A unicorn in monoceros: the 3 M⊙ 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 ($d\sim 460\, \rm pc$), bright (V ≃ 8.3 mag), evolved (T_{eff, giant} ≃ 4440 K, and L_giant ≃ 173 L_⊙) red giant in a high mass function, f(M) = 1.72 ± 0.01 M_⊙, nearly circular binary (P = 59.9 d, e ≃ 0). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey, Kilodegree Extremely Little Telescope, and Transiting Exoplanet Survey Satellite 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 $87.0^{\circ ^{+1.7^\circ }}_{-1.4^\circ }$, a mass ratio of q ≃ 0.33 ± 0.02, a companion mass of M_comp = 3.04 ± 0.06 M_⊙, a stellar radius of R_giant = 24.9 ± 0.7 R_⊙, and a giant mass of M_giant = 1.00 ± 0.07 M_⊙. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of ${\sim }63{{\ \rm per\ cent}}$ and ${\sim }24{{\ \rm per\ cent}}$, 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 discs. The X-ray luminosity of the system is $L_{\rm X}\simeq 7.6\times 10^{29}~\rm ergs~s^{-1}$, corresponding to L/L_edd ~ 10^-9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the 'mass gap'.