On Finding Black Holes in Photometric Microlensing Surveys

TL;DR

This paper introduces a new probabilistic classification method for identifying black hole candidates in microlensing surveys, leveraging lightcurve data and Galactic models, leading to the discovery of 23 high-probability candidates.

Contribution

The authors develop a flexible, full-distribution-based classification framework for microlensing events, implemented in the popclass Python package, improving black hole detection strategies.

Findings

Identified 23 high-probability black hole candidates from OGLE data.

Found the only known isolated black hole to be an outlier in current models.

Highlighted the limitations of previous simple cut-based classification methods.

Abstract

There are expected to be millions of isolated black holes in the Galaxy resulting from the death of massive stars. Measuring the abundance and properties of this remnant population would shed light on the end stages of stellar evolution and the evolution paths of black hole systems. Detecting isolated black holes is currently only possible via gravitational microlensing which has so far yielded one definitive detection. The difficulty in finding microlensing black holes lies in having to choose a small subset of events based on characteristics of their lightcurves to allocate expensive and scarce follow-up resources to confirm the identity of the lens. Current methods either rely on simple cuts in parameter space without using the full distribution information or are only effective on a small subsets of events. In this paper we present a new lens classification method. The classifier takes in posterior constraints on lightcurve parameters and combines them with a Galactic simulation to estimate the lens class probability. This method is flexible and can be used with any set of microlensing lightcurve parameters making it applicable to large samples of events. We make this classification framework available via the popclass python package. We apply the classifier to $\sim10,000$ microlensing events from the OGLE survey and find $23$ high-probability black hole candidates. Our classifier also suggests that the only known isolated black hole is an observational outlier according to current Galactic models and allocation of astrometric follow-up on this event was a high-risk strategy.