On-Axis Orphan Afterglows

On axis orphan afterglows are afterglows with no preceding γ-ray emission that arise within the initial GRB jet. This could happen if the GRB and afterglow emission are beamed differently in different wavelengths, as would be the case if the relativistic Lorentz factors varies strongly with angle within GRB jets. 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 suggest that the x-ray beaming factor (400 and 20 minutes after the burst respectively) is 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 an interesting limit on the optical beaming factor of GRB afterglows.