麦哲伦型星系NGC 55中的瞬态X射线源群
Transient X-ray source population in the Magellanic-type galaxy NGC 55
摘要：
我们提出了15个候选的瞬态X射线源检测档案{ \it XMM-牛顿}和{ \it钱德拉}观测附近的麦哲伦型，SB（s）m星系NGC 55的光谱和时间特性。根据X射线颜色分类法，大部分的来源可以被确定为X射线双星（XRB），还有六个来源是软的，包括一个可能的超新星遗迹。我们对两个明亮的候选XRB的数据进行了详细的光谱和变异性分析。这两个源显示出强烈的短期X射线变化，其X射线光谱和硬度比是一致的XRB。这些结果，结合它们的高X射线光度（$\sim 10^{38}~\rm erg~s^{-1}$），强烈表明它们是黑洞（BH）双星。七个不太明亮的光源在正常和高速吸积模式下的光谱特性与中子星或BH XRB一致，但其中一个可能对应于背景星系（因为位置重合）。通过光谱分析，我们发现这6个软源都是候选的超软源，主要发射在软（0.3- 2 keV）X射线波段。7个来源的存档{ \it Hubble Space Telescope}光学图像可用，数据表明其中大多数可能是高质量XRB。我们的分析揭示了NGC 55中瞬态人口的异质性（6个高质量XRB，1个低质量XRB，6个SSS，1个活动星系核），帮助建立该星系的X射线特性与其他麦哲伦型星系的相似性。
英文摘要：
We present the spectral and temporal properties of 15 candidate transient X-ray sources detected in archival {\it XMM-Newton} and {\it Chandra} observations of the nearby Magellanic-type, SB(s)m galaxy NGC 55. On the basis of an X-ray color classification scheme, the majority of the sources may be identified as X-ray binaries (XRBs), and six sources are soft, including a likely supernova remnant. We perform a detailed spectral and variability analysis of the data for two bright candidate XRBs. Both sources displayed strong short-term X-ray variability, and their X-ray spectra and hardness ratios are consistent with those of XRBs. These results, combined with their high X-ray luminosities ($\sim 10^{38}~\rm erg~s^{-1}$), strongly suggest that they are black hole (BH) binaries. Seven less luminous sources have spectral properties consistent with those of neutron star or BH XRBs in both normal and high-rate accretion modes, but one of them is the likely counterpart to a background galaxy (because of positional coincidence). From the spectral analysis, we find that the six soft sources are candidate super soft sources (SSSs), with dominant emission in the soft (0.3--2 keV) X-ray band. Archival {\it Hubble Space Telescope} optical images for 7 sources are available, and the data suggest that most of them are likely to be high-mass XRBs. Our analysis has revealed the heterogeneous nature of the transient population in NGC 55 (6 high-mass XRBs, 1 low-mass XRBs, 6 SSSs, 1 active galactic nucleus), helping establish the similarity of the X-ray properties of this galaxy to those of other Magellanic-type galaxies.
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