伽玛暴及其早期X射线余辉的观测特征

The Characteristics of the Prompt Gamma-rays and Early X-ray Afterglows of Gamma-ray Bursts

Swift卫星的X射线望远镜观测揭示部分伽玛暴的早期余辉光变曲线有一个缓慢衰减的成分,而相当一部分却没有这样的成分.研究比较这两种暴的观测性质发现两类暴的持续时间、伽玛辐射总流量、谱指数、谱硬度比峰值能量等物理量均没有显著差异.然而有该成分的那些伽玛暴谱比较软、早期X射线余辉比较弱、伽玛射线辐射效率显著高于没有这个成分的那些暴.结果表明两类暴的前身星和中心机制一致,是否呈现这个缓慢衰减成分可能取决于外部介质.

After the successful launch of NASA＇s dedicated gamma-ray burst （GRB） mission, Swift, very early X-ray afterglow data for a large sample of GRBs have been recorded by the on board X-ray Telescope （XRT）. The data show that the early X-ray afterglow light curves of some GRBs have a shallow decay component, but some have not. The physics mechanisms of this component are debated. We compare the characteristics of the prompt gamzna-rays and the early X-ray afterglows for the two kinds of GRBs with a sample of 29 GRBs. Our results show that the properties of the prompt gamma-rays, including burst duration, gamma-ray flux, gamma-ray spectral index, hardness ratio, and peak energy of the νfν spectrum, have no significant difference for the two kinds of bursts. The early X- ray afterglows, however, show significant difference. Those bursts that have shallow decay components tend to have soft and dark X-ray afterglows, while the X-ray afterglows of those bursts that without shallow decay components tend to be hard and bright, and the GRB efficiency of the two kinds of bursts have also significant difference. The efficiency of those bursts with a shallow decay component is significantly higher that that of the bursts without this component. These results might suggest that the two kinds of GRBs have the same progenitor, and their central engines are similar. The early shallow decay component is possibly due the surrounding medium.