A unicorn in monoceros: the 3M(circle dot) 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 similar to 460 pc), bright (V similar or equal to 8.3mag), evolved (T-eff,T- giant similar or equal to 4440 K, and L-giant similar or equal to 173 L-circle dot) red giant in a high mass function, f(M) = 1.72 +/- 0.01 M-circle dot, nearly circular binary (P = 59.9 d, e similar or equal to 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 , a mass ratio of q similar or equal to 0.33 +/- 0.02, a companion mass of M-comp = 3.04 +/- 0.06 M-circle dot, a stellar radius of R-giant = 24.9 +/- 0.7 R-circle dot, and a giant mass of M-giant = 1.00 +/- 0.07 M-circle dot. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of similar to 63 per cent and similar to 24 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-X similar or equal to 7.6 x 10(29) , corresponding to L/L-edd similar to 10(-9). The simplest explanation for the massive companion is a single compact object, most likely a black hole in the 'mass gap'.