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

A black hole candidate orbiting a luminous star in the Large Magellanic Cloud young cluster NGC 1850 (similar to 100 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-initial similar to 4-5 M-circle dot and M-current similar to 1-2 M-circle dot) 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-2,K- revised = 176 +/- 3 kms(-1) versus K-2,K- original = 140 +/- 3 kms(-1)) 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(revised) = 2.83 M(circle dot)versus f(original) = 1.42 M-circle dot). By exploiting the spectral disentangling technique, we place an upper limit of 10 percent 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 disc 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 >= 4.7 M-circle dot is present in the system (given the inclination and secondary mass constraints), it must be hidden in a optically thick disc to be undetected in the MUSE spectra.