Abstract

We present the results of a stacking analysis of Swift detected GRBs with XRT localizations that fell within the LAT field-of-view at the time of trigger prior to Feb 1st, 2012. We perform two types of stacking analysis on this sample of 82 GRBs. The first is a composite likelihood analysis in which the likelihood profiles for each individual burst is calculated separately and added together to obtain an upper limit on the combined flux and spectral index of the sample. The second method uses an estimated background to compare the total observed counts to the total expected counts extracted from the GRB location within a fixed 100s duration and 0.75-10 GeV energy range. We find no evidence for a significant detection in the co-added counts and likelihood maps. We discuss the implication of this non-detection on the intrinsic shape of the GRB spectra and on the ubiquity of extra high-energy spectral components above 75 MeV.