2021MNRAS.504.2577J

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby (d∼460 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^○^+1.7○-1.4^○^, 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 ∼63 per cent and ∼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_≃7.6×10²⁹∼ 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'.