2021/01/06 by T. Jayasinghe, K. Z. Stanek, Todd A. Thompson +18 · 1 voice
Physics and Astronomy · #astro-ph.SR #astro-ph.HE #gr-qc
paper · pdf · doi:10.1093/mnras/stab907
arxiv published 2021/01/06 · arxiv updated 2021/03/26
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 \rm pc), bright (V≃8.3~mag), evolved (T\rm eff, giant≃4440~K, and L\rm giant≃173~L_\odot) red giant in a high mass function, f(M)=1.72± 0.01~M_\odot, 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 (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^∘^+1.7∘-1.4∘ , a mass ratio of q≃0.33±0.02, a companion mass of M\rm comp=3.04±0.06~M_\odot, a stellar radius of R\rm giant=24.9±0.7~R_\odot, and a giant mass of M\rm giant=1.00±0.07~ M_\odot. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V-band of ∼63% and ∼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≃7.6×1029~\rm ergs~s-1, corresponding to L/L\rm edd∼10-9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the "mass gap".