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

• Published in: arXiv.org 2021-03
• Main Authors: Jayasinghe, T, Stanek, K Z, Thompson, Todd A, Kochanek, C S, Rowan, D M, Vallely, P J, Strassmeier, K G, Weber, M, Hinkle, J T, F -J Hambsch, Martin, D, Prieto, J L, Pessi, T, Huber, D, Auchettl, K, Lopez, L A, Ilyin, I, Badenes, C, Howard, A W, Isaacson, H, Murphy, S J
• Format: Article
• Language: English
• Subjects: Accretion disks; Black holes; Companion stars; Eclipsing binary stars; Light curve; Luminosity; Neutron stars; Neutrons; Red giant stars; Stellar temperature; Variable stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2101.02212

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby (\(d\sim460\,\rm pc\)), bright (\(V\simeq8.3\)~mag), evolved (\(T_{\rm eff, giant}\simeq4440\)~K, and \(L_{\rm giant}\simeq173~L_\odot\)) red giant in a high mass function, \(f(M)=1.72\pm 0.01~M_\odot\), 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 \(87.0^\circ{}^{+1.7^{\circ}}_{-1.4^{\circ}} \), a mass ratio of \(q\simeq0.33\pm0.02\), a companion mass of \(M_{\rm comp}=3.04\pm0.06~M_\odot\), a stellar radius of \(R_{\rm giant}=24.9\pm0.7~R_\odot\), and a giant mass of \(M_{\rm giant}=1.00\pm0.07~ M_\odot\). 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 \(L_{\rm X}\simeq7.6\times10^{29}~\rm ergs~s^{-1}\), corresponding to \(L/L_{\rm edd}{\sim}10^{-9}\). The simplest explanation for the massive companion is a single compact object, most likely a black hole in the "mass gap".