Astronomy & Telescopes

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. IV. Lens Mass and Distance Measurements

By Keith Cowing

Status Report

astro-ph.EP

October 20, 2025

Filed under astronomy, exoplanet, Galactic Bulge Time Domain Survey (GBTDS), imaging, microlensing, Nancy Grace Roman Galactic Exoplanet Survey, Nancy Grace Roman Space Telescope

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. IV. Lens Mass and Distance Measurements — A representative sample of nine historical events from Table 5. The top row shows nearby (< 4 kpc) events for which a combination of all three mass-distance relations (θE, πE, lens flux) give an unambiguously measured lens mass and distance. The yellow data with error bars shows the best fit result from image-constrained modeling, the blue contours give the posterior distribution (central 39%, 86%, 99%) from pyLIMASS. The middle row shows more distant events (> 4 kpc) for which the estimate of lens mass and distance is still relatively accurate but less precise. The bottom row shows problematic events for which there are degeneracies that remain (MB10328; Vandorou et al. 2025b), or the identification of the lens is dubious (MB11262; Terry et al. 2025b), or the significantly overlapping θE and πE relations result in large errors (OB141760; Rektsini et al. 2025).– astro-ph.EP

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.

Sean K. Terry, Etienne Bachelet, Farzaneh Zohrabi, Himanshu Verma, Alison Crisp, Macy Huston, Carrisma 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

Comments: 27 pages, 7 figures, 6 tables, submitted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2510.13974 [astro-ph.EP (or arXiv:2510.13974v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2510.13974
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Submission history
From: Sean Terry
[v1] Wed, 15 Oct 2025 18:02:17 UTC (5,612 KB)
https://arxiv.org/abs/2510.13974

Astrobiology, Astronomy,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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