Identifying quiescent compact objects in massive Galactic single-lined spectroscopic binaries

Abstract

Context. The quest to detect dormant stellar-mass black holes (BHs) in massive binaries (i.e. OB+BH systems) is challenging; only a few candidates have been claimed to date, all of which must still be confirmed. Aims. To search for these rare objects, we study 32 Galactic O-type stars that were reported as single-lined spectroscopic binaries (SB1s) in the literature. In our sample we include Cyg X-1, which is known to host an accreting stellar-mass BH, and HD 74194, a supergiant fast X-ray transient, in order to validate our methodology. The final goal is to characterise the nature of the unseen companions to determine if they are main-sequence (MS) stars, stripped helium stars, triples, or compact objects such as neutron stars (NSs) or stellar-mass BHs. Methods. After measuring radial velocities and deriving orbital solutions for all the systems in our sample, we performed spectral disentangling to extract putative signatures of faint secondary companions from the composite spectra. We derived stellar parameters for the visible stars and estimated the mass ranges of the secondary stars using the binary mass function. Variability observed in the photometric TESS light curves was also searched for indications of the presence of putative companions, degenerate or not. Results. In 17 of the 32 systems reported as SB1s, we extract secondary signatures, down to mass ratios of ∼ 0.15. For the 17 newly detected double-lined spectroscopic binaries (SB2s), we derive physical properties of the individual components and discuss why they have not been detected as such before. Among the remaining systems, we identify nine systems with possible NS or low-mass MS companions. For Cyg X-1 and HD 130298, we are not able to extract any signatures for the companions, and the minimum masses of their companions are estimated to be about 7 M . Our simulations show that secondaries with such a mass should be detectable from our dataset, no matter their nature: MS stars, stripped helium stars or even triples. While this is expected for Cyg X-1, confirming our methodology, our simulations also strongly suggest that HD 130298 could be another candidate to host a stellar-mass BH. Conclusions. The quest to detect dormant stellar-mass BHs in massive binaries is far from over, and many more systems need to be scrutinised. Our analysis allows us to detect good candidates, but confirming the BH nature of their companions will require further dedicated monitorings, sophisticated analysis techniques, and multi-wavelength observations.

The Tarantula Massive Binary Monitoring

“…It is interesting to note that the orbit of the confirmed O+BH binary VFTS 243 is near-circular, implying negligible kick or ejecta for the BH progenitor (Shenar et al 2022). This is in contrast to the recent report of a BH in the eccentric (e ≈ 0.5) binary HD 130298, which otherwise shares a similar orbital period and spectral type to VFTS 243 (Mahy et al 2022). The orbit of the OB+BH candidate VFTS 779 is near-circular, while VFTS 514 is eccentric.…”

“…We assume a maximum inclination of 90 • for all targets. The minimum inclination i crit is estimated by equating the measured equatorial rotation velocity sin i/ sin i crit with the critical rotation velocity eq,crit of the luminous components (Mahy et al 2022). For simplicity, we adopt a typical value of the critical equatorial velocity as 500 km s −1 (e.g.…”

“…In principle, one can perform simulations to narrow down the possible types of companions (e.g. faint main sequence stars, helium stars; see Shenar et al 2022 andMahy et al 2022). One would then need to combine this with assumptions regarding the likelihood of these companions, which involves the initial mass function and binary evolution.…”

A red giant orbiting a black hole

“…Red and blue points show low-and high-mass X-ray binaries, whose parameters we take from Remillard & McClintock (2006) and the BlackCAT catalog of X-ray transients introduced by Corral- Santana et al (2016). We also show the binaries VFTS 243 (in the LMC; Shenar et al 2022) and HD 130298 (in the Milky Way; Mahy et al 2022), which both are single-lined binaries containing ∼ 25 𝑀 O stars and unseen companions suspected to be BHs. In magenta, we show two binaries with suspected BH companions in the globular cluster NGC 3201, discovered with MUSE (Giesers et al 2018(Giesers et al , 2019.…”