[论文解读] The γ-ray sky seen at X-ray energies. II. The Swift hunt of Fermi BL Lac objects among unidentified γ-ray sources

E. J. Marchesini, A. Paggi+4 more|arXiv (Cornell University)|Jun 1, 2020

Astrophysics and Cosmic Phenomena参考文献 93被引用 2

一句话总结

本研究利用Swift/XRT的X射线数据，在费米未关联伽马射线源（UGSs）中识别潜在的BL Lac候选体，结合X射线特性与多波段数据。研究确认了50个BL Lac候选体，其中包括12个缺乏典型中红外或射电BL Lac特征的源，证明X射线选则是发现UGSs中隐藏BL Lac天体的强有力方法。

ABSTRACT

Context. Nearly 50% of all sources detected by the Fermi Large Area Telescope are classified as blazars or blazar candidates, one of the most elusive classes of active galaxies. Additional blazars can also be hidden within the sample of unidentified or unassociated γ-ray sources (UGSs) that constitute about one-third of all gamma-ray sources detected to date. We recently confirmed that the large majority of Fermi blazars of the BL Lac subclass have an X-ray counterpart. Aims. Using the X-ray properties of a BL Lac training set and combining these with archival multifrequency information, we aim to search for UGSs that could have a BL Lac source within their γ-ray positional uncertainty regions. Methods. We reduced and analyzed the Swift X-ray observations of a selected sample of 327 UGSs. We then compared the X-ray fluxes and hardness ratios of all sources detected in the pointed fields with those of known Fermi BL Lacs. Results. We find at least one X-ray source, lying within the γ-ray positional uncertainty at 95% confidence level, for 223 UGSs and a total of 464 X-ray sources in all fields analyzed. The X-ray properties of a large fraction of them, eventually combined with radio, infrared, and optical information, exhibit BL Lac multi-frequency behavior, thus allowing us to select high-confidence BL Lac candidates; some of them were recently observed during our optical spectroscopic campaign which confirmed their nature. Conclusions. We find that out of 50 X-ray sources that were confirmed as BL Lacs through optical spectroscopy, 12 do not show canonical mid-infrared or radio BL Lac properties. This indicates that the selection of X-ray BL Lac candidates is a strong method to find new counterparts within Fermi UGSs. Finally, we pinpoint a sample of 32 Swift/XRT candidate counterparts to Fermi UGSs that are most likely BL Lac objects.

研究动机与目标

• 识别费米未关联伽马射线源（UGSs）中隐藏的BL Lac天体，这些天体可能解释了费米源中未被解释的部分。
• 检验已知费米BL Lacs的X射线特性是否可作为识别UGSs中新对应体的可靠选择标准。
• 结合X射线数据与档案中的射电、红外和光学信息，确认候选源的性质。
• 通过多波段分析减少未关联伽马射线源的数量，识别出高置信度BL Lac对应体。

提出的方法

• 对327个费米未关联伽马射线源（UGSs）的Swift/XRT观测数据进行降噪与分析，以在其95%置信度伽马射线位置不确定区域内探测X射线对应体。
• 将检测到的X射线源的X射线通量与谱硬化指数，与一组已明确定义的已知费米BL Lacs训练样本进行比较。
• 整合多波段数据（射电、红外、光学）以评估与BL Lac特征一致的多频段行为。
• 对选定候选体进行光学光谱后续观测，以确认其为BL Lacs的性质。
• 使用统计标准，基于X射线与多波段特性将源分类为高置信度BL Lac候选体。
• 最终确定了32个最可能为BL Lac天体的高优先级Swift/XRT候选体。

实验结果

研究问题

• RQ1 仅凭X射线特性是否可用于识别未关联费米伽马射线源中的BL Lac候选体？
• RQ2 在未关联费米源的位置不确定区域内，有多少个源包含X射线发射的BL Lac天体？
• RQ3 是否有已确认的BL Lac候选体缺乏典型的中红外或射电BL Lac特征，从而挑战标准选择标准？
• RQ4 多波段数据（射电、红外、光学）在多大程度上提升了仅基于X射线数据的BL Lac候选体选择的可靠性？
• RQ5 通过基于X射线的识别方法，费米UGSs中有多少比例可由BL Lac天体解释？

主要发现

• 在327个未关联费米源中，有223个在其95%置信度伽马射线位置不确定区域中至少发现一个X射线源。
• 在所有分析的Swift/XRT场中总共检测到464个X射线源，其中许多表现出与BL Lacs一致的X射线特性。
• 通过光学光谱确认了50个X射线源为BL Lacs，其中包括12个缺乏典型中红外或射电BL Lac特征的源。
• 本研究证实，即使在其他方法无法识别的情况下，X射线选择仍是识别费米UGSs中BL Lac对应体的稳健方法。
• 确定了一个包含32个高置信度Swift/XRT候选体的样本，这些候选体最可能为BL Lac天体。
• 多波段数据的引入显著提升了仅依赖X射线特性时的BL Lac候选体选择可靠性。