Fermi-LAT Stacking Analysis of Swift Localized GRBs

Author: Daniel Kocevski

Abstract: We present a stacking analysis of Swift detected GRBs with XRT localizations that fell within the LAT field-of-view, but were undetected above 100 MeV. We examine a total of 65 GRBs by comparing the observed counts to that expected from an empirical background model, as well as by using a composite likelihood method, to place limits on the combined 0.1-30 GeV flux during the burst's prompt emission. We find no significant excess above the combined background during the prompt emission of the bursts when compared to performing the same analysis on blank regions of the sky. The large difference between out limits and the flux distribution of LAT detected GRBs may indicate that high energy cut-offs in GRB spectra may explain the lack of emission above 100 MeV for a majority of bursts.