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arXiv 0902.3504astro-ph.HE
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Swift/XRT数据的综合分析:IV。单次幂律衰减光变曲线与正则光变曲线及对X射线统一起源的启示

A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law Decaying Lightcurves vs. Canonical Lightcurves and Implications for a Unified Origin of X-rays

En-Wei Liang, Hou-Jun Lv, Shu-Jin Hou, Bin-Bin Zhang, Bing Zhang

摘要:

通过分析2009年7月之前观测到的雨燕/XRT光变曲线,发现19个事例在伽玛暴触发后的几十秒(或几百秒)到10^5秒内,以1 ~ 1.7的指数单调幂律衰减,明显不同于典型的由浅到正态衰减转变的光变曲线。两个样品的瞬发γ射线没有统计学差异,X射线性质也相似,尽管SPL样品的宿主星系的NH值略低,能量释放比典型样品大。在爆发标架中,SPL XRT光变曲线逐渐合并为一个汇流,而正则样品的正常衰变段也具有相同的特征。与正常衰变段类似,SPL光变曲线满足外激波模型的闭合关系。如果X射线是伽玛暴火球的余辉,我们的研究结果表明,浅衰变将是由于能量注入和总能量预算注入后的两个样本的伽玛暴是可比的。更有趣的是,我们发现山崎提出的一个先前的X射线发射模型更直接地解释数据。结果表明,X射线的零时间(T0)满足对数正态分布,SPL样品的T0可以忽略不计,这与T0分布在低端的尾部是一致的。以T0为参考,正则XRT光变曲线很好地跟踪SPL光变曲线。在我们的分析中,典型光变曲线的T0通常比已知的前兆从主伽玛暴的偏移大得多,这表明X射线可能是伽玛暴触发前外部冲击的发射分量。在大多数XRT光变曲线中没有检测到类似喷流的破裂,这意味着它的喷流的张开角通常很大。

英文摘要:

By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19 cases that monotonously decay as a single power law (SPL) with an index of 1 ~ 1.7 from tens (or hundreds) to ~ 10^5 seconds post the GRB trigger, apparently different from the canonical lightcurves characterized by a shallow-to-normal decay transition. No statistical difference is found in their prompt gamma-rays, and the X-ray properties of the two samples are also similar, although the SPL sample tend to have a slightly lower NH value of the host galaxies and larger energy release compared with the canonical sample. The SPL XRT lightcurves in the burst frame gradually merge into a conflux.The normal decay segment for the canonical sample has the same feature. Similar to the normal decay segment, the SPL lightcurves satisfy the closure relations of external shock models. If the X-rays are the afterglow of the GRB fireball, our results indicate that the shallow decay would be due to energy injection and the total energy budget after injection for both samples of GRBs is comparable. More intriguing, we find that a prior X-ray emission model proposed by Yamazaki is more straightforward to interpret the data. We show that the zero times (T0) of the X-rays atisfy a log-normal distribution, and the negligible T0's of the SPL sample are consistent with being the tail of T0 distributions at low end. Referenced to T0, the canonical XRT lightcurves well trace the SPL lightcurves. The T0's of the canonical lightcurves in our analysis are usually much larger than the offsets of the known precursors from the main GRBs, indicating the X-rays would be an emission component from external shocks prior to GRB trigger. The lack of detection of a jet-like break in most XRT lightcurves implies that the opening angle of its jet would be usually large.

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