On-axis orphan afterglows

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 gamma-ray emission and for the lower energy afterglow. In this case we should expect that some observer will see on-axis orphan afterglows: X-ray, optical and radio afterglows within the initial relativistic ejecta with no preceding GRB; the prompt gamma-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 gamma-ray beaming. The results of Ariel 5 are consistent with this interpretation. The RASS from ROSAT and HEAO-1 constrain the X-ray beaming factor at 400 and 20 min after the burst, respectively, to be comparable and certainly not much larger than the gamma-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. (C) 2002 Elsevier Science B.V. All rights reserved.