http://arxiv.org/abs/2203.08478
The Milky Way is believed to host hundreds of millions of quiescent stellar-mass black holes (BHs). In the last decade, some of these objects have been potentially uncovered via gravitational microlensing events. All these detections resulted in a degeneracy between the velocity and the mass of the lens. This degeneracy has been lifted, for the first time, with the recent astrometric microlensing detection of OB110462. However, two independent studies reported very different lens mass for this event. Sahu et al. (2022) inferred a lens mass of 7.1 $\pm$ 1.3 Msun, consistent with a BH, while Lam et al. (2022) inferred 1.6-4.2 Msun, consistent with either a neutron star or a BH. Here, we study the landscape of isolated BHs formed in the field. In particular, we focus on the mass and center-of-mass speed of four sub-populations: isolated BHs from single-star origin, disrupted BHs of binary-star origin, main-sequence stars with a compact object companion, and double compact object mergers. Our model predicts that most ($\gtrsim$ 50%) isolated BHs in the Milky Way are of binary origin. Moreover, the origin of low-speed (< 50 km/s) isolated BHs depends on their mass: at least $\approx$ 70% of low-mass ($\lesssim$ 10 Msun) BHs are from binary origin. Under the assumption that OB110462 is a free-floating compact object, we conclude that it is more likely to be a BH originally belonging to a binary system. Our results suggest that low-speed BH microlensing events can be useful to understand binary evolution of massive stars in the Milky Way.
A. Vigna-Gómez and E. Ramirez-Ruiz
Thu, 17 Mar 22
31/66
Comments: 10 pages, 6 figures, 1 table. Submitted
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