Tidal Effects on the Radial Velocities of V723 Mon: Additional Evidence for a Dark 3 M _? Companion

Jayasinghe et al. (2021) identified a dark $\approx 3\,M_\odot$ companion on nearly edge-on 60\,\mathrm{day}$ orbit around the red giant star V723 Monoceros as black hole candidate in mass gap. This scenario was shown to explain most of data presented by (2021), except for periodic radial velocity (RV) residuals from circular Keplerian model. Here we show that RV are explained orbital phase-dependent distortion absorption line profile associated with changing visible fractions approaching and receding sides star, whose surface is tidally deformed rotating synchronously companion. Our model constrains $M_\bullet = 2.95\pm0.17\,M_\odot$ inclination $i=82.9^{+7.0}_{-3.3}\,\mathrm{deg}$ (medians 68.3% highest density intervals marginal posteriors) adopting radius $24.0\pm0.9\,R_\odot$ constrained its SED distance. The analysis provides independent support ellipsoidal variations limits companion's luminosity absence eclipses both derived (2021). We also common scheme evaluate tidal signal flux-weighted mean field can significantly underestimate amplitude RVs measured cross-correlation technique, present modified prescription directly models distorted effects RVs. formulation will be useful estimating component masses inclinations other similar binaries.