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GRB 090926A and bright late-time Fermi large area telescope gamma-ray burst afterglows

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posted on 2019-10-17, 15:32 authored by CA Swenson, A Maxham, PWA Roming, P Schady, L Vetere, BB Zhang, B Zhang, ST Holland, JA Kennea, NPM Kuin, SR Oates, KL Page, M De Pasquale
GRB 090926A was detected by both the Gamma-ray Burst Monitor and Large Area Telescope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. Swift follow-up observations began ~13 hr after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long-lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+105 s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 gamma-ray bursts; nine of these bursts, including GRB 090926A, also have been observed by Swift. Of the nine Swift-observed LAT bursts, six were detected by UVOT, with five of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for five years and has detected nearly 500 bursts, but has only seen ~30% of bursts with optical afterglows that live longer than 105 s. We have calculated the predicted gamma-ray fluence, as would have been seen by the Burst Alert Telescope (BAT) on board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate that the LAT bursts are more energetic than 88% of all Swift bursts and also have brighter than average X-ray and optical afterglows.



ASTROPHYSICAL JOURNAL LETTERS, 2010, 718 (1), pp. L14-L18 (5)

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