Detecting High-Energy Emission from Gamma-Ray Bursts with EGRET and GLAST
Wren, David Nathan
Ritz, Steven M
Sullivan, Gregory W
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The research described in this dissertation explores the detection of high-energy emission from gamma-ray bursts (GRBs) with EGRET and GLAST. Data from the EGRET experiment were searched for evidence of ~1-250 MeV emission that preceded or followed gamma-ray bursts on a time scale of hours. This led to the discovery of a gamma-ray burst with high-energy, post-quiescent emission from the prompt phase that was coincident with lower-energy (keV) emission. To do detailed event filtering studies for the GLAST Large Area Telescope (LAT), the flight software event filter was embedded in the standard science analysis environment. The event trigger rate, reasons why it must be reduced, and hardware-level methods of reducing it are studied. Much work was done to improve the performance of the prototype event filter, and additional work was done to develop algorithms to allow the LAT to distinguish Earth albedo photons from celestial gamma-rays, and to eliminate albedo events from the data stream. It is shown that it is possible to reduce the background rate to meet LAT mission requirements, while simultaneously keeping the gamma-ray acceptance rate high enough to exceed the relevant LAT requirements for those events. Using the onboard event filter, real-time, onboard, gamma-ray burst detection was then studied. A detection algorithm had been developed by members of the LAT collaboration, but the algorithm required a lower onboard background rate than the basic LAT requirement for downlink, in addition to knowledge of incident gamma-ray directions. Therefore, several methods of reducing the background rate to acceptable levels were provided, and onboard track reconstruction methods were created and tested. GRB detection was tested for two background filters and two track reconstruction methods for simulated bursts that had realistic light curves and spectral characteristics. With prototype background cuts, track reconstruction, and burst detection algorithms, the LAT burst detection requirements were exceeded. Suggestions were offered about how to enhance burst detection performance in the coming months before GLAST is launched.