Title:

Obtaining the GBM Untargeted Search Confirmation Rate of GRBs

On August 17, 2017, the Fermi Gamma-ray Burst Monitor (GBM) witnessed a short gamma-ray burst approximately 2 seconds after gravitational waves were observed from the binary neutron star merger GW170817. The event proved that short gamma-ray bursts are the result of binary neutron star collisions. In the hopes of finding more bursts coincident with gravitational waves, subthreshold GBM events are being considered. The GBM Untargeted Search is being used to find subthreshold gamma-ray signals that are too weak or geometrically poor to automatically trigger the spacecraft. Using Python, we analyzed the candidate short gamma-ray bursts found by the Untargeted Search. These were then compared to GBM triggered bursts to verify that the Untargeted Search was successful in finding and localizing these events. Of 159 GBM short gamma-ray burst triggers between January 2013 and March 2017, the search found 153. The missing 6 were found to be the result of missing data or a very weak event. An updated version of the Untargeted Search yielded a 21% improvement in candidate localization. A modification of the requirements for a location match resulted in a 52% improvement in candidate localization.

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