Article A wide star–black-hole binary system from radial-velocity measurements https://doi.org/10.1038/s41586-019-1766-2 Jifeng Liu1,2,3*, Haotong Zhang1*, Andrew W. Howard4, Zhongrui Bai1, Youjun Lu1,2, Roberto Soria2,5, Stephen Justham1,2,6, Xiangdong Li7,8, Zheng Zheng9, Tinggui Wang10, Received: 1 March 2019 Krzysztof Belczynski11, Jorge Casares12,13, Wei Zhang1, Hailong Yuan1, Yiqiao Dong1, Yajuan Lei1, Accepted: 28 August 2019 Howard Isaacson14, Song Wang1, Yu Bai1, Yong Shao7,8, Qing Gao1, Yilun Wang1,2, Zexi Niu1,2, Kaiming Cui1,2, Chuanjie Zheng1,2, Xiaoyong Mu2, Lan Zhang1, Wei Wang3,15, Alexander Heger16, Published online: 27 November 2019 Zhaoxiang Qi1,17, Shilong Liao17, Mario Lattanzi18, Wei-Min Gu19, Junfeng Wang19, Jianfeng Wu19, Lijing Shao20, Rongfeng Shen21, Xiaofeng Wang22, Joel Bregman23, Rosanne Di Stefano24, Qingzhong Liu25, Zhanwen Han26, Tianmeng Zhang1, Huijuan Wang1, Juanjuan Ren1, Junbo Zhang1, Jujia Zhang26, Xiaoli Wang26, Antonio Cabrera-Lavers12,27, Romano Corradi12,27, Rafael Rebolo13,27, Yongheng Zhao1,2, Gang Zhao1,2, Yaoquan Chu10 & Xiangqun Cui28 All stellar-mass black holes have hitherto been identifed by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun1–4. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries5,6. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. Here we report radial-velocity measurements taken over two years of the Galactic B-type star, LB-1. We fnd that the motion of the B star and an accompanying Hα emission line +11 require the presence of a dark companion with a mass of 68−13 solar masses, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories. A radial-velocity monitoring campaign using the Large Aperture lines: stellar absorption lines with apparent periodic motion, a broad Multi-Object Spectroscopic Telescope7 (LAMOST) to discover and Hα emission line moving in anti-phase with much smaller amplitude, study spectroscopic binaries has been running since 2016, and has and interstellar absorption lines that are time-independent (see Fig. 1). obtained 26 measurements each for about 3,000 targets brighter The overall spectral shape of LB-1 suggests a B-type star character- than 14 mag in the Kepler K2-0 field of the sky8. One of the B-type stars ized by prominent Balmer absorption lines without a significant Balmer towards the Galactic Anti-Centre, hereafter LB-1, located at coordinates jump. The metallicity, as measured from the Si ii/Mg ii lines, is about (l, b) = (188.23526°, +02.05089°), where l is Galactic longitude and b is (1.2 ± 0.2)Z☉ (where the solar metallicity Z☉ = 0.017), consistent with Galactic latitude, with a V-band magnitude of about 11.5 mag, exhib- that expected for a young B-type star in the Galactic plane. TLUSTY11 ited periodic radial-velocity variation, along with a strong, broad Hα model fitting to the high-resolution Keck spectra leads to effective 9 emission line that is almost stationary. Subsequent GTC/OSIRIS and temperature Teff = 18,100 ± 820 K and logg = 3.43 ± 0.15, where g is the Keck/HIRES10 observations between 2017 December and 2018 April surface gravity. (The Hα and Hβ lines were excluded from the fit because have confirmed the periodic variations and the prominent Hα emission of contamination from emission.) Such values of Teff and logg fit stellar line with higher spectral resolution. The spectra reveal three types of models12 around the main-sequence turn-off points with mass 1Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China. 2School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing, China. 3WHU-NAOC Joint Center for Astronomy, Wuhan University, Wuhan, China. 4Department of Astronomy, Caltech, Pasadena, CA, USA. 5Sydney Institute for Astronomy, The University of Sydney, Sydney, New South Wales, Australia. 6The Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands. 7School of Astronomy and Space Science, Nanjing University, Nanjing, China. 8Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing, China. 9Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA. 10CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China. 11Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Warsaw, Poland. 12Instituto de Astrofísica de Canarias, La Laguna, Spain. 13Departamento de Astrofísica, Universidad de La Laguna, Santa Cruz de Tenerife, Spain. 14Astronomy Department, University of California, Berkeley, CA, USA. 15School of Physics and Technology, Wuhan University, Wuhan, China. 16Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Victoria, Australia. 17Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China. 18INAF–Osservatorio Astrofisico di Torino, Pino Torinese, Italy. 19Department of Astronomy, Xiamen University, Xiamen, China. 20Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China. 21School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai, China. 22Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing, China. 23Department of Astronomy, University of Michigan, Ann Arbor, MI, USA. 24Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. 25Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China. 26Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Chinese Academy of Sciences, Kunming, China. 27GRANTECAN, Breña Baja, Spain. 28Nanjing Institute of Astronomical Optics and Technology, Chinese Academy of Sciences, Nanjing, China. *e-mail: [email protected]; [email protected] 618 | Nature | Vol 575 | 28 November 2019 a c H He I 4,471 Å Na I 5,896 Å 1.2 α 1.0 1.0 A1 0.8 0.8 0.6 B3 Normalized ux 0.4 0.2 LB-1 0.6 0.0 4,000 5,000 6,000 7,000 b 0.4 1.0 0.8 0.2 0.6 Normalized ux 0.4 0.0 0.2 3,900 4,000 4,100 –2 02 –2 02 –2 02 Wavelength (Å) RV/100 (km s–1) Fig. 1 | Optical spectra of LB-1. a, LAMOST spectrum (thin black trace; highest logg of the model grid but still lower than the typical value (logg > 5) for R ≈ 1,800) with stellar templates (A1, red; B3, green; offset for clarity) a B subdwarf. The Balmer absorption lines from this model are much wider than overplotted. b, Keck/HIRES spectrum of the wavelength range boxed in the observed profiles. c, Phased line profiles from LAMOST (blue), GTC (red) a (black trace; R ≈ 60,000) with the best TLUSTY model (green trace; and Keck (green) observations of the Hα emission line (left), the He i −1 Teff = 18,100 K, logg = 3.43, Z = Z☉, vsini = 10 km s ) overplotted. The 90% absorption line at λ = 4,471 Å (‘He i λ4,471’) of the visible star (middle), and confidence level (CL) errors for the model are ΔTeff = 820 K and Δlogg = 0.15. interstellar Na i absorption lines (right). The dashed lines are plotted to guide Also overplotted is a comparison model with logg = 4.75 (blue), which is the the eye. The binary phase φ is for the period of P = 78.9 d. +0.9 MB−=8.2 1.2 M⊙ (where M☉ is the solar mass), radius RB = (9 ± 2)R☉ (where from a gaseous Keplerian disk, which can be around the B star, the black +13 R☉ is the solar radius) and age t =35M−7 yr. The best-fit model is a sub- hole, or the binary. However, the inferred gaseous disk cannot be giant B-type star about 0.2 Myr after the main-sequence turn-off point. around the B star, because the Hα emission line is not tracing the motion Its distance D and extinction E(B − V), where B and V are respectively of the B star, as clearly shown in Fig. 1b. The line profile is distinctly B-band and V-band magnitude, can be derived simultaneously from different from a simple double-horned profile for a Keplerian disk fitting its wide-band spectral energy distribution (SED), resulting in viewed at high inclinations. It shows a ‘wine-bottle’ shape with multiple D = 4.23 ± 0.24 kpc and E(B − V) = 0.55 ± 0.03 mag (see Methods). These peaks in the line centre, which correspond to substantial non-coherent values are consistent with the 3D extinction map13 along LB-1’s direc- scattering components from a disk viewed at low inclination14,15. A cir- tion, so supporting this model. A subdwarf star with a similar tem- cumbinary disk would have an inner radius truncated at 1.7 times the perature is strongly ruled out by the narrow Balmer lines, as shown in binary separation16, and its corresponding projected velocity is 1 Fig. 1a, and also by the SED fitting. ≈0.75 times that of the visible star, that is, about 40 km s−1. The 1.7 The radial motion of the star, as measured from the stellar absorption emission line from such a circumbinary disk will be confined to within lines in 26 LAMOST, 21 GTC and 7 Keck observations obtained over two ±40 km s−1, yet the observed line is three times wider with wings extended years, can be best fitted with a period of P = 78.9 ± 0.3 d (see Methods).