Journal Article

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

R.-F. Shen, R. Willingale, P. Kumar, P. T. O'Brien and P. A. Evans

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 393, issue 2, pages 598-606
Published in print February 2009 | ISSN: 0035-8711
Published online February 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14223.x
The dust scattering model cannot explain the shallow X-ray decay in GRB afterglows

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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, AV≳ 10, which contradicts current findings of AV= 0.1–0.7 from optical and X-ray afterglow observations. We conclude that the dust scattering model cannot explain the X-ray plateaus.

Keywords: radiation mechanisms: non-thermal; scattering; dust, extinction; gamma-rays: burst

Journal Article.  7026 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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