SRG/eROSITAprospects for the detection of GRB afterglows

Abstract

We discuss the potential of the eROSITA telescope on board the Spectrum-X-Gamma observatory (also known as SRG) to detect gamma-ray burst (GRB) X-ray afterglows during its 4 yr all-sky survey. The expected rate of afterglows associated with long-duration GRBs without any information on the bursts proper that can be identified by a characteristic power-law light curve in the eROSITA data is four to eight events per year. An additional small number, ≲2 per year, of afterglows may be associated with short GRBs, ultra-hard (GeV) GRBs and X-ray flashes. eROSITA can thus provide the first unbiased (unaffected by GRB triggering) sample of ≲40 X-ray afterglows, which can be used for statistical studies of GRB afterglows and for constraining the shape of the GRB log N - log S distribution at its low-fluence end. The total number of afterglows detected by eROSITA may be yet higher due to orphan afterglows and failed GRBs. The actual detection rate could thus provide interesting constraints on the properties of relativistic jets associated with the collapse of massive stars. Finally, eROSITA can provide accurate (≲30 arcsec) coordinates of newly discovered afterglows within a day after the event, early enough for scheduling further follow-up observations.