A non-interacting Galactic black hole candidate in a binary system with a main-sequence star

We describe the discovery of a solar neighborhood ($d=474$~pc) binary system consisting of a main-sequence sunlike star and a massive non-interacting black hole candidate. We selected this system from the \textit{Gaia} DR3 binary catalog based on its high mass ratio and location close to the main sequence. The spectral energy distribution (SED) of the visible star is well described by a single stellar model, indicating that no contribution from another luminous source is needed to fit the observed photometry. We derive stellar parameters from a high S/N Magellan/MIKE spectrum, classifying the star as a main-sequence star with $T_{\rm eff} = 5972~\rm K$, $\log{g} = 4.54$, and $M = 0.91$~\msun. The spectrum also shows no indication of a second luminous component. We have measured radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over the past three months, which we use to determine the spectroscopic orbit of the binary. We show that the velocity data are consistent with the \textit{Gaia} astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of the astrometric and spectroscopic data, we derive a companion mass of $11.9^{+2.0}_{-1.6}$\msun. We conclude that this binary system harbors a massive black hole on an eccentric $(e =0.45 \pm 0.02)$, long-period ($185.4 \pm 0.1$ d) orbit. The main-sequence star that orbits this black hole is moderately metal-poor ($\mbox{[Fe/H]} = -0.30$), on a Galactic orbit similar to thin disk stars. Our conclusions are independent of \cite{ElBadry2022Disc}, who recently reported the discovery of the same system, and find a marginally lower companion mass than we do here.