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尘埃散射模型不能解释GRB余辉中的浅X射线衰变

The Dust Scattering Model Can Not Explain The Shallow X-ray Decay in GRB Afterglows

Rong-Feng Shen, Richard Willingale, Pawan Kumar, Paul T. O'Brien, Phil A. Evans

摘要:

最近提出了一个尘埃散射模型来解释在伽马射线暴(GRB)早期余辉中观测到的浅X射线衰减(平台)。在这个模型中,平台区是由位于伽玛暴附近(大约10到几百秒差距)的尘埃发出的散射的即时X射线。在本文中,我们仔细研究了模型,发现散射发射经历了强烈的光谱软化随时间的推移,由于该模型的基本成分,较硬的X射线光子具有较小的散射角,因此到达较早,而较软的光子遭受较大的角度散射,到达较晚。该模型预测了一个重大变化,即,$Δ\b \sim 2 - 3$,在X射线光谱指数从平台开始到平台结束,而观察到的数据显示接近零软化期间的平台和平台到正常的过渡阶段。散射模型预测较硬X射线光曲线和较软X射线光曲线之间的大差异,即,较硬X射线中的平稳期比较软X射线中的平稳期结束得早得多。在数据中看不到这个特征。该模型所需的尘埃的大散射光学深度意味着强消光的光学,$A_V \gtrsim $ 10,这与目前的研究结果$A_V= 0.1 - 0.7$从光学和X射线余辉观测。我们得出结论,尘埃散射模型不能解释X射线平台。

英文摘要:

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, i.e., $Δ\b \sim 2 - 3$, in the X-ray spectral index from the beginning of the plateau toward the end of the plateau, while the observed data shows 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_V \gtrsim $ 10, which contradicts current findings of $A_V= 0.1 - 0.7$ from optical and X-ray afterglow observations. We conclude that the dust scattering model can not explain the X-ray plateaus.

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