Gamma-ray burst reprocessing in an accretion disk

Abstract

The observational consequences of the reprocessing of gamma-ray burst (GRB) photons by an accretion disk surrounding a neutron star are explored. Reprocessing of gamma rays by cold disks driven by dynamical viscosity of nonrelativistic, degenerate electrons results in an optical/UV fluence relative to the gamma-ray fluence at earth of greater than 0.001, for GRB energies of (1-10) x 10 to the 37th ergs, and distances of 15-250 pc. The spectrum peaks in the optical/UV with a soft X-ray cutoff due to the inner edge of the disk, which lies very near the surface of the neutron star for magnetic fields less than 1 TG. The optical/UV peak is partially due to reprocessing in the outer part of the cold disk, which remains optically thick to gamma rays. At these distances, a positive identification of a quiescent counterpart is expected at an IR (K) sensitivity less than 2 microJy. On the other hand, compact binary systems with standard alpha disks and accretion rates resulting from Roche-lobe overflow must be located at least several hundred pc away in order to have escaped detection down to the current IR sensitivity. The optical/UV fluence resulting from reprocessing in these systems is deficient by at least an order of magnitude compared to the observed values.