An isolated mass gap black hole or neutron star detected with astrometric microlensing (2202.01903v2)

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 $M_\odot$. 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 $<2 M_\odot$ 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 ($\sim 10^8$) and find reasonable agreement given the small sample size.

• The paper presents an astrometric detection of a mass gap object with a >1 mas shift, inferring a mass between 1.6 and 4.4 solar masses.
• It employs a PSPL microlensing model with Bayesian inference to analyze combined photometric and astrometric data, addressing binary-source scenarios.
• The findings align with theoretical predictions on compact object populations, highlighting astrometric microlensing’s power to reveal isolated black holes and neutron stars.

An Isolated Mass Gap Black Hole or Neutron Star Detected with Astrometric Microlensing

The paper by Casey Y. Lam et al. focuses on the astrometric microlensing detection of potential compact objects in the mass gap category, predominantly black holes or neutron stars, through observations conducted with the Hubble Space Telescope and ground-based surveys. This paper presents a detailed analysis of a sample of five black hole candidates identified through gravitational microlensing surveys and scrutinized using a combination of astrometric and photometric data.

Key Findings

1. Astrometric Microlensing Signal: Among the five black hole candidates examined, one target, OGLE-2011-BLG-0462/MOA-2011-BLG-191 (OB110462), displays a significant >1 mas astrometric shift with minimal lens flux. This target shows an inferred lens mass ranging from 1.6 to 4.4 solar masses, indicating that the lens could be a neutron star or a low-mass black hole.
2. Nature of the Lens: The OB110462 has a substantial probability of being a dark object, corroborated by a significant likelihood of it being a neutron star or a black hole given the data's current state. The paper indicates tension between models that fit the photometric data versus those fitting the astrometric data, suggesting a complex underlying mass structure with potential binary source geometry.
3. Candidate Evaluation: For the remaining four candidates, the inferred lens masses are less than 2 solar masses, making them unlikely candidates for black holes. Two are likely white dwarfs or neutron stars.
4. Comparison with Theoretical Expectations: The paper's findings align with theoretical predictions concerning the number of black holes in the Milky Way galaxy (~10^8), given the estimated detection rate from microlensing alone.

Methodology

The research employs a point-source, point-lens (PSPL) microlensing model that incorporates both photometric and astrometric data to infer lens mass and potential black hole candidacy. The best-fit model parameters are estimated using Bayesian inference methods, thereby allowing for robust statistical evaluation of the observed microlensing events.

Implications

The results illustrate the capability of astrometric microlensing in detecting isolated compact objects such as black holes and neutron stars. The discovery of OB110462 as a plausible mass gap object contributes significantly to our understanding of compact object population statistics, particularly in the elusive mass range between the heaviest neutron stars and lightest black holes.

Future Research Directions

• Further Observations: Continued monitoring, including further Hubble Space Telescope cycles, is recommended to refine the mass estimation of the candidate lens.
• Advanced Modeling: The paper suggests the necessity of implementing more complex microlensing models that consider binary source or lens scenarios to account for the observed data tension.
• Constraints on Black Hole Population: As more BH candidates are observed using astrometric microlensing, there is potential to impose more stringent constraints on the black hole mass function and population within the Milky Way.

This paper underscores the potential of astrometric microlensing as a tool for identifying isolated stellar mass black holes and contributes new insights into the mass distribution of compact objects, potentially altering our understanding of stellar evolution and compaction endpoints.