Searching for needles in haystacks—using the Fermi/GBM to Find GRB γ-rays with the Fermi/LAT detector

Abstract

From the launch of the Fermi Gamma-ray Space Telescope to 2010 July 9, the Gamma-ray Burst Monitor (GBM) has detected 497 probable GRB events. Twenty-two of these satisfy the simultaneous requirements of an estimated burst direction within 52° of the Fermi Large Area Telescope (LAT) boresight and a low energy fluence exceeding 5 μerg cm¯². Using matched filter techniques, the spatially correlated Fermi/LAT photon data above 100 MeV have been examined for evidence of bursts that have so far evaded detection at these energies. High energy emission is detected with great confidence for one event, GRB 090228A. Since the LAT has significantly better angular resolution than the GBM, real-time application of these methods could open the door to optical identification and richer characterization of a larger fraction of the relatively rare GRBs that include high energy emission.