天龙座矮球状星系场中低光度恒星X射线源的分类
Classification of low-luminosity stellar X-ray sources in the field of the Draco dwarf spheroidal galaxy
摘要：
先前对天龙座矮球状（dSph）星系场中X射线源的X射线光度函数的研究表明，在0.5-2 keV的软能量范围内存在一群未知的X射线源。2015年，在26个深XMM-牛顿中再次观察到天龙座dSph，观测为尚未分类的来源提供了新的研究机会。我们将之前对Draco dSph的X射线源的多波长研究中提出的分类标准应用于2009年和2015年XMM-牛顿数据集中检测到的源。这些判据是基于X射线研究和光学、近红外和中红外对应物的性质，使我们能够将背景活动星系核和星系与其他类型的X射线源区分开来。我们提出了X射线源的分类，其中对应物被确定为恒星对象根据我们的标准从多波长数据。我们在天龙座dSph中发现了三颗新的共生星，它们的X射线光度介于$\sim$ 3.5 $\times10^{34}$ erg s $^{-1}$和5.5 $\times10^{34}$ erg s $^{-1}$之间。X射线光谱分析表明，其中两颗共生星是$β$型。这是第一次在附近星系中发现这类共生星。在天龙座dSph的视野中，有八个源被归类为银河系M矮星。这些M矮星的距离在$\sim$ 140-800 pc之间，它们的X射线光度在$10^{28}-10^{29}$ erg与$^{-1}$之间，而X射线与辐射热光度的对数比log $(\frac{L_\text{X}}{L_\text{bol}})$在$-3.4$与$-2.1$之间。多重观测使我们能够研究M矮星的耀斑活动。此外，我们分类三个前景源，位于距离的顺序为$\sim$ 1-3千秒差距的领域的天龙座dSph。
英文摘要：
A previous study of the X-ray luminosity function of the X-ray sources in the field of the Draco dwarf spheroidal (dSph) galaxy indicated the presence of a population of unknown X-ray sources in the soft energy range of 0.5-2 keV. In 2015, Draco dSph was observed again in twenty-six deep XMM-Newton, observations providing an opportunity for a new study of the yet unclassified sources. We apply the classification criteria presented in our previous multi-wavelength study of the X-ray sources of the Draco dSph to the sources detected in the combined 2009 and 2015 XMM-Newton data set. These criteria are based on X-ray studies and properties of the optical, near-infrared, and mid-infrared counterparts and allows us to distinguish background active galactic nuclei~(AGNs) and galaxies from other types of X-ray sources. We present the classification of X-ray sources, for which the counterpart is identified as a stellar object based on our criteria from multi-wavelength data. We identify three new symbiotic stars in the Draco dSph with X-ray luminosities between $\sim$3.5$\times10^{34}$ erg s$^{-1}$ and 5.5$\times10^{34}$ erg s$^{-1}$. The X-ray spectral analysis shows that two of the classified symbiotic stars are $β$-type. This is the first identification of this class of symbiotic stars in a nearby galaxy. Eight sources are classified as Galactic M dwarfs in the field of the Draco dSph. The distances of these M dwarfs are between$\sim$140-800 pc, their X-ray luminosities are between $10^{28}-10^{29}$ erg s$^{-1}$ and the logarithmic ratio of X-ray to bolometric luminosity, log$(\frac{L_\text{X}}{L_\text{bol}})$, is between $-3.4$ to $-2.1$. The multiple observations allowed us to investigate flare activity of the M dwarfs. Moreover, we classified three foreground sources, located at distances of the order of $\sim$1-3 kpc in the field of the Draco dSph.
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