Not so fast: LB-1 is unlikely to contain a 70 $M_{\odot}$ black hole

Abstract

The recently discovered binary LB-1 has been reported to contain a $\sim$\,$70\,M_{\odot}$ black hole (BH). The evidence for the unprecedentedly high mass of the unseen companion comes from reported radial velocity (RV) variability of the H$α$ emission line, which has been proposed to originate from an accretion disk around a BH. We show that there is in fact no evidence for RV variability of the H$α$ emission line, and that its apparent shifts instead originate from shifts in the luminous star's H$α$ absorption line. If not accounted for, such shifts will cause a stationary emission line to appear to shift in anti-phase with the luminous star. We show that once the template spectrum of a B star is subtracted from the observed Keck/HIRES spectra of LB-1, evidence for RV variability vanishes. Indeed, the data rule out periodic variability of the line with velocity semi-amplitude $K_{\rm Hα} > 1.3\,\rm km\,s^{-1}$. This strongly suggests that the observed H$α$ emission does not originate primarily from an accretion disk around a BH, and thus that the mass ratio cannot be constrained from the relative velocity amplitudes of the emission and absorption lines. The nature of the unseen companion remains uncertain, but a "normal" stellar-mass BH with mass $5\lesssim M/M_{\odot}\lesssim 20 $ seems most plausible. The H$α$ emission likely originates primarily from circumbinary material, not from either component of the binary.