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

Abstract

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($d\sim 460\mathrm{pc}$), bright ($V\simeq 8.3$~mag), evolved ($T_{\mathrm{eff},\mathrm{giant}}\simeq 4440$~K, and $L_{\mathrm{giant}}\simeq 173L_{\odot }$) red giant in a high mass function, $f(M)=1.72\pm 0.01M_{\odot }$, nearly circular binary ($P=59.9$ 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 $M_{\mathrm{comp}}=3.04\pm 0.06M_{\odot }$, a stellar radius of $R_{\mathrm{giant}}=24.9\pm 0.7R_{\odot }$, and a giant mass of $M_{\mathrm{giant}}=1.00\pm 0.07M_{\odot }$. We identify a likely non-stellar, diffuse veiling component with contributions in the $B$ and $V$-band of $\sim 63\%$ and $\sim 24\%$, 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 $L_{\mathrm{X}}\simeq 7.6\times 10^{29}\mathrm{ergs}{\mathrm{s}}^{-1}$, corresponding to $L/L_{\mathrm{edd}}\sim 10^{-9}$. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the "mass gap".

28 pages, 16 figures. Accepted for publication in MNRAS