presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its X-ray and radio afterglow. Here we study the process of repeated bulk Compton scatterings of shock breakout thermal photons by the mildly relativistic ejecta. During the shock breakout process, a fraction of the thermal photons would be repeatedly scattered between the pre-shock material and the shocked material as well as the mildly relativistic ejecta and, as a result, the thermal photons get boosted to increasingly higher energies. This bulk motion Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes, which could account for the prompt gamma-ray burst emission in low-luminosity supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been developed to simulate this repeated scattering process, which confirms that a significant fraction of the thermal photons get "accelerated" to form a nonthermal component, with a dominant luminosity. This interpretation for the prompt nonthermal emission of GRB060218 may imply that either the usual internal shock emission from highly relativistic jets in these low-luminosity GRBs is weak, or alternatively, that there are no highly relativistic jets in this peculiar class of bursts.Comment: Accepted for publication in ApJ; Introduction expanded, references added, conclusions unchanged; total 7 pages including 2 color figures and 1 tabl