3 Detection of sub-second GRB X-ray counterpart

Any detection of GRB related X-ray emission is based on the analysis of WFC light curves. If positive, an image is made in order to determine the coordinates of the possible counterpart. At present, the WFC data analysis is performed up to a maximum temporal resolution of 1 s. However, the study of a sample of longer GRBs with counterparts detected in the WFCs shows that this procedure might well fail to detect sub-second burst counterparts with similar peak fluxes (Feroci 1998). At quick-look level a minimum integrated flux is required to detect positively a source in a WFC celestial image, which thus poses a lower limit on the detectable fluence. In the case of a short burst, the X-ray flux will be concentrated in a sub-second time interval. Significant detection of the burst above background in the WFC light curve requires a bin size in the order of the burst duration, i.e. $\leq1$ s. This demonstrates the need for an analysis of WFC light curves at higher temporal resolution in order to ensure an efficient detection of sub-second transients. To this end, the SOC software has been upgraded allowing analysis of WFC light curves at high time resolution and celestial images of sub-second integrations. This will form part of the new SOC procedure.

An analysis of sub-second WFC data in both a crowded and almost empty field has been performed. Preliminary results show that in either case the minimum required fluence for positive detection in a WFC image is $\sim$ 70 counts (although the influence of background still requires further analysis). In the case of the near empty field the current SOC procedure would be adequate to detect a possible sub-second counterpart in a WFC light curve and image. The new procedure, based on high time resolution analysis, would nevertheless improve the detection efficiency. However, in the case of the crowded field, only the high time resolution analysis would allow the detection of counterparts in WFC images. The reason is that in this case the higher background (created by all the other sources in the field of view) requires a sub-second bin size for significant detection of the burst in the light curve.

An upper limit to the number of sub-second GRB counterparts which can be detected in WFC images may be obtained by converting the above mentioned minimum X-ray fluence into the corresponding $\gamma$-ray fluence. Using a realistic GRB spectrum in the 2-25 keV band consisting of a power law with photon index 1.35 (Feroci 1998), and applying the average ratio of X-ray/$\gamma$-ray fluences of 0.24 (Strohmayer et al. 1998) to sub-second GRBs we find, as a rough estimate, a $\gamma$-ray fluence threshold of $4.0 \ 10^{-8}$ ergs/cm² (40-700 keV). Only more fluent GRBs may be detected in WFC images at BeppoSAX SOC. This includes 84% of sub-second bursts in the 4B BATSE catalogue.