Search for GRB X-ray afterglows in the ROSAT all-sky survey

We describe a search for X-ray afterglows from gamma-ray bursts using the ROSAT all-sky survey (RASS) data. If the emission in the soft X-ray band is significantly less beamed than in the gamma-ray band, we expect to detect many afterglows in the RASS. Our search procedure generated 23 afterglow candidates, where about 4 detections are predicted. However, follow-up spectroscopy of several counterpart candidates strongly suggests a flare star origin of the RASS events in many, if not all, cases. Given the small number of events we conclude that the ROSAT survey data are consistent with comparable beaming angles in the X-ray and gamma-ray bands. This result is perhaps not surprising, given that the data constrain the relative beaming fraction only within a few hours of the burst. However, models predicting a large amount of energy emerging as a nearly isotropic X-ray component of the early afterglow are severely constrained by the ROSAT data. In particular, a so far undetected class of "dirty fireballs" and delayed "rebursts" are constrained.