A Unicorn in Monoceros: the 3MβŠ™ 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=460 pc), bright evolved red giant in a high mass function nearly circular binary (𝑃 = 59Β•.9 d, e approx. 0). Analyses of the stellar spectra and spectral energy distribution (SED) give 𝑇eff =Β– 4440 K, 𝐿 = 173 𝐿sβŠ™ and 𝑅 = 22 π‘…βŠ™. Matching these parameters to MIST evolutionary models indicates a mass of the visible giant of 𝑀giant = 1.Β•07 +/- 0.Β•24 π‘€βŠ™. 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 𝑖 = 87Β• deg, a mass ratio of 0Β•.30 +/- 0Β•.02, and a Β companion mass of 𝑀comp = 2.Β•91 +/- 0Β•.08 π‘€βŠ™, a stellarΒ radius of the giant of 𝑅giant = 23.Β•6 +/-1.Β•0 π‘…βŠ™, and a giant mass of 𝑀giant = 0.Β•87 +/-0.Β•08 π‘€βŠ™ , consistent with our other estimates. We identify a likely non-stellar, diffuse veiling component with contributions in the 𝐡 and 𝑉-band of ~64% and ~23%, respectively, and a luminosity of ~20 πΏβŠ™. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object even if the companion is a binary. 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 𝐿X = 1Β• x 10^30 erg/s, corresponding to 𝐿/𝐿edd ~10^-9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the β€œmass gap”, although a double neutron star binary is possible.

LBT/PEPSI line profiles for the Balmer H𝛼, H𝛽, Ca I πœ†6439 and Ca I πœ†6463 lines (black). A model spectrum using the atmospheric parameters is shown in red. The blue lines show the velocity offset of the Balmer absorption lines (12 km/s) from the rest frame of the giant. PEPSI was used in its R=250,000 resolution mode.

Read more: Jayasinghe et al. 2021, MNRAS, 504, 2577