Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. IV. Lens Mass and Distance Measurements
Sean K. Terry, Etienne Bachelet, Farzaneh Zohrabi, Himanshu Verma, Alison Crisp, Macy Huston, Carissma McGee, Matthew Penny, Natasha S. Abrams, Michael D. Albrow, Jay Anderson, Fatemeh Bagheri, Jean-Phillipe Beaulieu, Andrea Bellini, David P. Bennett, Galen Bergsten, T. Dex Bhadra, Aparna Bhattacharya, Ian A. Bond, Valerio Bozza, Christopher Brandon, Sebastiano Calchi Novati, Sean Carey, Jessie Christiansen, William DeRocco, B. Scott Gaudi, Jon Hulberg, Stela Ishitani Silva, Sinclaire E. Jones, Eamonn Kerins, Somayeh Khakpash, Katarzyna Kruszynska, Casey Lam, Jessica R. Lu, Amber Malpas, Shota Miyazaki, Przemek Mroz, Arjun Murlidhar, David Nataf, Marz Newman, Greg Olmschenk, Rakek Poleski, Clement Ranc, Nicholas J. Rattenbury, Krzysztof Rybicki, Vito Saggese, Jennifer Sobeck, Keivan G. Stassun, Alexander P. Stephan, Rachel A. Street, Takahiro Sumi, Daisuke Suzuki, Aikaterini Vandorou, Meet Vyas, Jennifer C. Yee, Weicheng Zang, Keming Zhang
TL;DR
The paper evaluates whether RGES can satisfy the requirement to measure lens masses and distances for at least 40% of detected planet hosts with 20% precision. It uses simulations of 3,000 microlensing events toward GBTDS fields, applies Fisher matrix analysis to estimate light-curve parameter uncertainties, and combines these with Roman imaging observables (lens flux and relative lens-source proper motion) via the pyLIMASS tool to estimate lens properties. Assuming the Cassan et al. (2012) exoplanet mass function, it finds that >40% of hosts meet the 20% precision threshold, indicating the mission can meet the requirement. Validation against empirical mass/distance measurements from image-constrained light-curve modeling with Hubble and Keck follow-up shows agreement within about 1 sigma, supporting the robustness of the simulation framework. Together, the results reinforce the feasibility and scientific payoff of RGES for Galactic exoplanet demographics.
Abstract
As part of the Galactic Bulge Time Domain Survey (GBTDS), the Nancy Grace Roman Galactic Exoplanet Survey (RGES) will use microlensing to discover cold outer planets and free-floating planets unbound to stars. NASA has established several science requirements for the GBTDS to ensure RGES success. A key advantage of RGES is Roman's high angular resolution, which will allow detection of flux from many host stars. One requirement specifies that Roman must measure the masses and distances of 40% of detected planet hosts with 20% precision or better. To test this, we simulated microlensing events toward the GBTDS fields and used Fisher matrix analysis to estimate light curve parameter uncertainties. Combining these with Roman imaging observables (lens flux, relative lens-source proper motion), we estimated the achievable precision of lens mass and distance measurements. Using pyLIMASS, a publicly available code for estimating lens properties, we applied this analysis to 3,000 simulated events. Assuming the Cassan et al. (2012) exoplanet mass function, we find that >40% of host stars meet the required 20% precision threshold, confirming that the GBTDS can satisfy the mission requirement. We validated our approach by comparing our inferred lens masses and distances to empirical measurements from detailed image-constrained light curve modeling of historical microlensing events with Hubble and Keck follow-up imaging. Our results agree within roughly 1 sigma, demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.