在M51发现两个食X射线双星
Discovery of two eclipsing X-ray binaries in M51
摘要：
我们在M1，51中发现了两个发光的（且高度可变的）X射线源的食和陷。一个（CXOM 51 J132943.3 $+$ 471135）是超铝超软源，具有在约0.1keV的温度下的热谱和约$10^4$ km的特征黑体半径。另一个（CXOM 51 J132946.1 $+$ 471042）具有额外的热等离子体发射的双组分光谱;它在2005年和2012年爆发期间接近$10^{39}$ erg s $^{-1}$的X射线光度。根据一系列{ \it Chandra}观测中三次日食的时间，我们确定了CXOM 51 J132946.1 $+$ 471042的双星周期（$52.75 \pm 0.63$ hr）和日食分数（$22\% \pm 0.1\%$）。我们还确定了档案{ \it哈勃太空望远镜}图像中的蓝色光学对应物，与大质量施主星（质量为$\sim$ 20- 35 $M_{\odot}$）一致。通过结合X射线光变曲线参数和施主星的光学约束，我们证明了系统中的质量比必须是$M_2/M_1 \gtrsim ~18$，因此致密天体很可能是中子星（在爆发时超过其爱丁顿极限）。我们的结果的一般意义是，我们说明了一种方法（适用于高倾角源）识别发光中子星X射线双星，在没有X射线脉动或相位分辨光谱。最后，我们讨论了不同的X射线光谱外观预期从超爱丁顿中子星和黑洞在高视角。
英文摘要：
We discovered eclipses and dips in two luminous (and highly variable) X-ray sources in M\,51. One (CXOM51 J132943.3$+$471135) is an ultraluminous supersoft source, with a thermal spectrum at a temperature of about 0.1 keV and characteristic blackbody radius of about $10^4$ km. The other (CXOM51 J132946.1$+$471042) has a two-component spectrum with additional thermal-plasma emission; it approached an X-ray luminosity of $10^{39}$erg s$^{-1}$ during outbursts in 2005 and 2012. From the timing of three eclipses in a series of {\it Chandra} observations, we determine the binary period ($52.75 \pm 0.63$ hr) and eclipse fraction ($22\% \pm 0.1\%$) of CXOM51 J132946.1$+$471042. We also identify a blue optical counterpart in archival {\it Hubble Space Telescope} images, consistent with a massive donor star (mass of $\sim$20--35 $M_{\odot}$). By combining the X-ray lightcurve parameters with the optical constraints on the donor star, we show that the mass ratio in the system must be $M_2/M_1 \gtrsim ~18$, and therefore the compact object is most likely a neutron star (exceeding its Eddington limit in outburst). The general significance of our result is that we illustrate one method (applicable to high-inclination sources) of identifying luminous neutron star X-ray binaries, in the absence of X-ray pulsations or phase-resolved optical spectroscopy. Finally, we discuss the different X-ray spectral appearance expected from super-Eddington neutron stars and black holes at high viewing angles.
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