GeV photons from up-scattering of supernova shock breakout X-rays by an outside GRB jet

Abstract

Shock breakout X-ray emission has been reported for the first time from a supernova connected with a gamma-ray burst, namely GRB060218/SN2006aj. The gamma-ray emission and the power-law decaying X-ray afterglow are ascribed to a highly relativistic jet, while the thermal soft X-rays are thought to be produced when the radiation-dominated shock breaks from the optically thick stellar wind. We study the inverse Compton emission of the breakout thermal soft X-rays scattered by relativistic electrons in the jet forward shock, which is expected to be at larger radii than the breakout shock. This IC emission produces sub-GeV to GeV photons, which may be detectable by GLAST. The detection of such GeV photons simultaneously with the supernova shock breakout emission would give evidence for the presence of a GRB jet ahead of the shock while the shock is breaking out. The anisotropic scattering between the X-rays and relativistic electrons may lead to large angle emission outside of the jet opening angle. This has implications for the detection of GeV photons from burstless type Ib/c hypernova shock breakout, which due to its more isotropic emission might be observed with wide-field X-ray cameras such as LOBSTER.