Resolving Microlensing Events with Triggered VLBI

TL;DR

This paper proposes using triggered VLBI observations of radio-bright Mira variables during microlensing events to precisely measure the properties of lensing objects, including black holes, significantly advancing Galactic black hole studies.

Contribution

It introduces a novel method combining radio interferometry with microlensing to accurately determine lens mass, distance, and velocity, especially for black holes.

Findings

Microlensing events with radio-bright Mira stars occur at about 2 per year.

The method can measure lens properties with better than 15% precision.

Future surveys could increase black hole detections to 10 per year.

Abstract

Microlensing events provide a unique capacity to study the stellar remnant population of the Galaxy. Optical microlensing suffers from a near complete degeneracy between the mass, the velocity and the distance. However, a subpopulation of lensed stars, Mira variable stars, are also radio bright, exhibiting strong SiO masers. These are sufficiently bright and compact to permit direct imaging using existing very long baseline interferometers such as the Very Long Baseline Array (VLBA). We show that these events are relatively common, occurring at a rate of $\approx 2~{\rm yr^{\ -1}}$ of which $0.1~{\rm yr^{-1}}$ are associated with Galactic black holes. Features in the associated images, e.g., the Einstein ring, are sufficiently well resolved to fully reconstruct the lens properties, enabling the measurement of mass, distance, and tangential velocity of the lensing object to a precision better than 15%. Future radio microlensing surveys conducted with upcoming radio telescopes combined with modest improvements in the VLBA could increase the rate of Galactic black hole events to roughly 10~${\rm yr}^{-1}$, sufficient to double the number of known stellar mass black holes in a couple years, and permitting the construction of distribution functions of stellar mass black hole properties.