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

• Published in: Monthly notices of the Royal Astronomical Society 2023-05, Vol.521 (2), p.3162-3171
• Main Authors: Saracino, S, Shenar, T, Kamann, S, Bastian, N, Gieles, M, Usher, C, Bodensteiner, J, Kochoska, A, Orosz, J A, Sana, H
• Format: Article
• Language: English
• Subjects: binaries: spectroscopic; globular clusters: individual: NGC 1850; techniques: imaging spectroscopy; techniques: photometric; techniques: radial velocities
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/stad764

ABSTRACT A black hole candidate orbiting a luminous star in the Large Magellanic Cloud young cluster NGC 1850 (∼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 (Minitial ∼ 4–5 M⊙ and Mcurrent ∼ 1–2 M⊙) 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 (K2, revised = 176 ± 3 km s−1 versus K2, original = 140 ± 3 km s−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 (frevised = 2.83 M⊙versus foriginal = 1.42 M⊙). By exploiting the spectral disentangling technique, we place an upper limit of 10 per cent 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⊙ 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.