# A wide star–black-hole binary system from radial-velocity measurements

## Abstract

All stellar-mass black holes have hitherto been identified 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 Sun1,2,3,4. 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 find that the motion of the B star and an accompanying Hα emission line require the presence of a dark companion with a mass of $${68}_{-13}^{+11}$$ 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.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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## Author information

J.L. and H.Z. are equally responsible for supervising the discovery and follow-up observations. H.Z. and Z.H. proposed the LAMOST monitoring campaign, and H.Z.’s group reduced the LAMOST data with meticulous efforts. J.L. proposed the GTC/Keck/Chandra observations, and his and H.Z.’s groups carried out subsequent data reduction and analysis. J.L. wrote the manuscript with help mainly from H.Z., Y. Lu, R.S., S.W., X.L., Y.S., T.W., Y.B., Z.B., W.Z., Q.G., Y.W., Z.Z., K.B. and J.C. W.W., A.H., W.M.G., J. Wang, J. Wu, L.S., R.S., X.W., J.B., R.D.S. and Q.L. also contributed to the physical interpretation and discussion. H.Y., Y.D., Y. Lei, Z.N., K.C., C.Z., X.M., L.Z., T.Z., H.W., J.R., Junbo Zhang, Jujia Zhang and X.W. also contributed to data collection and reduction. A.W.H. and H.I. contributed to collecting and reducing Keck data. A.C.L., R.C. and R.R. contributed to collecting and reducing GTC data. Z.Q., S.L. and M.L. contributed to utilization of Gaia data. Y.Z., G.Z., Y.C. and X.C. contributed to the implementation of LAMOST. All contributed to the paper in various forms.

Correspondence to Jifeng Liu or Haotong Zhang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Using isochrones from PARSEC models.

The grid of logg and Teff was constructed using the PARSEC isochrones. The colour bar represents initial stellar mass. The black ellipse indicates the 90% uncertainty of the Teff and logg of the B star; all points inside it are considered as acceptable models for the B star.

### Extended Data Fig. 2 SED fitting results for the B star.

a, E(B − V) versus distance, both of which are determined from the SED fitting. The colour bar indicates χ2. b, Distance versus stellar mass, the latter being determined from the acceptable PARSEC models of the B star. The colour bar indicates χ2. c, E(B − V) versus distance. The colour bar indicates logg, while the colour bar indicates χ2 in a. d, Several examples of the SED fitting. The black squares are the data from the UCAC4, 2MASS and AllWISE catalogues. The diamonds with different colours indicate magnitudes from different models. See Methods for details.

### Extended Data Fig. 3 Variation of E(B − V) with distance in the direction of LB-1.

The black circles represent the extinction values corresponding to different distances from the 3D dust map. The green points are the extinction and distances from SED fitting for each acceptable model of the B star. The red cross marks the extinction value from the 3D dust map at 4.23 kpc, while the red dashed line shows the Gaia distance of 2.14 kpc.

### Extended Data Fig. 4 Search for periodicities for LB-1 with the Lomb–Scargle method.

The radial-velocity curve from LAMOST, GTC and Keck observations is being used here. The highest peak corresponds to the orbital period of ~78.9 d.

### Extended Data Fig. 5 Separation a as a function of MB and MBH.

Here a is calculated from Kepler’s third law for each pair of MB (B-star mass) and MBH (black-hole mass). The contours and colours both represent the values of a. The white dashed lines in the contour plot outline a valid region of the separation of the binary system. It comes from the limitations on MB, (7–9.1)M, and on MBH, (55–79)M. See Methods for details.

### Extended Data Fig. 6 Semi-major axis of the orbit of the B-star aB as a function of MB and MBH.

Here aB is calculated from Kepler’s third law for each pair of MB (B-star mass) and MBH (black-hole mass). The contours and colours both represent the values of aB. The white dashed lines in the contour plot outline a valid region for the semi-major axis of the B star. It comes from the limitations on MB, (7–9.1)M, and on MBH, (55–79)M. See Methods for details.

### Extended Data Fig. 7 Black-hole mass versus initial mass in the zero age main sequence (ZAMS) for single stars.

For standard wind mass-loss prescriptions, only low-mass black holes are predicted: MBH < 15M (pink curve). However, for reduced wind mass loss, much heavier black holes are formed: MBH = 30M for winds reduced to 50% (blue curve) and MBH = 60M for winds reduced to 30% (red curve) of the standard values. Note that to reach MBH = 80M (black curve) it is necessary to switch off pair-instability pulsation supernovae (PPSN) or pair-instability supernovae (PSN), which severely limit black-hole masses.

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Liu, J., Zhang, H., Howard, A.W. et al. A wide star–black-hole binary system from radial-velocity measurements. Nature 575, 618–621 (2019). https://doi.org/10.1038/s41586-019-1766-2

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• ### A stripped helium star in the potential black hole binary LB-1

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