Himanshu Verma | Lens Flux Analysis of Gravitational Microlensing Events with Roman Space Telescope

The constraints on the mass and distance of the lens inferred from the microlensing light curve can be significantly improved with a well-informed prior on the lens flux contribution to the total flux. These constraints can be further refined by including measurements of the lens's proper motion relative to the source. In this study, we aim to estimate the lens flux contribution and relative proper motion using a 2D image of the source-lens system captured at a much later epoch, well after the microlensing event, by the Roman Space Telescope. This late observation provide a maximally separated point spread functions (PSFs), corresponding to the lens and the source, due to their relative motion and hence maximizes the detectability of the lens in the image. We derive analytical estimates of the uncertainties in the lens flux and relative proper motion measurements based on 2D image observables, such as the centroid, spread, and skewness of the combined PSFs, assuming a Gaussian approximation. Additionally, we exploit the color-dependent centroid shift observable through multi-band photometry of Roman and demonstrate how it can further constrain lens properties. We discuss how our error estimates can be applied to assess the likelihood of accurately measuring the mass and distance of the host lens in planetary microlensing events, thereby resolving the degeneracy in determining the planet's mass and its separation from the host star. Furthermore, our framework can set upper limits on the flux of dark lenses, contributing to the discovery and characterization of a population of dark compact objects in the Galaxy.