Updated radial velocities and new constraints on the nature of the unseen source in NGC1850 BH1

Abstract

A black hole candidate orbiting a luminous star in the Large Magellanic Cloud young cluster NGC 1850 (\(\sim100\)Myr) has recently been reported based on radial velocity and light curve modelling. Subsequently, an alternative explanation has been suggested for the system: a bloated post-mass transfer secondary star (M\(_{\rm initial} \sim 4-5M_{\odot}\), M\(_{\rm current} \sim 1-2M_{\odot}\)) with a more massive, yet luminous companion (the primary). Upon reanalysis of the MUSE spectra, we found that the radial velocity variations originally reported were underestimated (\(K_{\rm 2,revised} = 176\pm3\)km/s vs \(K_{\rm 2,original} = 140\pm3\)km/s) because of the weighting scheme adopted in the full-spectrum fitting analysis. The increased radial velocity semi-amplitude translates into a system mass function larger than previously deduced (\(f_{\rm revised}\)=2.83\(M_{\odot}\) vs \(f_{\rm original}\)=1.42\(M_{\odot}\)). By exploiting the spectral disentangling technique, we place an upper limit of 10\% of a luminous primary source to the observed optical light in NGC1850 BH1, assuming that the primary and secondary are the only components contributing to the system. Furthermore, by analysing archival near-infrared data, we find clues to the presence of an accretion disk in the system. These constraints support a low-mass post-mass transfer star but do not provide a definitive answer whether the unseen component in NGC1850 BH1 is indeed a black hole. These results predict a scenario where, if a primary luminous source of mass M \(\ge 4.7M_{\odot}\), is present in the system (given the inclination and secondary mass constraints), it must be hidden in a optically thick disk to be undetected in the MUSE spectra.