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

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 not bound 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 and 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 3000 simulated events. Assuming the A. Cassan et al. 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 σ , demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.

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Publication Details

Journal
The Astronomical Journal
Published
2026-03-09
DOI
https://doi.org/10.3847/1538-3881/ae43e6
Primary Topic
Stellar, planetary, and galactic studies
Type
article
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article

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

Sean K. Terry, William DeRocco, Amber Malpas, Sinclaire Jones et al.
The Astronomical Journal
Stellar, planetary, and galactic studies
article

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

Sean K. Terry, William DeRocco, Amber Malpas, Sinclaire Jones, Natasha S. Abrams, Farzaneh Zohrabi, Marz Newman, I. A. Bond, Somayeh Khakpash, Fatemeh Bagheri, Vito Saggese, Jennifer Sobeck, Macy Huston, V. Bozza, Alexander P. Stephan, K. Kruszyńska, Matthew T. Penny, E. Kerins, Jessie L. Christiansen, David M. Nataf, S. Carey, E. Bachelet, Andrea Bellini, Meet J. Vyas, Himanshu Verma, Shota Miyazaki, Alison L. Crisp, J K Yee, D Suzuki, Christopher Brandon, Jon Hulberg, Arjun Murlidhar, Clément Ranc, Carissma McGee, Michael D. Albrow, Jessica R. Lu, Keming Zhang, Nicholas J. Rattenbury, Jean-Philippe Beaulieu, Stela Ishitani Silva, T. Dex Bhadra, B. Scott Gaudi, Casey Lam, Krzysztof Rybicki, Keivan G. Stassun, (Roman Galactic Exoplanet Survey Project Infrastructure Team), Przemek Mróz, Takahiro Sumi, David P. Bennett, Galen Bergsten, Aparna Bhattacharya, Weicheng Zang, Jay Anderson, Rachel A. Street, Greg Olmschenk, Radek Poleski, Sebastiano Calchi Novati, Aikaterini Vandorou
article en

Abstract

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 not bound 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 and 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 3000 simulated events. Assuming the A. Cassan et al. 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 σ , demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.

The Astronomical JournalVol. 171(4)
Louisiana State University (US), California Institute of Technology (US), University of Iowa (US), Las Cumbres Observatory Global Telescope Network (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), University of Tasmania (AU), Goddard Space Flight Center (US), University of Salerno (IT), Johns Hopkins University (US), University of Auckland (NZ), University of America (US), University of Canterbury (NZ), Lehigh University (US), Carnegie Institution for Science (US), Vanderbilt University (US), Japan Aerospace Exploration Agency (JP), Institut d'Astrophysique de Paris (FR), University of Manchester (GB), Sorbonne Université (FR), Kavli Institute for Particle Astrophysics and Cosmology (US), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Osaka Health Science University (JP), Université Bourgogne Franche-Comté (FR), Institut UTINAM (FR), Center for Astrophysics Harvard & Smithsonian (US), Carnegie Observatories (US), Institute of Space and Astronautical Science (JP), Infrared Processing and Analysis Center (US), American Institute of Aeronautics and Astronautics (US), University of Warsaw (PL), Massey University (NZ), The Ohio State University (US), Ahmedabad University (IN), Massachusetts Institute of Technology (US), University of Maryland, College Park (US), University of Naples Federico II (IT), Catholic University of America (US), University of California, Berkeley (US), The University of Osaka (JP)
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Stellar, planetary, and galactic studies
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