A Unicorn in Monoceros: the 3MโŠ™ 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 (๐‘ƒ = 59ย•.9 d, e approx. 0). Analyses of the stellar spectra and spectral energy distribution (SED) give ๐‘‡eff =ย– 4440 K, ๐ฟ = 173 ๐ฟsโŠ™ and ๐‘… = 22 ๐‘…โŠ™. Matching these parameters to MIST evolutionary models indicates a mass of the visible giant of ๐‘€giant = 1.ย•07 +/- 0.ย•24 ๐‘€โŠ™. 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 ๐‘– = 87ย• deg, a mass ratio of 0ย•.30 +/- 0ย•.02, and a ย companion mass of ๐‘€comp = 2.ย•91 +/- 0ย•.08 ๐‘€โŠ™, a stellarย radius of the giant of ๐‘…giant = 23.ย•6 +/-1.ย•0 ๐‘…โŠ™, and a giant mass of ๐‘€giant = 0.ย•87 +/-0.ย•08 ๐‘€โŠ™ , consistent with our other estimates. We identify a likely non-stellar, diffuse veiling component with contributions in the ๐ต and ๐‘‰-band of ~64% and ~23%, respectively, and a luminosity of ~20 ๐ฟโŠ™. 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 ๐ฟX = 1ย• x 10^30 erg/s, corresponding to ๐ฟ/ย๐ฟedd ~10^-9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the โ€œmass gapโ€, although a double neutron star binary is possible.

LBT/PEPSI line profiles for the Balmer H๐›ผ, H๐›ฝ, Ca I ๐œ†6439 and Ca I ๐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, in arXiv ย (submitted to MNRAS)