A Be-type star with a black-hole companion
A Be-type star with a black-hole companion
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DOI：
10.1038/nature12916
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发表时间：
2014-01-16
期刊：
影响因子：
64.8
通讯作者：
Simon-Diaz, S.
中科院分区：
文献类型：
--
作者：
Casares, J.;Negueruela, I.;Simon-Diaz, S.
恒星质量的黑洞都是通过X射线发射发现的，这是由于它们的双星伴星的气体吸积（这种气体要么是从低质量恒星剥离的，要么是从大质量恒星的风中提供的）。双星演化模型也预测了黑洞的存在，这些黑洞是从快速旋转的Be型恒星的赤道包层吸积而来的（Be型恒星是热蓝不规则变星，显示出氢的特征光谱发射线）。然而，在银河系已知的大约80颗Be X射线双星中，只有脉动中子星被发现是伴星（2-4）。黑洞被正式认为是Be星星MWC 656的伴星（参考文献5;也被称为HD 215227），尽管这一结论是基于Be星星的单一径向速度曲线、错误的光谱分类（6）和对倾角的粗略估计。在这里，我们报告的意见，吸积盘线镜像轨道的MWC 656。通过拟合赤道盘上的Fe II轮廓得到了Be星星的径向速度曲线，并对Be进行了精细分类（B1.5-B2 III星星的分类），这表明有一个太阳质量为3.8到6.9的黑洞围绕MWC 656运行，MWC 656是伽马射线源AGL J2241+4454的候选对应物（参考文献5，6）。该黑洞是X射线静止的，由辐射效率低下的吸积流供给，其光度小于爱丁顿光度的1.6x10（-7）倍。这意味着在传统的X射线巡天中，很难探测到有黑洞伴星的Be双星。
Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars(1-3) (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions(2-4). A black hole was formally allowed as a solution for the companion to the Be star MWC656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification(6) and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe II profiles from the equatorial disk, and a refined Be classification (to that of a B1.5-B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC656, the candidate counterpart of the gamma-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6x10(-7) times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.