根据两个有紧密伴星CD -30 11223和PG 1232-136的sdB双星X射线观测对热亚矮星风的约束
Constraints on the winds of hot subdwarf stars from X-ray observations of two sdB binaries with compact companions: CD -30 11223 and PG 1232-136
摘要：
关于B光谱型（sdB）热亚矮星的弱星风的观测数据很少。由sdB星和致密天体（白矮星、中子星或黑洞）组成的近距离双星系统可以作为吸积动力的X射线源被探测到。研究sdB星的X射线发射可以探测由于其低光度而不能直接从光谱学中导出的sdB星的线驱动风的性质。在这里，我们报告的第一个敏感的X射线观测的两个sdB双星紧凑的同伴。CD-30 11223是已知轨道周期最短（1.2小时）的sdB双星，它的伴星肯定是一颗白矮星。PG 1232-136是一颗sdB双星，被认为是拥有黑洞伴星的最佳候选者。我们分别在2013年8月和2009年7月用XMM-牛顿观测了这些恒星的50 k和36 k。它们都没有被检测到，我们推导出CD-30 11223的光度上限约为1.5x10^29 erg/s，PG 1232-136的光度上限约为1.5x10^29 erg/s。由于黑洞吸积的效率未知，PG 1232-136的相应质量损失率约束不佳。另一方面，在CD-30 11223的情况下，在合理的假设下，我们可以推导出sdB星风质量损失率的上限约为3 × 10 ^-13太阳质量/年。这是少数几个观测限制弱风预期在这类低质量的热恒星。我们还报告了在CD-30 11223领域偶然发现的星系团的X射线发射的结果。
英文摘要：
Little observational data are available on the weak stellar winds of hot subdwarf stars of B spectral type (sdB). Close binary systems composed of an sdB star and a compact object (white dwarf, neutron star or black hole) could be detected as accretion-powered X-ray sources. The study of their X-ray emission can probe the properties of line-driven winds of sdB stars that can not be derived directly from spectroscopy because of the low luminosity of these stars. Here we report on the first sensitive X-ray observations of two sdB binaries with compact companions. CD -30 11223 is the sdB binary with the shortest known orbital period (1.2 h) and its companion is certainly a white dwarf. PG 1232-136 is an sdB binary considered the best candidate to host a black hole companion. We observed these stars with XMM-Newton in August 2013 for 50 ks and in July 2009 for 36 ks, respectively. None of them was detected and we derived luminosity upper limits of about 1.5x10^29 erg/s for CD -30 11223 5x10^29 erg/s for PG 1232-136. The corresponding mass loss rate for PG 1232-136 is poorly constrained, owing to the unknown efficiency for black hole accretion. On the other hand, in the case of CD -30 11223 we could derive, under reasonable assumptions, an upper limit of about 3x10^-13 solar masses/yr on the wind mass loss rate from the sdB star. This is one of the few observational constraints on the weak winds expected in this class of low mass hot stars. We also report the results on the X-ray emission from a cluster of galaxies serendipitously discovered in the field of CD -30 11223.
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