3 Selection on X-ray intensity

Since the WFC sensitivity is not a priori matched with the GRBM sensitivity a selection in X-ray intensity is naturally expected. Given the poor positioning capabilities of the gamma-ray experiments there is no obvious method to definitely check whether the BeppoSAX sample is X-ray selected or not. However, the following considerations suggest that such a selection is not effective.

First, all the GRBs that are known (through a BATSE accurate localization, i.e. LOCBURST, [Kippen et al. 1997]) to have occurred within the WFC field of view have shown a detectable X-ray counterpart. Second, we have taken a small sample of GRBs detected by the GRBM and extrapolated the best-fit to the average 40-700 keV energy spectra downwards to the WFC energy range (2-20 keV). Of course, this procedure includes large uncertainties in the spectral parameter extrapolation due both to statistical errors and to the assumption that the same law holds at low energies. However, the results are reported in Table 1 and show the expected fluxes for these bursts. In all cases they are in a range of quite easy detectability with the WFC.

Even if the above considerations are in no way conclusive, they suggest that the WFC sensitivity happens to be well matched with the GRBM and BATSE sensitivity.

  

Table 1: Extrapolated 2-20 keV flux of GRBM detected events, obtained extrapolating the spectral best-fit law (spectral GRBM analysis from G. Ghirlanda 1999, Thesis)

^a Parameters from the phenomenological law after Band et al. (1993).
^b GRB duration.
^c 10^-8 erg cm^-2 s^-1 in 40-700 keV (average).
^d 10^-8 erg cm^-2 s^-1 in 2-10 keV (extrapolated).
^e Fit with a simple power law.