Abstract
The mass distribution of black holes identified through X-ray emission suggests a paucity of black holes in the mass range of 3 to 5 solar masses. Modified theories have been devised to explain this mass gap, and it is suggested that natal kicks during a supernova explosion can more easily disrupt binaries with lower-mass black holes. Although recent Laser Interferometer Gravitational-Wave Observatory observations reveal the existence of compact remnants within this mass gap, the question of whether low-mass black holes can exist in binaries remains a matter of debate. Such a system is expected to be non-interacting and without X-ray emission, and can be searched for using radial-velocity and astrometric methods. Here we report on Gaia Data Release 3 (DR3) 3425577610762832384, which is a wide binary system that includes a red giant star and an unseen object, exhibiting an orbital period of approximately 880 days and a near-zero eccentricity. Through the combination of radial-velocity measurements from the Large Aperture Multi-Object Spectroscopic Telescope and astrometric data from Gaia DR2 and DR3 catalogues, we determine a mass of \(3.{6}_{-0.5}^{+0.8}\,{M}_{\odot }\) of the unseen component. If the unseen companion is a black hole, its mass would fall within the gap and it would strongly suggest the existence of binary systems containing low-mass black holes. More notably, the formation of its surprisingly wide circular orbit challenges current binary evolution and supernova explosion theories.
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Data availability
The LAMOST spectra used in this paper are available from the LAMOST database at https://www.lamost.org/lmusers. The RV data, stellar parameters of the visible star and parameters of known black holes are listed in tables in Supplementary Information. The other 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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Acknowledgements
This work is supported by the National Science Foundation of China (NSFC) under grant numbers 11988101/11933004 (J.L.), 12273057 (S.W.), 12041301/12121003 (Xiangdong Li), U2031205/12233002 (Q.L.), 12288102/12125303 (X.C.), 12173081 (H.G.), 123B2045 (S.G.), 12041303 (P.W.), 12333008 (X.M.), 12473066 (F.F.) and 12103086 (Zhenwei Li). It is also supported by the National Key Research and Development Program of China (NKRDPC) under grant numbers 2023YFA1607900 (W.-M.G.), 2023YFA1607901 (S.W.), 2023YFA1607902 (Y.S.), 2016YFA0400804 (J.L.), 2021YFA0718500 (Xiangdong Li), 2021YFA0718500 (Q.L.) and 2021YFA1600403 (X.C.). In addition, it is supported by the Strategic Priority Program of the Chinese Academy of Sciences under grant numbers XDB41000000 (S.W.) and XDB0550300 (Y.S.). F.F. thanks the Shanghai Jiao Tong University 2030 Initiative. H.G. thanks the Key Research Programme of Frontier Sciences, CAS, no. ZDBS-LY-7005, Yunnan Fundamental Research Projects (grant no. 202101AV070001). X.M. thanks the Yunnan Fundamental Research Projects (grant nos. 202401BC070007 and 202201BC070003) and the International Centre of Supernovae, Yunnan Key Laboratory (grant no. 202302AN360001). Zhenwei Li thanks the Yunnan Fundamental Research Projects (grant no. 202401AT070139). P.W. thanks the CAS Youth Interdisciplinary Team, the Youth Innovation Promotion Association CAS (ID 2021055) and the Cultivation Project for FAST Scientific Payoff and Research Achievement of CAMS-CAS. This work is made possible with the LAMOST, which is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular, the institutions participating in the Gaia MultiLateral Agreement (MLA). We acknowledge the support of the staff of the Xinglong 2.16 m telescope. This work was partially supported by the Open Project Program of the CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences.
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S.W., F.F. and J.L. were equally responsible for supervising the discovery. S.W. selected G3425 from Gaia DR3 sample. S.W. and X.Z. reduced the LAMOST data and performed RV and stellar parameter analysis. F.F. did the joint fitting of RV and astrometric data. H.G., Y.S., Y.C., S.G. and L.Z. performed binary evolution simulations. S.W. and X.Z. wrote the paper with help mainly from J.L., F.F., H.G., Y.S., Y.C., S.G. and Y.H. P.W., X.L., Z.B., Hailong Yuan, Haibo Yuan, Z.Z., T.Y., J.Z., T.L., M.X., H.H., M.Z. and D.F. also contributed to data analysis. Xiangdong Li, X.C., Zhengwei Liu, X.M., Q.L., H.Z., W.-M.G. and Zhenwei Li also contributed to the binary formation interpretation and discussion. All authors contributed to the paper in various forms.
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Extended data
Extended Data Fig. 1 Best-fit RV curves.
Top panel: best-fit RV curve (purple line) using the blue-band RV data, compared with the RV curve from Gaia NTBO catalog (dashed line). The RV data from LAMOST LRS and MRS and 2.16 m telescope are marked by three cyan squares, 27 blue dots, and five grey diamonds, respectively. The error bars represent 1σ uncertainties. The RV data given by 2.16 m telescope on BJD ≈ 2460322 were combined to enhance the accuracy. The grey area shows the observation time of the data used by Gaia, from July 25th 2014 to May 28th 2017. Bottom panel: best-fit RV curve (purple line) using the red-band RV data, compared with the RV curve from Gaia NTBO catalog (dashed line). The RV data from LAMOST LRS and MRS and 2.16 m telescope are marked by orange squares, red dots, and grey diamonds, respectively.
Extended Data Fig. 2 Optimal orbital solutions for G3425.
The panels from left to right respectively display the best fits to RV data, Gaia GOST data, and the predicted G3425 position on January 1st, 2024. In the left panel, the best-fit RV curve is represented by the red line, overplotted with the LAMOST LRS and MRS RV data (30 green points). The error bars represent 1σ uncertainties. In the middle panel, the optimal fit to the Gaia GOST data is shown, along with a comparison between best-fit and catalog proper motions and positions at Gaia DR2 (GDR2) and GDR3 reference epochs. The shaded regions represent the uncertainty in position and proper motion. Each segment represents the best-fit position and proper motion offsets induced by the reflex motion at a specific reference epoch. The right panel displays the predicted companion position on January 1st, 2024, with a 1σ contour line to indicate the prediction uncertainty.
Extended Data Fig. 3 Model fit to the five-parameter astrometry of GDR2 and GDR3.
The astrometry of the barycenter is subtracted from the five-parameter solutions (represented by dots with error bars) and from the model predictions based on posterior sampling (represented by box plots). For the box plot, the lower and upper hinges correspond to the 25th and 75th percentiles. The upper whisker extends from the hinge to the largest value within 1.5*IQR (inter-quartile range). The lower whisker extends to the smallest value within 1.5*IQR. Data beyond the whiskers are ‘outliers’.
Extended Data Fig. 4 Position of G3425 in Hertzsprung-Russell diagram.
The isochrones (from top to bottom) from PARSEC models have an age of 108.4/108.6/108.8/109.0/109.2/109.4 yr, respectively. The red and blue stars are calculated with the distances from Gaia DR3 (1442 pc) and our joint fitting (1786 pc), respectively. The grey background points are selected from Gaia DR2 with distances of d < 200pc, Gmag between 5–16 mag, and galactic latitude ∣b∣ > 10°. No extinction correction was applied to these background stars.
Extended Data Fig. 5 SED and Gaia XP spectrum of G3425.
Top panel: SED fitting for G3425 with a distance of \(178{6}_{-248}^{+342}\) pc. The observed data come from APASS (three black dots), Gaia (three yellow dots), 2MASS (three red dots) and WISE (two green dots). The grey line is the best-fit model. The error bars represent 1σ uncertainties. Bottom panel: Comparison of the observed spectrum (flux-calibrated Gaia XP spectrum) and the PHOENIX template spectrum \(({T}_{{\rm{eff}}}=5000\,{\rm{K}},\log g=2.5,[{\rm{Fe}}/{\rm{H}}]=0.0)\), with multi-band magnitudes overplotted.
Extended Data Fig. 6 Example of spectral disentangling (q = 0.75) of G3425 using the blue band and red band of the LAMOST MRS.
The vertical panels show the spectra at phases of 0.31, 0.61, and 0.77. The blue lines are the separated spectra of the visible star (primary); the red lines show the spectra from the other component; the green lines represent the sum of blue lines and red lines. The black lines are the spectra from LAMOST MRS observations.
Extended Data Fig. 7 Estimated number distribution of the Galactic detached black hole binaries with a (sub)giant companion in the plane of orbital period versus eccentricity.
Only the systems with 3.1M⊙ < MBH < 4.1M⊙ and 1.7M⊙ < Mgiant < 3.7M⊙ are included. The top and bottom panels correspond to the delayed and stochastic supernova-explosion mechanisms, respectively. The left and right panels correspond to common envelope ejection efficiencies of αCE = 1 and αCE = 5, respectively. In each panel, the black star marks the position of G3425.
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Supplementary Sections 1–10, Tables 1–8 and Figs. 1–5.
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Wang, S., Zhao, X., Feng, F. et al. A potential mass-gap black hole in a wide binary with a circular orbit. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02359-9
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DOI: https://doi.org/10.1038/s41550-024-02359-9
