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The dust scattering model cannot explain the shallow X-ray decay in GRB afterglows

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journal contribution
posted on 24.10.2012, 09:21 by R-F. Shen, P. Kumar, R. Willingale, P. T. O'Brien, P. A. Evans
A dust scattering model was recently proposed to explain the shallow X-ray decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows. In this model, the plateau is the scattered prompt X-ray emission by the dust located close (about 10 to a few hundred pc) to the GRB site. In this paper, we carefully investigate the model and find that the scattered emission undergoes strong spectral softening with time, due to the model's essential ingredient that harder X-ray photons have smaller scattering angle thus arrive earlier, while softer photons suffer larger angle scattering and arrive later. The model predicts a significant change, that is Δβ∼ 2–3, in the X-ray spectral index from the beginning of the plateau towards the end of the plateau, while the observed data show close to zero softening during the plateau and the plateau-to-normal transition phase. The scattering model predicts a big difference between the harder X-ray light curve and the softer X-ray light curve, i.e. the plateau in harder X-rays ends much earlier than in softer X-rays. This feature is not seen in the data. The large scattering optical depths of the dust required by the model imply strong extinction in optical, A[subscript: V]≳ 10, which contradicts current findings of A[subscript: V]= 0.1–0.7 from optical and X-ray afterglow observations. We conclude that the dust scattering model cannot explain the X-ray plateaus.

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Citation

Monthly Notices of the Royal Astronomical Society, 2009, 393 (2), pp. 598-606

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Oxford University Press

issn

0035-8711

eissn

1365-2966

Copyright date

2009

Available date

24/10/2012

Publisher version

http://mnras.oxfordjournals.org/content/393/2/598

Language

en

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