快速膨胀调查：动机、策略和首次X射线结果
The Swift Bulge Survey: Motivation, Strategy, and First X-ray Results
摘要：
极微弱X射线瞬变（VFX T）是峰值X射线亮度（$L_X$）为$L_X\lesssim10^{36}$ erg/s的X射线瞬变，尚未得到充分理解。我们与斯威夫特天文台一起对银河系凸出部的16平方度进行了调查，使用短（60秒）曝光，并在2017-18年的19个时期内每2周返回一次（从2017年11月到2018年2月，凸出部处于太阳约束状态）。我们的主要目标是探测和研究银河系隆起中各种类型的X射线源的VFXT行为。在这项工作中，我们解释了观测策略的调查，比较我们的结果与预期数量的源检测每类，并讨论了我们的调查银河系VFXT人口的限制。我们检测到91个X射线源，其中25个明显变化了至少10倍。这些X射线源中有45个已知的对应物：17颗色球活跃星，12颗X射线双星，5颗激变变星（和4个候选变星），3个共生系统，2个射电双星，1个活动星系核和一个年轻星团。其他46个是以前未确定的性质。我们利用X射线硬度比，搜索光学/红外同行在公布的目录，和流量比从静止到爆发，以约束未知来源的性质。在这46颗星中，有7颗是新发现的硬瞬变星，很可能是VFXT X射线双星。此外，我们发现强有力的新证据的共生性质的4个源在我们的完整的样本，和新的证据吸积功率在6个X射线源与光学同行。我们的研究结果表明，很大一部分VXFT可能是由共生系统组成的。
英文摘要：
Very faint X-ray transients (VFXTs) are X-ray transients with peak X-ray luminosities ($L_X$) of $L_X\lesssim10^{36}$ erg/s, which are not well-understood. We carried out a survey of 16 square degrees of the Galactic Bulge with the Swift Observatory, using short (60 s) exposures, and returning every 2 weeks for 19 epochs in 2017-18 (with a gap from November 2017 to February 2018, when the Bulge was in sun-constraint). Our main goal was to detect and study VFXT behaviour in the Galactic Bulge across various classes of X-ray sources. In this work, we explain the observing strategy of the survey, compare our results with the expected number of source detections per class, and discuss the constraints from our survey on the Galactic VFXT population. We detected 91 X-ray sources, 25 of which have clearly varied by a factor of at least 10. 45 of these X-ray sources have known counterparts: 17 chromospherically active stars, 12 X-ray binaries, 5 cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio pulsars, 1 AGN, and a young star cluster. The other 46 are of previously undetermined nature. We utilize X-ray hardness ratios, searches for optical/infrared counterparts in published catalogs, and flux ratios from quiescence to outburst to constrain the nature of the unknown sources. Of these 46, 7 are newly discovered hard transients, which are likely VFXT X-ray binaries. Furthermore, we find strong new evidence for a symbiotic nature of 4 sources in our full sample, and new evidence for accretion power in 6 X-ray sources with optical counterparts. Our findings indicate that a large subset of VXFTs is likely made up of symbiotic systems.
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