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 ($\sim 100$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${}_{\mathrm{initial}}\sim 4-5M_{\odot }$, M${}_{\mathrm{current}}\sim 1-2M_{\odot }$) 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 ($K_{2,\mathrm{revised}}=176\pm 3$km/s vs $K_{2,\mathrm{original}}=140\pm 3$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 ($f_{\mathrm{revised}}$=2.83$M_{\odot }$ vs $f_{\mathrm{original}}$=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.

10 pages, 8 Figures and 2 Tables. Accepted for publication by MNRAS

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