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

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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. We found 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 cases. We conclude that the RASS data are consistent with comparable beaming angles in the X-ray (within a few hours of the burst) and gamma-ray bands. 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.