The Prime Focus Infrared Microlensing Experiment (PRIME): First Results
Takahiro Sumi, David A. H. Buckley, Alexander S. Kutyrev, Motohide Tamura, David P. Bennett, Ian A. Bond, Giuseppe Cataldo, Joseph M. Durbak, S. Bradley Cenko, Dale Fixsen, Orion Guiffreda, Ryusei Hamada, Yuki Hirao, Asahi Idei, Dan Kelly, Markus Loose, Gennadiy N. Lotkin, Eric I. Lyness, Stephen Maher, Shuma Makida, Noriyuki Matsunaga, Shota Miyazaki, Gregory Mosby, Samuel H. Moseley, Tutumi Nagai, Togo Nagano, Seiya Nakayama, Mayu Nishio, Kansuke Nunota, Ryo Ogawa, Ryunosuke Oishi, Yui Okumoto, Nicholas J. Rattenbury, Yuki K. Satoh, Elmer H. Sharp, Daisuke Suzuki, Takuto Tamaoki, Eleonora Troja, Sarah V. White, Hibiki Yama
TL;DR
The PRIME paper introduces the first dedicated wide-field near-infrared microlensing survey targeting the inner Galactic bulge, enabled by a 1.8 m NIR prime-focus telescope at SAAO and a multi-institution collaboration tied to NASA's Roman Space Telescope; its goals include measuring exoplanet occurrence and mass functions in dense bulge environments and performing concurrent space-based parallax measurements to directly determine lens masses, including free-floating planets. Since beginning GB observations in 2024, PRIME has identified hundreds of microlensing candidates and over a thousand variable stars, providing real-time alerts for follow-up and planning all-sky and ToO observations for transients. The project aims to map microlensing event rates and optical depths to constrain Galactic structure, inform Roman field choices, and enable comparative studies of planet formation across different bulge environments, while also expanding variable-star catalogs in highly reddened regions. This work demonstrates PRIME's potential to deliver mass measurements and environmental context for exoplanets and stellar populations in the Galactic center region, advancing both exoplanet science and Galactic archaeology.
Abstract
We present the description of the instruments and the first results of the PRime-focus Infrared Microlensing Experiment (PRIME). PRIME is the first dedicated near-infrared (NIR) microlensing survey telescope located at the South African Astronomical Observatory (SAAO) in Sutherland, South Africa. Among its class, it offers one of the widest fields of view in the NIR regime. PRIME's main goals are (1) To study planetary formation by measuring the frequency and mass function of planets. In particular, we compare results from the central Galactic bulge (GB), accessible only in the NIR by PRIME, with those from the outer GB by optical surveys. (2) To conduct concurrent observations with NASA's Nancy Grace Roman Space telescope. Due to the different lines of sight between the ground and space, we detect slight variations in light curves, known as ``Space-based parallax." This effect allows us to measure the mass of lens systems and their distance from the Earth. It is the only method to measure the mass of the free-floating planets down to Earth-mass. We began the GB survey in February 2024 and analyzed images through June 1, 2025, identifying 486 microlensing candidates and over a thousand variable stars, including Mira variables, which are useful to study the Galactic structure. We issue real-time alerts for follow-up observations, supporting exoplanet searches, and the chemical evolution studies in the GB. During the off-bulge season, we conduct an all-sky grid survey and Target of Opportunity (ToO) observations of transients, including gravitational wave events, gamma-ray bursts, and other science.