KMT-2019-BLG-0371 and the Limits of Bayesian Analysis

TL;DR

This paper examines the limitations of Bayesian analysis in microlensing event interpretation, showing that inferred masses mainly depend on the Einstein radius and are insensitive to other parameters.

Contribution

It demonstrates the dominant influence of measured Einstein radius on Bayesian mass estimates and explores constraints affecting Bayesian inference in microlensing.

Findings

Mass estimates are primarily determined by $ heta_{ m E}$.

Bayesian results are insensitive to event direction and proper motion.

The interpretation of the secondary as a planet relies solely on Bayesian analysis.

Abstract

We show that the perturbation at the peak of the light curve of microlensing event KMT-2019-BLG-0371 is explained by a model with a mass ratio between the host star and planet of $q \sim 0.08$. Due to the short event duration ($t_{\rm E} \sim 6.5\ $ days), the secondary object in this system could potentially be a massive giant planet. A Bayesian analysis shows that the system most likely consists of a host star with a mass $M_{\rm h} = 0.09^{+0.14}_{-0.05}M_{\odot}$ and a massive giant planet with a mass $M_{\rm p} = 7.70^{+11.34}_{-3.90}M_{\rm Jup}$. However, the interpretation of the secondary as a planet (i.e., as having $M_{\rm p} < 13 M_{\rm Jup}$) rests entirely on the Bayesian analysis. Motivated by this event, we conduct an investigation to determine which constraints meaningfully affect Bayesian analyses for microlensing events. We find that the masses inferred from such a Bayesian analysis are determined almost entirely by the measured value of $\theta_{\rm E}$ and are relatively insensitive to other factors such as the direction of the event $(\ell, b)$, the lens-source relative proper motion $\mu_{\rm rel}$, or the specific Galactic model prior.