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FERMI LAT STACKING ANALYSIS OF SWIFT LOCALIZED GRBs
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Number of Authors: 112
2016 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 822, no 2, 68Article in journal (Refereed) Published
Abstract [en]

We perform a comprehensive stacking analysis of data collected by the Fermi. Large Area Telescope (LAT) of gamma-ray bursts (GRBs) localized by the Swift. spacecraft, which were not detected by the LAT but which fell within the instrument's field of view at the time of trigger. We examine a total of 79 GRBs by comparing the observed counts over a range of time intervals to that expected from designated background orbits, as well as by using a joint likelihood technique to model the expected distribution of stacked counts. We find strong evidence for subthreshold emission at MeV to GeV energies using both techniques. This observed excess is detected during intervals that include and exceed the durations typically characterizing the prompt emission observed at 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 a burst's prompt gamma-ray and afterglow X-ray flux both correlate with the strength of the subthreshold emission, the X-ray afterglow flux measured by Swift's X-ray Telescope at 11 hr post trigger correlates far more significantly. Overall, the extended nature of the subthreshold emission and its connection to the burst's afterglow brightness lend. further support to the external forward shock origin of the late-time emission detected by the LAT. These results suggest that the extended high-energy emission observed by the LAT may be a relatively common feature but remains undetected in a majority of bursts owing. to instrumental threshold effects.

Place, publisher, year, edition, pages
2016. Vol. 822, no 2, 68
Keyword [en]
gamma-ray burst: general, gamma rays: general, methods: data analysis, X-rays: bursts
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-131993DOI: 10.3847/0004-637X/822/2/68ISI: 000377204900011OAI: oai:DiVA.org:su-131993DiVA: diva2:952430
Available from: 2016-08-12 Created: 2016-07-05 Last updated: 2016-08-12Bibliographically approved

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Conrad, JanLarsson, StefanLi, Liang
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Department of PhysicsThe Oskar Klein Centre for Cosmo Particle Physics (OKC)Department of Astronomy
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