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GRB 130925A: an ultralong gamma ray burst with a dust-echo afterglow, and implications for the origin of the ultralong GRBs

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posted on 19.02.2015, 16:23 by P. A. Evans, R. Willingale, J. P. Osborne, P. T. O'Brien, N. R. Tanvir, D. D. Frederiks, V. D. Pal'shin, D. S. Svinkin, A. Lien, J. Cummings, S. Xiong, B-B. Zhang, D. Goetz, V. Savchenko, H. Negoro, S. Nakahira, K. Suzuki, K. Wiersema, R. L. C. Starling, A. J. Castro-Tirado, A. P. Beardmore, R. Sanchez-Ramirez, J. Gorosabel, S. Jeong, J. A. Kennea, D. N. Burrows, N. Gehrels
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.

Funding

This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. PAE, JPO, KW and APB acknowledge UK Space Agency support. The Konus-Wind experiment is partially supported by a Russian Space Agency contract, RFBR grants 12-02-00032a and 13-02-12017 ofi-m. DNB and JAK acknowledge support from NASA contract NAS5-00136. This work includes observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. This work was partially supported by the Spanish Ministry project AYA2012-29727-C03-01.

History

Citation

Monthly Notices of the Royal Astronomical Society (October 11, 2014) 444 (1): 250-267.

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society (October 11

Publisher

Oxford University Press (OUP), Royal Astronomical Society

issn

0035-8711

eissn

1365-2966

Copyright date

2014

Available date

19/02/2015

Publisher version

http://mnras.oxfordjournals.org/content/444/1/250

Language

en