标题： 伽马射线暴环境中的时间依赖性光电吸收、光致电离和荧光线发射 显示英文标题

标题： Time-dependent photoelectric absorption, photoionization and fluorescence line emission in gamma-ray burst environments

摘要： 如果伽马射线暴与年轻星系中的密集恒星形成区域有关，密集的环绕爆发天体物质（CBM）将发生光电吸收。 随着爆发的发展，周围物质被光致电离，导致荧光线发射和减少的光电吸收不透明度。 我们详细分析了这一过程，考虑了时间依赖的光电吸收、CBM的光致电离和荧光线发射。 我们发现，即使伽马射线暴发生在密集的恒星形成区域，伽马射线暴环境的光致电离会导致在爆发后几周到几年的时间尺度上出现恒定但非常微弱的延迟荧光线发射。 一个随时间变化的铁~K边缘吸收特征可以作为诊断工具，揭示CBM的密度结构，并可能提供红移测量的机会。 我们还研究了光电吸收是否可能是某些明亮BATSE $\gamma$-ray爆发低能谱斜率光谱演化的因素，这些爆发在峰值能量以下表现出极其硬的光谱，与光学薄同步加速器冲击模型不一致。 我们发现，为了解释低于几百keV的观测到的硬光谱及其时间演化，伽马射线暴环境中需要非常强的金属富集（约100倍于太阳系丰度），并且CBM的空间分布需要相当特殊。 显示英文摘要

摘要： If gamma-ray bursts are associated with dense star-forming regions in young galaxies, photoelectric absorption by the dense circumburster material (CBM) will occur. As the burst evolves, the surrounding material is photoionized, leading to fluorescence line emission and reduced photoelectric absorption opacity. We have analyzed this process in detail, accounting for the time-dependent photoelectric absorption, photoionization and fluorescence line emission from the CBM. We find that even if GRBs are hosted in dense star-forming regions, photoionization of the GRB environment leads to a constant, but very weak level of delayed fluorescence line emission on timescales of weeks to years after the burst. A temporally evolving iron~K edge absorption feature can serve as diagnostic tool to reveal the density structure of the CBM and may provide an opportunity for redshift measurements. We also investigated whether photoelectric absorption could be responsible for the spectral evolution of the low-energy slopes of some bright BATSE $\gamma$-ray bursts displaying extremely hard spectra below the peak energy, inconsistent with the optically-thin synchrotron shock model. We find that a very strong metal enrichment (~ 100 times solar-system abundances) in the gamma-ray burst environment and a rather peculiar spatial distribution of the CBM would be necessary in order to account for the observed hard spectra below a few 100 keV and their temporal evolution.