Identifying young gamma-ray burst fossils

Abstract

The recent reports of temporal and spectral peculiarities in the early stages of some afterglows suggest that we may be wrong in postulating a central engine that becomes dormant after the burst itself. A continually decreasing post-burst relativistic outflow, such as put out by a decaying magnetar, may continue to be emitted for periods of days or longer, and we argue that it can be efficiently reprocessed by the ambient soft photon field radiation. Photons produced either by the post-explosion expansion of the progenitor stellar envelope or by a binary companion provide ample targets for the relativistic outflow to interact and produce high-energy γ-rays. The resultant signal may yield luminosities high enough to be detected with the recently launched INTEGRAL and the GLAST experiment now under construction. Its detection will surely offer important clues for identifying the nature of the progenitor and possibly constraining whether some route other than single-star evolution is involved in producing a rapidly rotating helium core which in turn, at collapse, triggers a burst.