An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing (2201.13296v4)

Abstract: We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E~270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 Msun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a 'natal kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.

• The paper reports the first unambiguous detection of an isolated stellar-mass black hole using astrometric microlensing and HST data.
• It measures a black hole mass of approximately 7.1±1.3 M☉, with a distance of about 1.58 kpc and distinct proper motion.
• The findings pave the way for identifying dark compact objects and refining stellar evolution models using space-based observations.

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

The research conducted by Sahu et al. reports a significant milestone in the field of astrophysics, presenting the first unambiguous detection and mass determination of an isolated stellar-mass black hole (BH) using astrometric microlensing. This paper utilizes data from the Hubble Space Telescope (HST) to ascertain the properties of an isolated black hole, MOA-2011-BLG-191/OGLE-2011-BLG-0462, located in the direction of the Galactic bulge.

Key Findings and Methodology

This research leverages observations over six years from the HST, capturing the astrometric shift of a distant source star due to a passing compact object. The paper unveils a clear relativistic astrometric deflection of a source star, indicating the presence of a massive non-luminous object with no detectable emission, confirming its nature as a black hole.

• Mass Measurement: The combination of astrometric data from HST and the ground-based light curve facilitated the determination of the black hole's mass, calculated to be approximately $7.1 \pm 1.3\,M_\odot$. The inclusion of parallactic effects further corroborated this calculation.
• Distance Estimation: The analysis suggests the black hole is approximately $1.58 \pm 0.18$ kpc away from Earth, considerably closer than the Galactic bulge sources, confirming its status as an isolated entity.
• Proper Motion: The proper motion of the black hole deviates from the mean motion of Galactic-disk stars at similar distances, suggesting that it might have received a "kick" during its progenitor's supernova explosion. Its relative transverse space velocity is estimated to be around $45\,$ km/s.
• Non-Detectability in Light: The astrometric microlensing technique demonstrated the non-luminous nature of the black hole without detectable light emission in optical wavelengths. This characteristic is critical in differentiating the black hole from other stellar remnants like neutron stars or white dwarfs.

Implications and Future Work

This paper provides a crucial methodology for identifying isolated black holes, contributing to our understanding of black hole demographics in the Milky Way.

• Practical Implications: These results open new avenues for the identification of other stellar-mass black holes that remain unseen by traditional electromagnetic-based observations, thus offering a comprehensive census of such objects. The findings also suggest the potential for discovering other compact objects utilizing space-based astrometric facilities.
• Theoretical Contributions: This work aids in refining models of stellar evolution and black hole formation, particularly concerning the population synthesis of isolated black holes which may not have undergone common-envelope evolution in binary systems.
• Prospects for AI and Automation: With the advent of large-scale astronomical surveys, the incorporation of AI and machine learning techniques for data processing and anomaly detection in astrometric data can significantly enhance future efforts to discover stellar-mass black holes.

Further investigations leveraging upcoming space missions like the Nancy Grace Roman Space Telescope and the Vera C. Rubin Observatory are anticipated to dramatically increase the number of detected black holes, enrich binary evolution modeling, and refine the constraints on massive star remnants. Additionally, future observations might further elucidate the dynamics of isolated BHs in differing Galactic environments and their interaction with the interstellar medium, potentially through multi-wavelength investigations.

In conclusion, the pioneering work by Sahu et al. exemplifies the potential of astrometric microlensing as a robust technique to unearth dark and elusive cosmic entities, enriching our astronomical toolkit and deepening our understanding of compact objects in the universe.