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Abstract 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 masses for this event. Sahu et al. inferred a lens mass of 7.1 ± 1.3M⊙, consistent with a BH, while Lam et al. inferred 1.6–4.2M⊙, 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 subpopulations: 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 (≳70%) isolated BHs in the Milky Way are of binary origin. However, noninteractions lead to most massive BHs (≳15–20M⊙) being predominantly of single 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-mass BH microlensing events can be useful to understand binary evolution of massive stars in the Milky Way, while high-mass BH lenses can be useful to probe single stellar evolution.
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This content will become publicly available on December 1, 2025
First Resolution of Microlensed Images of a Binary-lens Event
Abstract We resolve the multiple images of the binary-lens microlensing event ASASSN-22av using the GRAVITY instrument of the Very Large Telescope Interferometer (VLTI). The light curves show weak binary-lens perturbations, complicating the analysis, but the joint modeling with the VLTI data breaks several degeneracies, arriving at a strongly favored solution. Thanks to precise measurements of the angular Einstein radiusθE= 0.724 ± 0.002 mas and microlens parallax, we determine that the lens system consists of two M dwarfs with masses ofM1= 0.258 ± 0.008M⊙andM2= 0.130 ± 0.007M⊙, a projected separation ofr⊥= 6.83 ± 0.31 au, and a distance ofDL= 2.29 ± 0.08 kpc. The successful VLTI observations of ASASSN-22av open up a new path for studying intermediate-separation (i.e., a few astronomical units) stellar-mass binaries, including those containing dark compact objects such as neutron stars and stellar-mass black holes.
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- Award ID(s):
- 2307385
- PAR ID:
- 10628098
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- AAS Journals
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 977
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 229
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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