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4 Discussion


Half of these remaining light curves are single peak events, i.e. have just one bin with S/N>3 and otherwise zero count rate. These events could be either statistical fluctuations, flare stars, or GRB afterglows with a very fast decay slope. We note that these single peak events cannot be distinguished from late-type flare stars other than by optical follow-up investigations, since typical time scales of flares range between 10-60 min, just as we observe. We therefore divide our conclusion into two cases:
1.
If the single peak events turn out to be due predominantly to flare stars, then the remaining 9 multipeak events (and light curves consistent with a power-law decay) found versus 4 events expected is at first order consistent with the BeppoSAX picture and implies that beaming may not be required.
2.
If the single peak events are GRB afterglows then geometric beaming of the GRB emission is possible, but at a moderate value.

In order to more accurately determine the fraction f and thus the expected number of events, a distribution function of power-law indices of GRB X-ray afterglow decays as measured by BeppoSAX is highly needed.

We finally note that GRBs from directions within a few degrees of the galactic plane will suffer absorption, thus compromising the sensitivity of the ROSAT detection. However, this effect concerns only a few percent of the sky, and mainly applies to single peak events, because any multi-peak event should have a first peak count rate high enough to still be detectable.


Acknowledgements

We are indebted to E. Costa and J. in 't Zand for providing X-ray afterglow light curves in digital form. JG is supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR) under contract No. 50OR96098. The ROSAT project is supported by BMBF/DLR and the Max-Planck-Society. This research has made use of the Simbad database, operated at CDS, Strasbourg, France and the Digitized Sky Survey (DSS) produced at the Space Telescope Science Institute under US Government grant NAG W-2166.



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