[论文解读] A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law Decaying XRT lightcurves and Implications for the Unified Origin of the X-rays

En-Wei Liang, Hou-Jun Lv+2 more|arXiv (Cornell University)|Feb 20, 2009

Gamma-ray bursts and supernovae被引用 1

一句话总结

本研究分析了17个伽马射线暴（GRBs）的Swift/XRT光 lightcurves，其X射线衰减呈单一幂律（SPL）衰减，发现其在10⁵秒时的X射线光度服从以log L = 45.5 ± 0.7为中心的正态分布。结果表明，SPL与经典浅衰减至正常衰减光曲线之间看似存在的差异，可能并非源于内在物理差异，而是初始时间参考点（T₀）选择所致，暗示两类GRBs的X射线辐射具有统一的起源。

ABSTRACT

By systematically analyzing the Swift/XRT lightcurves detected before 2008 October, we find 17 lightcurves that monotonously decay as a single power law (SPL) with an index of 1 ∼ 1.7 from tens (or hundreds) of seconds to ∼ 10 5 seconds post the GRB trigger. They are apparently different from the canonical lightcurves that are characterized by a shallow-to-normal decay transition. We compare the distributions of the observables of the prompt gamma-rays in the Swift/BAT band for the two groups of GRBs, but no statistical difference is found. The spectral characteristics of the X-rays, including the integrated X-ray spectral index, the column density of neutral hydrogen (NH) of the host galaxies, and the spectral evolution feature, between the two groups of GRBs are also consistent with each other. Interestingly, the SPL XRT lightcurves in the burst frame gradually merge into a conflux, and their luminosities at 10 5 seconds are normally distributed at log L/ergs s −1 = 45.5 ± 0.7. For the GRBs with canonical (shallow-to-normal) lightcurves, the normal decay segment has the similar feature. These results likely suggest that both the prompt gammarays and the X-rays for the two groups of GRBs may share the similar physical origins, and the apparent difference between the XRT lightcurves of two groups of GRBs may not be intrinsic. If the shallow decay is due to energy injection into the fireball, this suggests that the total energy budget after injection for both types of GRBs is similar. More intriguingly, the picture is consistent with the scenario that the apparent shallow-to-normal behavior is an artifact because of the improper choice of the zero time point (T0), as suggested by Yamazaki. To

研究动机与目标

• 调查GRBs中呈现单一幂律（SPL）衰减的X射线光曲线是否与具有经典浅衰减至正常衰减的光曲线属于不同的物理群体。
• 检验X射线光曲线形态的差异是否反映内在物理差异，还是观测中的伪影。
• 检查两类GRBs在喷发伽马射线性质和X射线光谱特征方面是否存在显著差异。
• 评估SPL光曲线的光度演化是否支持两类GRB之间存在共同的辐射机制。
• 评估浅衰减阶段是否如Yamazaki所提出的那样，是由于T₀选择不当所致。

提出的方法

• 对2008年10月前收集的Swift/XRT光曲线进行系统分析，重点关注呈现单调幂律衰减的光曲线。
• 从触发后数十秒至约10⁵秒期间，识别出17个呈现单一幂律（SPL）衰减的GRBs。
• 比较SPL GRBs与经典GRBs在Swift/BAT波段中的喷发伽马射线性质（如峰值光度、持续时间）的差异。
• 测量并比较两类组的X射线谱指数、中性氢柱密度（NH）及光谱演化特征。
• 计算SPL GRBs在10⁵秒时的X射线光度，并评估其分布特征。
• 评估T₀时间参考点对光曲线形态的影响，检验Yamazaki提出的假设：即浅衰减是由于T₀误标所致。

实验结果

研究问题

• RQ1 具有单一幂律X射线衰减的GRBs与具有浅衰减至正常衰减光曲线的GRBs在喷发伽马射线发射特性方面是否存在物理差异？
• RQ2 具有SPL和经典光曲线的GRBs在X射线光谱特征（包括谱指数和NH）方面是否存在统计学上的显著差异？
• RQ3 SPL衰减GRBs在10⁵秒时的X射线光度是否服从正态分布，且该分布是否与经典GRBs的正常衰减阶段一致？
• RQ4 观测到的浅衰减至正常衰减行为是否可能是由于初始时间参考点（T₀）选择不当所致的伪影？
• RQ5 光度和光谱演化模式是否表明两类GRB的X射线辐射具有统一的物理起源？

主要发现

• 具有单一幂律（SPL）衰减的GRBs在10⁵秒时的X射线光度服从以log L = 45.5 ± 0.7 erg s⁻¹为中心的正态分布。
• 在SPL和经典光曲线的GRBs之间，喷发伽马射线性质（如峰值光度、持续时间）未发现统计学上显著的差异。
• 两类GRBs的X射线谱指数、柱密度（NH）及光谱演化特征一致。
• 在爆发参考系中，SPL光曲线逐渐融合为一个共同的光度演化模式。
• 经典GRBs的正常衰减阶段在10⁵秒时也表现出类似的光度分布，暗示其具有共同的物理起源。
• 结果与假设一致：即看似存在的浅衰减至正常衰减行为是由于T₀选择不当所致，支持X射线辐射具有统一的辐射机制。