Abstract:
Launched on 2008 June 11, the Large Area Telescope (LAT) instrument on board the Fermi Gamma-ray Space
Telescope has provided a rare opportunity to study high-energy photon emission from gamma-ray bursts (GRBs).
Although the majority of such events (27) have been identified by the Fermi-LAT Collaboration, four were uncovered
by using more sensitive statistical techniques. In this paper, we continue our earlier work by finding three more
GRBs associated with high-energy photon emission, GRB 110709A, 111117A, and 120107A. To systematize our
matched filter approach, a pipeline has been developed to identify these objects in nearly real time. GRB 120107A
is the first product of this analysis procedure. Despite the reduced threshold for identification, the number of GRB
events has not increased significantly. This relative dearth of events with low photon number prompted a study of the
apparent photon number distribution. We find an extremely good fit to a simple power law with an exponent of −1.8
± 0.3 for the differential distribution. As might be expected, there is a substantial correlation between the number
of lower energy photons detected by the Gamma-ray Burst Monitor (GBM) and the number observed by LAT.
Thus, high-energy photon emission is associated with some but not all of the brighter GBM events. Deeper studies
of the properties of the small population of high-energy emitting bursts may eventually yield a better understanding
of these entire phenomena.
