恒星级黑洞的搜寻与研究进展

Search and research advances on stellar-mass black holes

迄今为止,银河系内发现的约20颗恒星级黑洞都是通过黑洞吸积伴星物质所发出的X射线来识别.若黑洞的X射线辐射低于望远镜探测极限,人们仍可以通过监测伴星的视向运动来发现黑洞.我们利用LAMOST望远镜对银河系的一颗B型恒星进行速度测量,发现它在围绕一颗不可见天体——大质量恒星级黑洞做周期性运动.这是国际上利用视向速度监测发现黑洞的一次成功实践.尽管引力波探测实验已经发现相似质量的恒星级黑洞,但在富金属环境中形成如此大质量的黑洞对目前的恒星演化理论形成了巨大的挑战.本文介绍了恒星级黑洞的几种探测方法,并对近些年国内外搜寻黑洞所做的努力进行了回顾,最后对未来利用视向速度监测方法和天体测量方法寻找黑洞进行了展望.

All stellar-mass black holes in the Milky Way have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. The masses of the black holes in these ~20 systems are all less than 30 times that of the Sun. Theory predicts, however, that there are hundreds of millions of stellar-mass black holes and X-rayemitting systems form only a minority of the total population of star-black-hole binaries. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. A radial-velocity monitoring campaign using the Large Aperture Multi-Object Spectroscopic Telescope(LAMOST) to discover and study spectroscopic binaries has been running since 2016. Among about 3000 objects, we find that one B-type star, called LB-1,exhibits periodic radial-velocity variation, along with a strong, broad Hα emission line that is almost stationary. Combined with the GTC and Keck spectral observations, we find that the motion of the B star and the spectral features require the presence of a dark companion, indicating a massive stellar-mass black hole. We further conducted a high-resolution,phase-resolved spectroscopic study of its Paschen lines, using the Calar Alto 3.5 m telescope. We find that Paβ and Paγ(after subtraction of the stellar absorption component) are well fitted with a standard double-peaked disk profile,confirming its nature as one stellar-mass black hole. The discovery of LB-1 suggests that future similar campaigns will probe a quiescent black-hole population different from the X-ray-bright one. The gravitational-wave experiments LIGO and Virgo have discovered about 50 gravitational-wave events caused by binary black hole mergers. Although they have detected black holes of similar mass, the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories. In this article, we introduce several methods for discovering stellar-mass black holes, including X-ray emission(searching for star-black-hole binaries), gravitational wave(searching for binary black holes mergers), micro-lensing(searching for isolated black holes), and radial-velocity monitoring(searching for star-black-hole binaries). We review the research advances on recent discoveries on stellar-mass black holes. Until now, less than five stellar-mass black holes were discovered by radial-velocity monitoring. In addition,since September 2018, LAMOST starts a new 5-year medium-resolution spectroscopic survey. This survey will observe about two million stellar spectra with R≈7500 and limiting magnitude of around G=15 mag. Moreover, it can also provide about 200 thousand stars with averagely 60-epoch observations and limiting magnitude of G≈14 mag. The radial velocity uncertainty can be less than 1 km/s. The time-domain spectral data can be quite helpful in discovering the X-ray-quiescent black holes. In the other hand, Gaia transit data in the next data release will reach an astrometric error as small as 0.1 milliarcsecond. The full orbit and the parallax of one star-black-hole binary can be solved simultaneously from the combination of radial velocity measurements and Gaia transit data. This will give the total binary mass and the mass ratio,simultaneously. We expect in the future, the combination of radial-velocity monitoring and astrometry can play a key role on this intriguing area.