A naked-eye triple system with a nonaccreting black hole in the inner binary

Abstract

Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple. A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit. The radial-velocity semi-amplitude of 61.3 km s−1 of the inner star and its minimum (probable) mass of 5.0 M⊙ (6.3 ± 0.7 M⊙) imply a mass of the unseen object of ≥4.2 M⊙ (≥5.0 ± 0.4 M⊙), that is, a black hole (BH). The spectroscopic time series is stunningly similar to observations of LB-1. A similar triple-star architecture of LB-1 would reduce the mass of the BH in LB-1 from ∼70 M⊙ to a level more typical of Galactic stellar remnant BHs. The BH in HR 6819 probably is the closest known BH to the Sun, and together with LB-1, suggests a population of quiet BHs. Its embedment in a hierarchical triple structure may be of interest for models of merging double BHs or BH + neutron star binaries. Other triple stars with an outer Be star but without BH are identified; through stripping, such systems may become a source of single Be stars.

Is HR 6819 a triple system containing a black hole?

“…The second system that was recently proposed to contain a stellar-mass BH is HR 6819 (Rivinius et al 2020). It was reported to be variable already by Buscombe & Morris (1960).…”

“…Recently, Rivinius et al (2020) suggested that HR 6819 hosts the closest stellar-mass BH to the Sun yet reported. They argued that the emission profiles attributed to the rapidly-rotating Be star are stationary on the time scale of years, and that the Be star seen in the spectrum must therefore be a distant tertiary in an outer orbit.…”

“…They argued that the emission profiles attributed to the rapidly-rotating Be star are stationary on the time scale of years, and that the Be star seen in the spectrum must therefore be a distant tertiary in an outer orbit. Based on the binary mass function ( f M = 0.96 M ) derived from the SB1 RV curve and the mass estimate of the narrow-lined star (M ≥ 5 M ), which the authors classified as B3 III, Rivinius et al (2020) concluded that the unseen secondary star in the 40 d orbit must be more massive than 4 M . Given this mass estimate, they concluded that the unseen companion must be a BH.…”

“…Given this mass estimate, they concluded that the unseen companion must be a BH. The system was thus interpreted by Rivinius et al (2020) as a hierarchical triple containing a B+BH inner binary on a 40 d orbit and a Be star on an unconstrained wider orbit. They suggested a similar configuration for LB-1 which is, however, contradicted by Shenar et al (2020) and Liu et al (2020).…”

“…In this paper we present an alternative explanation to HR 6819 that does not invoke an inferred stellar-mass BH or a tertiary component on a wider orbit. Our study is based on the same observational data as used by Rivinius et al (2020), which is described in Sect. 2.…”