Geminga and the search for optical counterparts of γ-ray-burst sources

Abstract

THE nature of gamma-ray-burst (GRB) sources has remained a mystery, in part because of the lack of any optical identification. Recently, deep CCD photometry has identified the optical counterpart G″ to the γ-ray source Geminga, whose location on a colour-magnitude diagram is unique¹. Although the X-rays from G″ are probably due to thermal emission from the neutron star, the implied column density (≳ 5 x 10²⁰) is inconsistent with the distance (D < 100 pc) required to fit the optical flux to the same spectral component. Here I show that the optical emission could be due instead to a cold accretion disk with accretion rate Ṁ ≈ 10¹¹ g s⁻¹. This has promising implications for the search for the optical counterparts to GRB sources whose basic physical parameters may resemble those of radio pulsars and related objects such as Geminga. I argue that a similar search in the field of a GRB location should produce candidates similar to G″ if 20 pc ≲ D_GRB ≲ 500 pc, as predicted by the disk-reprocessing model for the associated optical transients. This possibility is discussed in the light of the fact that the first optical counterpart to a GRB source may already have been found, which, if it is confirmed, may be used to test the accretion-disk model.