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 candi- dates 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 com- parable 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.