A Study on Event Screening for Downward Gamma-Rays with the GLAST Prototype Balloon Data

The triggering and data analysis for the GLAST balloon prototype (Thompson et al. 2002), like that of the GLAST gamma-ray telescope (LAT) planned for launch in 2006, is much more flexible than was the case for the CGRO/EGRET instrument. Furthermore, the large data volume expected from GLAST/LAT precludes the type of manual event review that was done for EGRET data. Thus a great deal of attention must be given to the software ``cuts'' that are used to eliminate the huge background of events produced by charged cosmic rays, without reducing unnecessarily the efficiency for gamma-ray detection. We have examined event by event the data from the August 2001 balloon flight and designed some candidate filters to cut events which were clearly charged particle background, and also others that are suspected of being non-downward-gamma-ray events. The cuts are based in part on experience with data from the SAS-2 and EGRET, but are modified to take into account the structural differences between GLAST/LAT and those earlier instruments. These empirically-developed filters demonstrate the feasibility of reducing the background so that the number of remaining events, nominally initiated by downward gamma rays, is at least roughly compatible with the number expected due to atmospheric gamma rays. We describe the cuts being studied, as well as their effects on both the background and the gamma-ray efficiency. To check the performance of the filters and estimate the amount of remaining background, the filters are applied to events from detailed Monte-Carlo simulations. We have simulated celestial gamma rays and an appropriate mix of proton and electron background including primaries, secondaries, albedo, etc. (Mizuno et al. 2002). The filters presented here for the prototype, a partially-instrumented single tower, are substantially different from those that have been used for the full GLAST/LAT. The preliminary results for the prototype suggest some complementary approaches to filtering that may be applicable to the flight instrument.