ROSAT is sensitive enough to detect a GRB X-ray afterglow for a few hours within its 10-30 s exposure time per sky location per scan. Figure 1 shows the one-scan sensitivity of ROSAT relative to the measured X-ray afterglow decay curves. The fraction f of afterglows detectable during the RASS depends on three critical parameters. First, the fraction of GRBs displaying X-ray afterglows: Previous observations suggest this fraction to be close to one. Second, the possible relation of X-ray flux to -ray peak flux or fluence: So far, the observed X-ray afterglow fluxes at about 100 s after the GRB are spread within a factor of 10 only, while the GRB fluxes range over a factor of > 1000. Third, the slope of the X-ray intensity decay: observed values range between t-1.8... t-2.5. The effect of the combination of the latter two factors is difficult to assess in an accurate manner given the low statistics at present, so we base our estimate of f on the observed X-ray afterglow intensities. A comparison with the ROSAT PSPC sensitivity suggests that we would detect practically all GRB afterglows in 3 subsequent scans, and 80% in 5 scans (see Fig. 1). Thus, we conservatively adopt f=0.8 in the following.
|Figure 1: Decay light curves of some observed GRB X-ray afterglows in the 2-10 keV range (GRB 970111: Feroci et al. 1998; GRB 970228: Costa et al. 1997; GRB 970402: Nicastro et al. 1998; GRB 970508: Piro et al. 1998; GRB 980329: in 't Zand et al. 1998) and their corresponding brightness extrapolated into the ROSAT band (scale on the right; assuming a power law with photon index of -2 and no absorption). The horizontal line gives the sensitivity of the ROSAT PSPC during one scan, and the vertical lines mark the time windows for the possible coverage of a GRB location by ROSAT during its scanning mode. Thus, one may expect an afterglow at an intensity of up to several hundred cts/s during the first scan, between 0.3-8 cts/s during the second scan, <2 cts/s during the third scan and so on|
The number of detectable X-ray afterglows from a GRB beamed towards us (based on the BATSE detection rate) during the RASS is
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