Systematic errors as a source of mass discrepancy in black hole microlensing event OGLE-2011-BLG-0462

TL;DR

This paper identifies systematic errors and blending effects as key factors causing discrepancies in black hole mass measurements from microlensing event OGLE-2011-BLG-0462, ultimately confirming the black hole's mass at about 7.88 solar masses.

Contribution

It demonstrates that systematic errors and blending significantly impact microlensing mass estimates, providing a refined measurement of the black hole's mass and highlighting challenges in astrometric microlensing.

Findings

Systematic errors caused discrepancies in mass measurements.

Blending by source companions affects astrometric microlensing accuracy.

Confirmed the black hole's mass as approximately 7.88 solar masses.

Abstract

Two independent groups reported the discovery of an isolated dark stellar remnant in the microlensing event OGLE-2011-BLG-0462 based on photometric ground-based observations coupled with astrometric measurements taken with the Hubble Space Telescope. These two analyses yielded discrepant mass measurements, with the first group reporting that the lensing object is a black hole of 7.1 +/- 1.3 solar masses whereas the other concluded that the microlensing event was caused by either a neutron star or a low-mass black hole (1.6-4.4 solar masses). Here, we scrutinize the available photometric and astrometric data and conclude that systematic errors are a cause of the discrepant measurements. We find that the lens is an isolated black hole with a mass of 7.88 +/- 0.82 solar masses located at a distance of 1.49 +/- 0.12 kpc. We also study the impact of blending on the accuracy of astrometric microlensing measurements. We find that low-level blending by source companions is a major, previously unrecognized, challenge to astrometric microlensing measurements of black hole masses.