Zhang 2006 用"内激波余辉"和曲率效应联合解释早期X余辉的陡降阶段

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astro-ph/0612246 [abs, ps, pdf, other] :

Title: Spectral Evolution of GRB Tails: Central Engine and Internal Shock Afterglows?
Authors: Bin-Bin Zhang, En-Wei Liang, Bing Zhang
Comments: 11 pages,2 figures

A steep decay segment tens to hundreds of seconds after the gamma-ray burst (GRB) prompt emission is commonly observed in the {\em Swift} XRT light curves, which is regarded as the tail emission of the prompt gamma-rays. The most straightforward interpretation is the curvature effect due to delay of propagation of photons from larger angles with respect to the line of sight. Prompted by the observed strong spectral evolution in the tails of GRB 060218 and GRB 060614, we present a systematic time-resolved spectral analysis of 17 bright GRB tails observed by XRT. While 7 tails in our sample have no spectral evolution and can be explained with the curvature effect, the other 10 tails all show significant hard-to-soft spectral evolution. A toy model that combines the curvature effect with an underlying putative central engine afterglow component can roughly explain the observed light curves and spectral evolutions for 7 of them. The suggested central engine afterglow is typically soft ($\beta=2.5\sim 6.4$) and decays as normal GRB afterglows (typically $\alpha=0.8\sim 1.5$), similar to the late afterglow of GRB 060218. There are 3 cases (GRB 050724, GRB 060218, and GRB 060614) that cannot be described by this model. We suggest that these tails may be interpreted as an internal shock afterglow due to cooling of the shock-heated region. More detailed physical models are called for to understand these two possibly new types of afterglows.