On-Axis Orphan Afterglows

Abstract

In many GRB inner engine models the highly relativistic GRB jets are engulfed by slower moving matter. This could result in different beaming for the prompt $γ$-ray emission and for the lower energy afterglow. In this case we should expect that some observer will see {\it on-axis orphan afterglows}: X-ray, optical and radio afterglows within the initial relativistic ejecta with no preceding GRB; The prompt $γ$-ray emission is pointing elsewhere. We show that the observations of the WFC on BeppoSAX constrain with high certainty the prompt X-ray beaming factor to be less than twice the prompt $γ$-ray beaming. The results of Ariel 5 are consistent with this interpretation. The RASS from ROTSE and HEAO-1 constrain the X-ray beaming factor at 400 and 20 minutes after the burst respectively to be comparable and certainly not much larger than the $γ$-ray beaming factor. There is no direct limit on the optical beaming. However, we show that observations of several months with existing hardware could result in a useful limit on the optical beaming factor of GRB afterglows.

19 pages 5 figures. A revised and extended version, 3 new figures added