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Title: GRB 130925A: an ultralong gamma ray burst with a dust-echo afterglow, and implications for the origin of the ultralong GRBs
Authors: Evans, P. A.
Willingale, R.
Osborne, J. P.
O'Brien, P. T.
Tanvir, N. R.
Frederiks, D. D.
Pal'shin, V. D.
Svinkin, D. S.
Lien, A.
Cummings, J.
Xiong, S.
Zhang, B-B.
Goetz, D.
Savchenko, V.
Negoro, H.
Nakahira, S.
Suzuki, K.
Wiersema, K.
Starling, R. L. C.
Castro-Tirado, A. J.
Beardmore, A. P.
Sanchez-Ramirez, R.
Gorosabel, J.
Jeong, S.
Kennea, J. A.
Burrows, D. N.
Gehrels, N.
First Published: 14-Aug-2014
Publisher: Oxford University Press (OUP), Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society (October 11, 2014) 444 (1): 250-267.
Abstract: GRB 130925A was an unusual gamma ray burst (GRB), consisting of three distinct episodes of high-energy emission spanning ∼20 ks, making it a member of the proposed category of ‘ultralong’ bursts. It was also unusual in that its late-time X-ray emission observed by Swift was very soft, and showed a strong hard-to-soft spectral evolution with time. This evolution, rarely seen in GRB afterglows, can be well modelled as the dust-scattered echo of the prompt emission, with stringent limits on the contribution from the normal afterglow (i.e. external shock) emission. We consider and reject the possibility that GRB 130925A was some form of tidal disruption event, and instead show that if the circumburst density around GRB 130925A is low, the long duration of the burst and faint external shock emission are naturally explained. Indeed, we suggest that the ultralong GRBs as a class can be explained as those with low circumburst densities, such that the deceleration time (at which point the material ejected from the nascent black hole is decelerated by the circumburst medium) is ∼20 ks, as opposed to a few hundred seconds for the normal long GRBs. The increased deceleration radius means that more of the ejected shells can interact before reaching the external shock, naturally explaining both the increased duration of GRB 130925A, the duration of its prompt pulses, and the fainter-than-normal afterglow.
DOI Link: 10.1093/mnras/stu1459
ISSN: 0035-8711
eISSN: 1365-2966
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Archived with reference to SHERPA/RoMEO and publisher website. This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society (C) 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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