Black Hole, Neutron Star and White Dwarf Candidates from Microlensing with OGLE-III

TL;DR

This study identifies 13 microlensing events in the OGLE-III data that are consistent with being caused by isolated stellar remnants like white dwarfs, neutron stars, or black holes, providing insights into their masses and distribution.

Contribution

The paper presents the first identification of candidate isolated stellar remnants through microlensing events in OGLE-III data, including mass estimates and implications for the mass gap.

Findings

13 candidate events consistent with stellar remnants

Black hole candidate with 9.3 solar masses at 2.4 kpc

Mass distribution shows no gap between neutron stars and black holes

Abstract

Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III database of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 MSun and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission.