银河系球状星团M12（NGC 6218）的X射线源及其光学对应物
X-ray Sources and Their Optical Counterparts in the Galactic Globular Cluster M12 (NGC 6218)
摘要：
我们研究了钱德拉X射线天文台ACIS-S对银河系球状星团M12的观测。在26 ks的曝光时间下，我们在半质量半径（2.16弧分）内探测到6个X射线源，其中两个在星团的核心半径（0.72弧分）内。如果我们假设这些源都与球状星团M12有关，那么这些源的亮度L_{X}在0.3-7.0 keV之间的变化范围大致为10^{30}至10^{32}尔格·秒^{-1}。为了识别，我们还分析了哈勃太空望远镜（HST）高级巡天相机（ACS）和宽视场和行星相机2（WFPC 2）的数据，并确定了HST ACS视场内五个X射线源的光学对应物。根据X射线和光学特征，在HST ACS视场内发现了2-5个候选活动双星或激变变星和0-3个背景星系。基于X射线源的数量与相遇率和球状星团质量成正比的假设，我们认为M12内部存在2个X射线源，这与我们的观测结果是一致的。因此，M12中存在已确认的X射线源（可能是CV或AB），表明球状星团中X射线源的原始起源，这与前人的研究结果一致。
英文摘要：
We study a Chandra X-ray Observatory ACIS-S observation of the Galactic globular clusterM12. With a 26 ks exposure time, we detect 6 X-ray sources inside the half-mass radius (2.16 arcminutes) of which two are inside the core radius (0.72 arcminutes) of the cluster. If we assume these sources are all associated with globular cluster M12, the luminosity L_{X} among these sources between 0.3-7.0 keV varies roughly from 10^{30} to 10^{32} ergs s^{-1}. For identification, we also analyzed the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field and Planetary Camera 2 (WFPC2) data and identified the optical counterparts to five X-ray sources inside the HST ACS field of view. According to the X-ray and optical features, we found 2-5 candidate active binaries (ABs) or cataclysmic variables (CVs) and 0-3 background galaxies within the HST ACS field of view. Based on the assumption that the number of X-ray sources scales with the encounter rate and the mass of the globular cluster, we expect 2 X-ray source inside M12, and the expectation is consistent with our observational results. Therefore, the existence of identified X-ray sources (possible CVs or ABs) in M12 suggests the primordial origin of X-ray sources in globular clusters which is in agreement with previous studies.
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