双星演化模型预测，从快速旋转的‘B-发射’ 或称 ‘Be’ 星（年轻的、蓝色的不规则变星，它们具有氢的特征光谱发射线）的赤道包络的吸积会形成黑洞。然而迄今为止，在Be X-射线双星中发现的紧凑伴星大多数都被认为是中子星。少数含有“亚矮星”，但此前没有一个被发现包含黑洞。这种情况随着在‘Be’ 星 MWC 656的伴星周围的一个吸积盘的光谱中一条前所未有的He II 4,686 Å光谱发射线的发现而被改变。这条线结合其他信息表明，存在一个质量为3.8到6.9个太阳质量的黑洞。
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 (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. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (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 classification6 and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. 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 MWC 656, the candidate counterpart of the γ-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 1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.