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.
FERMI LAT STACKING ANALYSIS OF SWIFT LOCALIZED GRBs / Ackermann M; Ajello M; Anderson B; Atwood WB; Axelsson M; Baldini L; Barbiellini G; Bastieri D; Bellazzini R; Bhat PN; Bissaldi E; Bonino R; Bottacini E; Brandt TJ; Bregeon J; Bruel P; Buehler R; Buson S; Caliandro GA; Cameron RA; Caragiulo M; Caraveo PA; Cecchi C; Charles E; Chekhtman A; Chiang J; Chiaro G; Ciprini S; Claus R; Cohen-Tanugi J; Conrad J; Cutini S; DAmmando F; de Angelis A; de Palma F; Desiante R; Di Venere L; Drell PS; Favuzzi C; Focke WB; Franckowiak A; Funk S; Fusco P; Gargano F; Gasparrini D; Gehrels N; Giglietto N; Giordano F; Giroletti M; Godfrey G; Grenier IA; Grove JE; Guiriec S; Hewitt JW; Hill AB; Horan D; Johannesson G; Kocevski D; Kouveliotou C; Kuss M; Larsson S; Li J; Li L; Longo F; Loparco F; Lovellette MN; Lubrano P; Mayer M; Mazziotta MN; McEnery JE; Michelson PF; Mizuno T; Monzani ME; Morselli A; Murgia S; Nemmen R; Nuss E; Ohno M; Ohsugi T; Omodei N; Orienti M; Orlando E; Paneque D; Perkins JS; Pesce-Rollins M; Piron F; Pivato G; Porter TA; Racusin JL; Raino S; Rando R; Razzano M; Reimer A; Reimer O; Schaal M; Schulz A; Sgro C; Siskind EJ; Spada F; Spandre G; Spinelli P; Takahashi H; Thayer JB; Tibaldo L; Tinivella M; Torres DF; Tosti G; Troja E; Vianello G; von Kienlin A; Werner M; Wood KS. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - STAMPA. - 822:2(2016), pp. 68.68-68.80. [10.3847/0004-637X/822/2/68]
FERMI LAT STACKING ANALYSIS OF SWIFT LOCALIZED GRBs
D'AMMANDO, FILIPPO;
2016
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
