A Unicorn in Monoceros: the $3M_{\odot }$ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($d\sim 460\mathrm{p}\mathrm{c}$), bright ($V\simeq 8.3$~mag), evolved ($T_{\mathrm{e}\mathrm{f}\mathrm{f},\mathrm{g}\mathrm{i}\mathrm{a}\mathrm{n}\mathrm{t}}\simeq 4440$~K, and $L_{\rm giant}\simeq173~L_\odot$)redgiantinahighmassfunction,$f(M)=1.72\pm 0.01~M_\odot$,nearlycircularbinary($P=59.9$d,$e\simeq 0$). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT), and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities and stellar temperature give an inclination of ${87.0}^{\circ }{}_{-{1.4}^{\circ }}^{+{1.7}^{\circ }}$, a mass ratio of $q\simeq 0.33\pm 0.02$, a companion mass of $M_{\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}}=3.04\pm 0.06M_{\odot }$, a stellar radius of $R_{\mathrm{g}\mathrm{i}\mathrm{a}\mathrm{n}\mathrm{t}}=24.9\pm 0.7R_{\odot }$, and a giant mass of $M_{\mathrm{g}\mathrm{i}\mathrm{a}\mathrm{n}\mathrm{t}}=1.00\pm 0.07M_{\odot }$. We identify a likely non-stellar, diffuse veiling component with contributions in the $B$ and $V$-band of $\sim 63\mathrm{\%}$ and $\sim 24\mathrm{\%}$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion disks. The X-ray luminosity of the system is $L_{\rm X}\simeq7.6\times10^{29}~\rm ergs~s^{-1}$,correspondingto$L/L_{\rm edd}{\sim}10^{-9}$. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the “mass gap”.