GeV Photons from the Upscattering of Supernova Shock Breakout X-Rays by an Outside Gamma-Ray Burst Jet

Shock breakout X-ray emission has been reported for the first time from a supernova connected with a gamma-ray burst, namely, GRB 060218/SN 2006aj. 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 (IC) 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 provide us with 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.