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We perform a comprehensive stacking analysis of data collected by the FermiLarge Area Telescope (LAT) of gamma-ray bursts (GRB) localized by the Swiftspacecraft, which were not detected by the LAT but which fell within theinstrument's field of view at the time of trigger. We examine a total of 79GRBs by comparing the observed counts over a range of time intervals to thatexpected from designated background orbits, as well as by using a jointlikelihood technique to model the expected distribution of stacked counts. Wefind strong evidence for subthreshold emission at MeV to GeV energies usingboth techniques. This observed excess is detected during intervals that includeand exceed the durations typically characterizing the prompt emission observedat keV energies and lasts at least 2700 s after the co-aligned burst trigger.By utilizing a novel cumulative likelihood analysis, we find that although aburst's prompt gamma-ray and afterglow X-ray flux both correlate with thestrength of the subthreshold emission, the X-ray afterglow flux measured bySwift's X-ray Telescope (XRT) at 11 hr post trigger correlates far moresignificantly. Overall, the extended nature of the subthreshold emission andits connection to the burst's afterglow brightness lend further support to theexternal forward shock origin of the late-time emission detected by the LAT.These results suggest that the extended high-energy emission observed by theLAT may be a relatively common feature but remains undetected in a majority ofbursts owing to instrumental threshold effects.

Fermi LAT Stacking Analysis of Swift Localized Gamma-ray Bursts