An isolated mass gap black hole or neutron star detected with astrometric microlensing

When stars that are more than 10x heavier than our Sun die, they collapse and leave behind neutron stars or black holes. There should be around 100 million black holes in our Milky Way Galaxy. However, we have only found about 2 dozen, mainly due to the fact that most black holes do not give off light and are therefore hard to find. The 2 dozen that have been found all have bright companions, which lets us infer the black hole’s presence. However, most of the 100 million black holes are expected to be single, isolated objects. To find isolated black holes, we use a phenomenon called gravitational microlensing.

The chance alignment on the sky of the black hole and background star allow us to detect the black hole and measure its mass. By measuring the changing brightness and position of the background star, we can learn about the properties of the unseen black hole.

For the first time, we report the detection of an isolated black hole or dark neutron star. Its name is OGLE-2011-BLG-0462/MOA-2011-BLG-191, nicknamed OB110462 for short. It’s around 1.6 - 4.4 times the mass of our Sun.

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When a distant background star passes behind a black hole, the gravitational field of the black hole acts like a lens, distorting and magnifying the star. In microlensing the distorted arcs are too small to be seen on the sky, appearing as a single point, but we are able to detect the increase in brightness and the slight shift in the apparent position of the star. Credit: Matthew Freeman (UC Berkeley/Moving Universe Lab), images courtesy NSO, NASA.

A "bird's eye view" of a gravitational microlensing event, if we could fly out of our Galaxy and see it from above. When two moving objects - a black hole/neutron star and a background star - come into near-perfect alignment, the black hole/neutron star will lens the background star which results in a magnification of the background star light as seen from Earth. Credit: Sean Terry (UC Berkeley, Moving Universe Lab)

As the unseen black hole/neutron star passes in front of a background star, the chance alignment causes the foreground black hole/neutron star to temporarily deflect the light of the background star, producing a transient brightening as observed from Earth. The top panel shows images stitched together, where the brightening of OB110462 (center) can be clearly observed. The bottom panel shows the brightness of OB110462 as a function of time. Credit: Casey Lam (UC Berkeley, Moving Universe Lab), data courtesy Optical Gravitational Lensing Experiment Collaboration

When a background star is lensed by a foreground black hole, the star's position appears to change. This difference in position is known as the astrometric shift. The position over time for OB110462 is shown when the background star is unlensed (gray) and lensed (red). Actual measurements from Hubble Space Telescope data are in black. The difference is pretty small and hard to measure (~1 milli-arcsecond). Credit: Natasha Abrams (UC Berkeley, Moving Universe Lab)

Check out the accepted version of the paper here:

https://arxiv.org/abs/2202.01903

Title: An isolated mass gap black hole or neutron star detected with astrometric microlensing

Authors: Casey Y. Lam, Jessica R. Lu, Andrzej Udalski, Ian Bond, David P. Bennett, Jan Skowron, Przemek Mroz, Radek Poleski, Takahiro Sumi, Michal K. Szymanski, Szymon Kozlowski, Pawel Pietrukowicz, Igor Soszynski, Krzysztof Ulaczyk, Lukasz Wyrzykowski, Shota Miyazaki, Daisuke Suzuki, Naoki Koshimoto, Nicholas J. Rattenbury, Matthew W. Hosek Jr., Fumio Abe, Richard Barry, Aparna Bhattacharya, Akihiko Fukui, Hirosane Fujii, Yuki Hirao, Yoshitaka Itow, Rintaro Kirikawa, Iona Kondo, Yutaka Matsubara, Sho Matsumoto, Yasushi Muraki, Greg Olmschenk, Clement Ranc, Arisa Okamura, Yuki Satoh, Stela Ishitani Silva, Taiga Toda, Paul J. Tristram, Aikaterini Vandorou, Hibiki Yama, Natasha S. Abrams, Shrihan Agarwal, Sam Rose, Sean K. Terry

Abstract: We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled lightcurves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant >1 mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6 - 4.4 M⊙. This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact object lens is relatively nearby (0.70-1.92 kpc) and has a slow transverse motion of <30 km/s. OB110462 shows significant tension between models well-fit to photometry vs. astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are <2M⊙ and they are unlikely to be black holes; two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (∼10^8) and find reasonable agreement given the small sample size.

MULab

An isolated mass gap black hole or neutron star detected with astrometric microlensing

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Check out the accepted version of the paper here:

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