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4 Sky distribution, untriggered GRBs, and BATSE location accuracy

Figure 6 shows a recent sky distribution of nearly 2300 GRBs observed with BATSE. The statistics are now such that the effect of earth-blockage is becoming apparent on the map. This shows as a slight deficiency along the celestial equator. Corrections for this blockage yield a map with a distribution consistent with isotropy.

Efforts have been made to both improve the location accuracy of bursts observed with BATSE, and to quantify this accuracy. A recent paper by Briggs et al. (1999), shows that the BATSE systematic error, as determined by a comparison with the Interplanetary Network (IPN) GRB locations, is best described by two near-Gaussian components. The larger component has a standard deviation of $\sim$1.8 deg.

  
\begin{figure}
\includegraphics [width=8.8cm]{R70_fig5.ps}\end{figure} Figure 5: An example of the hard-to-soft spectral evolution seen in many GRBs and many sub-pulses within bursts. The upper (bold) trace is time profile of the soft photons (25-50 keV), while the lower trace shows the profile of the hardest photons observed with the BATSE large-area detectors. This hard profile of the sub-pulse shows no detectable emission at later times after this sub-pulse. The pulse itself has a width (FWHM) less than half of the softer sub-pulse

A number of recent studies have used BATSE untriggered data to search for weak events in order to extend analyses (e.g. the peak flux distribution) to weaker GRBs, to verify the BATSE on-board burst trigger, and to identify other types of transient phenomena (Kommers et al. 1997; Kommers 1999; Kommers et al. 1999; Schmidt 1999; Stern et al. 1999). These studies appear to be capable of extending the BATSE sensitivity beyond that of the triggered GRBs. However, it is expected that weak events found in these searches may be contaminated by non-GRB fluctuations of the background, hard X-ray transients, and systematic effects. It is difficult to quantify the number of the false GRBs thus found and to accurately locate them.

  
\begin{figure}
\includegraphics [width=14.6cm]{R70_fig6.ps}\end{figure} Figure 6: The sky distribution shown in galactic coordinates of 2292 GRBs observed with BATSE. from over 7.7 years in operation. The celestial equator is shown as a long, curved dashed line. A slight deficiency of GRBs is seen in a broad region around this line due to the effects of Earth blockage

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