Resolving Microlensing Events with Triggered VLBI

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 $0.1~{\rm yr<sup>{-1}}$ 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~${\rm yr}<sup>{-1}$, 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.

• The paper demonstrates that triggered VLBI resolves microlensing events, enabling precise measurements of lens mass, distance, and velocity with better than 15% accuracy.
• The study identifies radio-bright Mira variables with SiO maser emissions as optimal targets to trigger VLBI observations of Galactic stellar remnants.
• The findings suggest that modest enhancements to VLBI sensitivity could double microlensing event detections, advancing our understanding of dark matter and stellar evolution.

Analysis of "Resolving Microlensing Events with Triggered VLBI"

The paper "Resolving Microlensing Events with Triggered VLBI" explores the intersection of gravitational microlensing events and very long baseline interferometry (VLBI) to obtain precise measurements of stellar remnant populations, such as black holes, in the Milky Way. The authors focus on leveraging optical triggers to initiate radio observations, especially targeting Mira variable stars that emit strong silicon monoxide (SiO) maser emissions. By employing the Very Long Baseline Array (VLBA), they aim to resolve the features in microlensed images, such as the Einstein ring, to precisely determine the mass, distance, and tangential velocity of the lensing object.

Key Research Insights

Microlensing presents a significant observational capability to probe stellar remnants and dark matter constituents of our galaxy. However, traditional optical microlensing encounters degeneracies involving the mass, velocity, and distance of the lensing object. These challenges are particularly pertinent when studying black holes within the Galaxy, as they lack luminous companions.

The paper identifies a subset of Mira variables, which are radio bright due to SiO maser activity, as suitable targets for VLBI. Statistical simulations detailed in sections \ref{sec:stats} and \ref{sec:rates} project the frequency of such lensing events, estimating that about 2 per year are detectable with current capabilities, revisiting this rate significantly with the incorporation of targeted radio surveys.

Implications on Observing Galactic Black Holes

The detection and detailed study of microlensing events associated with black holes are pivotal to understanding stellar evolution. Resolving the images with VLBI allows reconstruction of lens properties independently of direct photometric measurements, offering unprecedented insights into the cosmic census of stellar-mass black holes. Specifically, the measurement accuracy is better than 15%, a significant advancement over traditional methods.

The paper highlights the prospect of utilizing future radio telescope advancements to increase detection capabilities. By enhancing radio observation sensitivity with modest improvements to the VLBA, the rate of detected black hole events could rise to 10 per year, effectively doubling known stellar-mass black holes within a couple of years.

Future Directions and Challenges

In harnessing the synergy of optical and radio observations, there lies the potential to alleviate the limitations of present-day methods and categorize the remnants with precision, thus mapping their properties across the Galaxy. Moreover, the instrumentational evolution of VLBI, including increased bandwidth and sensitivity, can unlock a broader spectrum of radio sources that can be surveyed for microlensing.

Nevertheless, the implications extend beyond just stellar remnants. Gravitational lensing provides a method to test gravitational theories and explore dark matter distributions within the Galactic halo. In practice, optimizing surveys and overcoming intrinsic stellar variability (such as that of Miras) pose considerable analytical challenges.

Conclusion

The approach detailed in "Resolving Microlensing Events with Triggered VLBI" suggests a path forward for observational astrophysics that narrows down uncertainties in lensing-based mass measurements. It not only posits a method for discovering and analyzing stellar remnants such as black holes but also paves the way for broader, systematic insights into the Milky Way’s dark constituents. This progression is crucial, as it informs stellar evolution models, reinforces theoretical frameworks concerning dark matter, and sets a roadmap for future astronomical instruments and surveys to follow. The paper underscores a methodological innovation that bridges optical and radio astrophysics, unlocking a novel vantage in the study of our Galaxy.