HST pre-imaging of a free-floating planet candidate microlensing event

High-cadence microlensing observations uncovered a population of very short-timescale microlensing events, which are believed to be caused by the population of free-floating planets (FFPs) roaming the Milky Way. Unfortunately, the light curves of such events are indistinguishable from those caused by wide-orbit planets. To properly differentiate both cases, one needs high-resolution observations that would allow one to resolve a putative luminous companion to the lens long before or after the event. Usually, the baseline between the event and high-resolution observations needs to be quite long (∼10 yr), hindering potential follow-up efforts. However, there is a chance to use archival data if they exist. Here, we present an analysis of the microlensing event OGLE-2023-BLG-0524, the site of which was captured in 1997 with the Hubble Space Telescope (HST). Hence, we achieve a record-breaking baseline length of 25 years. A very short duration of the event ( t E = 0.346 ± 0.008 d) indicates an FFP as the explanation. Based on the finite-source effects in the light curve, we measure an angular Einstein radius value of θ E = 4.78 ± 0.23 μas, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Though we detect no potential companion to the lens in the HST imaging, we find that the HST imaging is insufficient to constrain beyond 25-48% of potential companions (depending on whether or not the occurrence rate of wide-orbit planets is correlated with the host mass); hence, we are unable to confidently confirm this event as an FFP. Despite this, our results show that archival high-resolution images should be available for many microlensing events, providing a potential new avenue to confirm FFP candidates in future observations.

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Journal
Astronomy and Astrophysics
Published
2025-12-11
DOI
https://doi.org/10.1051/0004-6361/202555863
Citations
1
Primary Topic
Astrophysics and Star Formation Studies
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article
Field-Weighted Citation Impact
1.15
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article

HST pre-imaging of a free-floating planet candidate microlensing event

Sang-Mok Cha, Yongseok Lee, I. Soszyński, Michael D. Albrow et al.
1 citations
Astronomy and Astrophysics
Astrophysics and Star Formation Studies
1.15
article

HST pre-imaging of a free-floating planet candidate microlensing event

Sang-Mok Cha, Yongseok Lee, I. Soszyński, Michael D. Albrow, Weicheng Zang, Hongjing Yang, M. Gromadzki, Kyu‐Ha Hwang, Krzysztof A. Rybicki, M. K. Szymański, J. Skowron, Byeong-Gon Park, R. Poleski, Yoon-Hyun Ryu, Yossi Shvartzvald, Seung‐Lee Kim, Chung‐Uk Lee, Cheongho Han, A. Udalski, Youn Kil Jung, Patryk Iwanek, P. Pietrukowicz, P. Mróz, K. Ulaczyk, In-Gu Shin, S. Kozłowski, Marcin Wrona, Mateusz Kapusta, W. Richard Pogge, Dong-Jin Kim, Dong-Joo Lee, Sun-Ju Chung, C. Jennifer Yee, Andrew Gould
article en
1 citations

Abstract

High-cadence microlensing observations uncovered a population of very short-timescale microlensing events, which are believed to be caused by the population of free-floating planets (FFPs) roaming the Milky Way. Unfortunately, the light curves of such events are indistinguishable from those caused by wide-orbit planets. To properly differentiate both cases, one needs high-resolution observations that would allow one to resolve a putative luminous companion to the lens long before or after the event. Usually, the baseline between the event and high-resolution observations needs to be quite long (∼10 yr), hindering potential follow-up efforts. However, there is a chance to use archival data if they exist. Here, we present an analysis of the microlensing event OGLE-2023-BLG-0524, the site of which was captured in 1997 with the Hubble Space Telescope (HST). Hence, we achieve a record-breaking baseline length of 25 years. A very short duration of the event ( t E = 0.346 ± 0.008 d) indicates an FFP as the explanation. Based on the finite-source effects in the light curve, we measure an angular Einstein radius value of θ E = 4.78 ± 0.23 μas, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Though we detect no potential companion to the lens in the HST imaging, we find that the HST imaging is insufficient to constrain beyond 25-48% of potential companions (depending on whether or not the occurrence rate of wide-orbit planets is correlated with the host mass); hence, we are unable to confidently confirm this event as an FFP. Despite this, our results show that archival high-resolution images should be available for many microlensing events, providing a potential new avenue to confirm FFP candidates in future observations.

Astronomy and AstrophysicsVol. 706
University of Science and Technology (YE), Planetary Science Institute (US), Goddard Space Flight Center (US), Chungbuk National University (KR), University of Canterbury (NZ), Korea Astronomy and Space Science Institute (KR), Laboratoire AstroParticule et Cosmologie (FR), National University of Science and Technology (ZW), Westlake University (CN), Kyung Hee University (KR), University of Warwick (GB), Max Planck Institute for Astronomy (DE), Center for Astrophysics Harvard & Smithsonian (US), Astronomical Observatory (RS), University of Warsaw (PL), The Ohio State University (US), Weizmann Institute of Science (IL), University of Maryland, College Park (US), Villanova University (US), Tsinghua University (CN)
Openalex Percentile: Top 14%
Astrophysics and Star Formation Studies
1.15
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