Another one (BH+OB pair) bites the dust

Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique), Allée du 6 Août 19c, B-4000 Sart Tilman, Liège, Belgium

^⋆ Corresponding author; ynaze@uliege.be

Received: 18 December 2024
Accepted: 25 February 2025

Abstract

Aims. Most (or possibly all) massive stars reside in multiple systems. From stellar evolution models, numerous systems with an OB star coupled to a black hole would be expected to exist. There have been several claimed detections of such pairs in recent years and this is notably the case of HD 96670.

Methods. Using high-quality photometry and spectroscopy in the optical range, we revisited the HD 96670 system. We also examined complementary X-ray observations to provide a broader view of the system properties.

Results. The TESS light curves of HD 96670 clearly show eclipses, ruling out the black hole companion scenario. This does not mean that the system is not of interest. Indeed, the combined analysis of photometric and spectroscopic data indicates that the system most likely consists of a O8.5 giant star paired with a stripped-star companion with a mass of ∼4.5 M_⊙, a radius of ∼1 R_⊙, and a surface temperature of ∼50 kK. While several B+sdOB systems have been reported in the literature, this would be the first case of a Galactic system composed of an O star and a faint stripped star. In addition, the system appears brighter and harder than normal OB stars in the X-ray range, albeit less so than for X-ray binaries. The high-energy observations provide hints of phase-locked variations, as typically seen in colliding wind systems. As a post-interaction system, HD 96670 actually represents a key case for probing binary evolution, even if it is not ultimately found to host a black hole.