Weikang Zheng, Carl W. Akerlof, Shashi B. Pandey, Timothy A. McKay, Binbin Zhang, Bing Zhang, Takanori Sakamoto
Launched on June 11, 2008, the LAT instrument onboard the $Fermi$ Gamma-ray Space Telescope has provided a rare opportunity to study high energy photon emission from gamma-ray bursts. Although the majority of such events (27) have been identified by the Fermi LAT Collaboration, four were uncovered by using more sensitive statistical techniques (Akerlof et al 2010, Akerlof et al 2011, Zheng et al 2012). In this paper, we continue our earlier work by finding four more GRBs associated with high energy photon emission, GRB 100718A, 110709A, 111117A and 120107A. To systematize our matched filter approach, a pipeline has been developed to identify these objects in near 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 $\pm$ 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 GBM and the number observed by the 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 this entire phenomena.
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http://arxiv.org/abs/1203.5113
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