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GRB 081028 and its late-time afterglow re-brightening

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journal contribution
posted on 24.10.2012, 09:06 by R. Margutti, G. Chincarini, C. Guidorzi, J. Mao, A. Moretti, F. Pasotti, F. Genet, J. Granot, R. B. Duran, P. Schady, T. Sakamoto, S. D. Barthelmy, N. Gehrels, A. Miller, J. S. Bloom, D. Starr, G. Olofsson, P. A. Evans, J. P. U. Fynbo, D. Malesani, D. N. Burrows, P. W. A. Roming
Swift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure E[subscript: γ,iso] measurements.

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Citation

Monthly Notices of the Royal Astronomical Society, 2010, 402 (1), pp. 46-64

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Oxford University Press (OUP)

issn

0035-8711

eissn

1365-2966

Copyright date

2009

Available date

24/10/2012

Publisher version

http://mnras.oxfordjournals.org/content/402/1/46

Language

en

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