Constraining the Circumburst Medium of Gamma-Ray Bursts with X-Ray Afterglows
Constraining the Circumburst Medium of Gamma-Ray Bursts with X-Ray Afterglows
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DOI：
10.3847/1538-4357/ac3de4
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发表时间：
2021-11
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影响因子：
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通讯作者：
Xiaoqi Tian;Y. Qin;Mei Du;S. Yi;Yanke Tang
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作者：
Xiaoqi Tian;Y. Qin;Mei Du;S. Yi;Yanke Tang
长伽马射线暴（GRB）被认为源自大质量恒星的核心塌缩。据信，余辉特性是由周围星际介质（ISM）中材料的密度决定的。因此，环爆密度可用于区分星际风 n(R) ∝ R -k 和常密度介质 (ISM) n(R)=const 。先前对不同余辉样本的研究表明，GRB的环爆介质既不是简单地受到星际风的支持，也不是完全受到ISM的青睐。在这项工作中，我们的新样本由 39 个早期 X 射线余辉中具有平滑开始的凹凸状特征的伽玛暴组成，其中 20 个伽玛暴具有红移测量值。通过使用平滑的破幂律函数来拟合 X 射线光曲线的凸块，我们推导出 FWHM 作为特征宽度 (ω)，以及凸块的上升和衰减时间尺度 (T r 和 T d )。 X射线凸点时间尺度之间的相关性与之前在光学余辉中发现的相关性类似。基于薄壳情况的火球前激波模型，得到电子能谱指数p的分布，并进一步约束介质密度分布指数k。发现新的推断 k 集中在 1.0，范围从 0.2 到 1.8。这一发现与之前的研究一致。我们对 X 射线余辉的详细调查的结论表明，所选伽马暴的环境介质并不均匀，即既不是 ISM，也不是典型的星际风。
Long gamma-ray bursts (GRBs) are considered to originate from core collapse of massive stars. It is believed that the afterglow property is determined by the density of the material in the surrounding interstellar medium (ISM). Therefore, the circumburst density can be used to distinguish between an interstellar wind, n(R) ∝ R −k , and a constant-density medium (ISM), n(R)=const . Previous studies with different afterglow samples show that the circumburst medium of GRBs is neither simply supported by an interstellar wind nor completely favored by an ISM. In this work, our new sample consists of 39 GRBs with smoothly onset bump-like features in early X-ray afterglows, in which 20 GRBs have the redshift measurements. By using a smooth broken power-law function to fit the bumps of X-ray light curves, we derive the FWHM as the feature width (ω), as well as the rise and decay timescales of the bumps (T r and T d ). The correlations between the timescales of X-ray bumps are similar to those found previously in the optical afterglows. Based on the fireball forward shock model of the thin shell case, we obtain the distribution of the electron spectral index p and further constrain the medium density distribution index k. The new inferred k is found to be concentrated at 1.0, with a range from 0.2 to 1.8. This finding is consistent with previous studies. The conclusion of our detailed investigation for X-ray afterglows suggests that the ambient medium of the selected GRBs is not homogeneous, i.e., neither ISM nor the typical interstellar wind.