[论文解读] Evidence for ultra-fast outflows in radio-quiet AGNs: I - detection and statistical incidence of Fe K-shell absorption lines

Francesco Tombesi, M. Cappi+5 more|arXiv (Cornell University)|Jun 14, 2010

Astrophysical Phenomena and Observations参考文献 89被引用 307

一句话总结

本研究通过统一分析42个射电宁静活动星系核（AGNs）的XMM-Newton数据，提供了超高速喷流（UFOs）存在的统计证据。在42个源中检测到22条蓝移的铁K壳层吸收线，能量高于7 keV，全局误报概率小于3×10⁻⁸，证实UFOs是一种普遍现象，覆盖分数约为0.4–0.6，速度高达约0.3c，很可能与吸积盘风有关。

ABSTRACT

We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton. We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies E>7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3x10^-8, and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability <10^-7. We identify the lines as Fe XXV and Fe XXVI K-shell resonant absorption. They are systematically blue-shifted, with a velocity distribution ranging from zero up to 0.3c, with a peak and mean value at 0.1c. We detect variability of the lines on both EWs and blue-shifted velocities among different observations even on time-scales as short as a few days, possibly suggesting somewhat compact absorbers. Moreover, we find no significant correlation between the cosmological red-shifts of the sources and the lines blue-shifted velocities, ruling out any systematic contamination by local absorption. If we define Ultra-fast Outflows (UFOs) those highly ionized absorbers with outflow velocities higher than 10^4 km/s, then the majority of the lines are consistent with being associated to UFOs and the fraction of objects with detected UFOs in the whole sample is at least 35%. This fraction is similar for Type 1 and Type 2 sources. The global covering fraction of the absorbers is consequently estimated to be in the range C=0.4-0.6, thereby implying large opening angles. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper.

研究动机与目标

• 统计量化在大规模、统一分析的射电宁静AGNs样本中蓝移Fe K壳层吸收线的出现率和特性。
• 通过分析XMM-Newton观测的完整样本，而非依赖先前报告的检测结果，克服发表偏倚。
• 确定所观测到的吸收线是否为AGNs的本征特征，而非局部吸收或随机涨落所致。
• 评估高度电离喷流的全局覆盖分数和速度分布，若速度v > 10⁴ km/s则定义为UFO。
• 确立这些特征作为AGN核心区域吸积盘风潜在标志的显著性。

提出的方法

• 在42个射电宁静AGNs的101次XMM-Newton EPIC pn观测中，盲搜高于6.4 keV的窄吸收特征。
• 采用统一的谱模型：吸收幂律谱加上高斯发射和吸收线。
• 应用F检验并进行大量蒙特卡洛模拟，评估检测显著性及误报概率。
• 利用XMM-Newton的MOS数据交叉验证检测结果，确认其稳健性。
• 通过线能量偏移计算蓝移速度，并与宇宙学红移比较，以排除局部吸收的可能性。
• 将UFO定义为速度大于10⁴ km/s的吸收物质，并估算其全局覆盖分数。

实验结果

研究问题

• RQ1 在统一分析的射电宁静AGNs样本中，蓝移Fe K壳层吸收线的统计出现率是多少？
• RQ2 所检测到的吸收线是否具有统计显著性，且不太可能是随机涨落？
• RQ3 蓝移速度是否与宇宙学红移相关，表明是局部吸收还是AGN本征喷流？
• RQ4 有多少比例的射电宁静AGNs拥有速度v > 10⁴ km/s的超高速喷流（UFO）？
• RQ5 吸收物质的全局覆盖分数是多少？这对其喷流几何结构有何启示？

主要发现

• 在101次XMM-Newton EPIC pn观测中总共检测到36条窄吸收线，其中22条在本征能量高于7 keV。
• 这22条高能线由随机涨落引起的全局概率小于3×10⁻⁸，证实其统计显著性。
• MOS数据的独立验证得到的误报概率小于10⁻⁷，进一步强化了检测的可靠性。
• 这些吸收线被识别为Fe XXV和Fe XXVI K壳层共振吸收，系统性蓝移，速度范围为0至约0.3c，峰值位于约0.1c。
• 样本中至少35%的源拥有UFO（v > 10⁴ km/s），且类型1与类型2 AGNs之间无显著差异。
• 吸收体的全局覆盖分数估计为C ~ 0.4–0.6，表明喷流物质具有较大的张角。