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arXiv · 2302.07880

A red giant orbiting a black hole

Kareem El-Badry·Hans-Walter Rix·Yvette Cendes·Antonio C. Rodriguez·Charlie Conroy·Eliot Quataert·Keith Hawkins·Eleonora Zari·Melissa Hobson·Katelyn Breivik·Arne Rau·Edo Berger·Sahar Shahaf·Rhys Seeburger·Kevin B. Burdge·David W. Latham·Lars A. Buchhave·Allyson Bieryla·Dolev Bashi·Tsevi Mazeh·Simchon Faigler
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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 $\sim 1M_{\odot}$ red giant and a dark companion with mass $M_2 = 8.9\pm 0.3\,M_{\odot}$ that is very likely a BH. The orbital period, $P_{\rm orb} = 1277$ days, is much longer than that of any previously studied BH binary. Our radial velocity (RV) follow-up over a 7-month period spans more than 90% of the orbit's dynamic range in RV and is in excellent agreement with predictions of the Gaia solution. UV imaging and high-resolution optical spectra rule out all 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 strongly enhanced in $\alpha-$elements, with $\rm [\alpha/Fe] = +0.26$, but the system's Galactocentric orbit is typical of the thin disk. We obtained X-ray and radio nondetections 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 likely significantly outnumber their close, X-ray bright cousins, but their formation pathways remain uncertain.

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Kareem El-Badry, Hans-Walter Rix, Yvette Cendes, Antonio C. Rodriguez, Charlie Conroy, Eliot Quataert, Keith Hawkins, Eleonora Zari, Melissa Hobson, Katelyn Breivik, Arne Rau, Edo Berger, Sahar Shahaf, Rhys Seeburger, Kevin B. Burdge, David W. Latham, Lars A. Buchhave, Allyson Bieryla, Dolev Bashi, Tsevi Mazeh, Simchon Faigler. 2023-02-15. A red giant orbiting a black hole. https://doi.org/10.1093/mnras/stad799

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