Article

A red giant orbiting a black hole

Kareem El-BadryCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USAHans-Walter RixMax-Planck Institute for Astronomy , Königstuhl 17, D-69117 Heidelberg, GermanyY. CendesCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USAAntonio C. RodriguezDepartment of Astronomy, California Institute of Technology , Pasadena, CA 91125, USACharlie ConroyCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USAEliot QuataertDepartment of Astrophysical Sciences, Princeton University , Princeton, NJ 08544, USAKeith HawkinsDepartment of Astronomy, The University of Texas at Austin , 2515 Speedway Boulevard, Austin, TX 78712, USAEleonora ZariMax-Planck Institute for Astronomy , Königstuhl 17, D-69117 Heidelberg, GermanyMélissa J. HobsonMax-Planck Institute for Astronomy , Königstuhl 17, D-69117 Heidelberg, GermanyKatelyn BreivikCenter for Computational Astrophysics, Flatiron Institute , 162 Fifth Ave, New York, NY 10010, USAA. RauMax-Planck-Institut für extraterrestrische Physik , Gießenbachstraße 1, D-85748 Garching, GermanyE. BergerCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USAS. ShahafDepartment of Particle Physics and Astrophysics, Weizmann Institute of Science , Rehovot 7610001, IsraelRhys SeeburgerMax-Planck Institute for Astronomy , Königstuhl 17, D-69117 Heidelberg, GermanyKevin B. BurdgeMIT-Kavli Institute for Astrophysics and Space Research 77 Massachusetts Ave. , Cambridge, MA 02139, USADavid W. LathamCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USALars A. BuchhaveDTU Space, National Space Institute, Technical University of Denmark , Elektrovej 328, DK-2800 Kgs. Lyngby, DenmarkAllyson BierylaCenter for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USADolev BashiSchool of Physics and Astronomy, Tel Aviv University , Tel Aviv 6997801, IsraelT. MazehSchool of Physics and Astronomy, Tel Aviv University , Tel Aviv 6997801, IsraelS. FaiglerSchool of Physics and Astronomy, Tel Aviv University , Tel Aviv 6997801, Israel

2023en

ABI

Abstract

ABSTRACT We report spectroscopic and photometric follow-up of a dormant black hole (BH) candidate from Gaia DR3. The system, which we call Gaia BH2, contains a ∼1 M⊙ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\, {\rm M}_{\odot }$ that is very likely a BH. The orbital period, Porb = 1277 d, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans &gt;90 per cent of the orbit’s RV range and is in excellent agreement with the Gaia solution. UV imaging and high-resolution optical spectra rule out plausible luminous companions that could explain the orbit. The star is a bright (G = 12.3), slightly metal-poor ($\rm [Fe/H]=-0.22$) low-luminosity giant ($T_{\rm eff}=4600\, \rm K$; $R = 7.8\, R_{\odot }$; $\log \left[g/\left({\rm cm\, s^{-2}}\right)\right] = 2.6$). The binary’s orbit is moderately eccentric (e = 0.52). The giant is enhanced in α-elements, with $\rm [\alpha /Fe] = +0.26$, but the system’s Galactocentric orbit is typical of the thin disc. We obtained X-ray and radio non-detections of the source near periastron, which support BH accretion models in which the net accretion rate at the horizon is much lower than the Bondi–Hoyle–Lyttleton rate. At a distance of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit – like that of Gaia BH1 – seems too wide to have formed through common envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia BH2 significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain.

Identifiers

DOI: 10.1093/mnras/stad799

Citations and references

Cited by 30 references

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