Searching for Compact Objects in the Single-Lined Spectroscopic Binaries of the Young Galactic Cluster NGC 6231

Context. Recent evolutionary computations predict that a few percent of massive O or early-B stars in binary systems should have a dormant stellar-mass black hole (BH) as a companion. However, despite several reported candidate X-ray quiet OB+BH systems over the last couple of years, finding them with certainty remains challenging. Yet these have great importance as they can be gravitational wave (GW) source progenitors, and they are landmark systems in constraining supernova kick physics. Aims. This work aims to characterise the hidden companions to the single-lined spectroscopic binaries (SB1s) identified in the B star population of the young open Galactic cluster NGC 6231 to find candidate systems for harbouring compact object companions. Methods. With the orbital solutions for each SB1 constrained in a previous study, we applied Fourier spectral disentangling to multi-epoch optical VLT/FLAMES spectra of each target to extract a potential signature of a faint companion, and to identify newly disentangled double-lined spectroscopic binaries (SB2s). For targets where the disentangling does not reveal any spectral signature of a stellar companion, we performed atmospheric and evolutionary modelling on the primary (visible) star to obtain constraints on the mass and nature of the unseen companion. Results. For seven of the 15 apparent SB1 systems, we extracted the spectral signature of a faint companion, resulting in seven newly classified SB2 systems with mass ratios down to near 0.1. From the remaining targets, for which no faint companion could be extracted from the spectra, four are found to have companion masses that lie in the predicted mass ranges of neutron stars (NSs) and BHs. Two of these targets have companion masses between 1 and 3.5 M-circle dot, making them potential hosts of NSs (or lower mass main sequence stars). The other two have mass ranges between 2.5 to 8 M-circle dot and 1.6 and 26 M-circle dot, respectively, and so are identified as candidates for harbouring BH companions. Conclusions. We present four SB1 systems in NGC 6231 that are candidates for harbouring compact objects, among which CD-41 11038 and CXOU J165421.3-415536 are the most convincing cases. We propose further observational tests involving photometric and interferometric follow-up observations of these objects.