The nature of the binary system NGC1850-BH1 still debated: a system with a 11 solar masses black hole (Saracino et al. 2021) or with a sdOB star (El-Badry et al. 2021)?
In the last years several observational surveys were designed in order to detect stellar mass black holes, including both those which are actively accreting and black holes in a quiescent status. These observations are crucial to understand the real extent of the population of black holes in the Milky Way, and to understand the formation of massive stellar black holes (more massive than 10 solar masses) and intermediate mass black holes (more massive than 100 solar masses), which are source of most of the gravitational waves detected so far. The most likely hypothesis on the formation of such black holes is by coalescence in stellar clusters which are dense enough to result in high chances of encounters. However, to date only a few candidate black holes were identified in stellar clusters and none of these detections is unambiguous.
In a recent paper published by the Monthly Notices of the Royal Astronomical Society (MNRAS), the team of astronomers led by S. Saracino (Liverpool John Moores University) presented a new technique which is aimed at finding black holes in binary systems. This method is based on the search of radial velocity signals in spectra of the star in the system (which are periodic oscillations of observed lines due to the Doppler effect and the orbit of the star around the black hole), together with photometric estimate of the mass of the stellar companion and the analysis of theoretical models of the system aimed at reproducing the observed light curve. Thanks to this method, the team has discovered a binary system in the star cluster NGC 1850 (in the Large Magellanic Cloud) with a 5 solar masses star and a 11 solar masses black hole. This is an important discovery, being the first time that a black hole is discovered in a stellar cluster. The study is described in the paper “A black hole detected in the young massive LMC cluster NGC 1850“, counting among the coauthors also the astronomer M. G. Guarcello of INAF – Astronomical Observatory of Palermo. The discovery has been the subject of a press release of the European Southern Observatory (link)·
However, the debate is at the base of scientific research. In fact, a few days after the discovery has been published, the astronomer K. El-Badry (Center for Astrophysics | Harvard & Smithsonian) presented a different interpretation of the properties of the binary system in NGC 1850, described in the paper: “NGC 1850 BH1 is another stripped-star binary masquerading as a black hole“, which has been submitted for publication to MNRAS. As shown by the authors, the binary system may not contain a 11 solar mass black hole, but an inflated sdOB (subdwarf OB) star instead. These are low-mass stars (0.5 solar masses in this case), exceptionally hot (spectral type O or B) and helium-rich. These stars can form in binary systems after the mass-transfer phase, during which the hydrogen rich outer envelope of the star has been stripped away. This interpretation, however, seems to fail in reproducing the observed radial velocity signal and the observed photometric light curve. Future observations are thus required in order to finally shed light on the nature of NGC1850-BH1.
The figure (click here to visualize the entire figure) shows an image of NGC 1850 acquired with MUSE instrument of the European Southern Observatory, with marked the position of the binary system. The right panel shows a zoomed image from the Hubble Space Telescope.
Mario Giuseppe Guarcello ( follow mguarce) ( youtube)
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