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Three Saturn-mass Microlensing Planets Identified through Signals from Peripheral-caustic Perturbations

Cheongho Han, Chung-Uk Lee, Andrzej Udalski, Ian A. Bond, Michael D. Albrow, Sun-Ju Chung, Andrew Gould, Youn Kil Jung, Kyu-Ha Hwang, Yoon-Hyun Ryu, Yossi Shvartzvald, In-Gu Shin, Jennifer C. Yee, Weicheng Zang, Hongjing Yang, Doeon Kim, Dong-Jin Kim, Sang-Mok Cha, Seung-Lee Kim, Dong-Joo Lee, Yongseok Lee, Byeong-Gon Park, Kyeongsoo Hong, Richard W. Pogge, Przemek Mróz, Michał K. Szymański, Jan Skowron, Radosław Poleski, Igor Soszyński, Paweł Pietrukowicz, Szymon Kozłowski, Krzysztof A. Rybicki, Patryk Iwanek, Krzysztof Ulaczyk, Marcin Wrona, Mariusz Gromadzki, Mateusz J. Mróz, Fumio Abe, David P. Bennett, Aparna Bhattacharya, Ryusei Hamada, Yuki Hirao, Asahi Idei, Stela Ishitani Silva, Shuma Makida, Shota Miyazaki, Yasushi Muraki, Tutumi Nagai, Togo Nagano, Seiya Nakayama, Mayu Nishio, Kansuke Nunota, Ryo Ogawa, Ryunosuke Oishi, Yui Okumoto, Greg Olmschenk, Clément Ranc, Nicholas J. Rattenbury, Yuki Satoh, Takahiro Sumi, Daisuke Suzuki, Takuto Tamaoki, Sean K. Terry, Paul J. Tristram, Aikaterini Vandorou, Hibiki Yama

Abstract

We present the discovery and analysis of three microlensing planets identified through brief positive anomalies on the wings of their light curves. The events, KMT-2021-BLG-0852, KMT-2024-BLG-2005, and KMT-2025-BLG-0481, were detected in high-cadence survey data from the KMTNet, OGLE, MOA, and PRIME collaborations. The anomaly morphologies are consistent with major-image perturbations induced by planetary-mass companions located near the peripheral caustic. A systematic exploration of model degeneracies, including binary-source scenarios, higher mass-ratio binary lenses, and the inner--outer caustic degeneracy, firmly establishes the planetary origin of each signal. Measurements of the angular Einstein radius and event timescale, combined with Bayesian priors from a Galactic model, yield the physical parameters of each system. The hosts are low-mass stars (0.12--0.75~$M_\odot$), while the companions are Saturn-mass planets (0.16--0.59 $M_{\rm J}$) projected at separations of 1.1--7.8 au, placing them beyond the snowline of their hosts. These results demonstrate the capability of microlensing to detect and characterize cold giant planets around low-mass stars at kpc distances, populating the critical transition region between ice giants and gas giants.

Three Saturn-mass Microlensing Planets Identified through Signals from Peripheral-caustic Perturbations

Abstract

We present the discovery and analysis of three microlensing planets identified through brief positive anomalies on the wings of their light curves. The events, KMT-2021-BLG-0852, KMT-2024-BLG-2005, and KMT-2025-BLG-0481, were detected in high-cadence survey data from the KMTNet, OGLE, MOA, and PRIME collaborations. The anomaly morphologies are consistent with major-image perturbations induced by planetary-mass companions located near the peripheral caustic. A systematic exploration of model degeneracies, including binary-source scenarios, higher mass-ratio binary lenses, and the inner--outer caustic degeneracy, firmly establishes the planetary origin of each signal. Measurements of the angular Einstein radius and event timescale, combined with Bayesian priors from a Galactic model, yield the physical parameters of each system. The hosts are low-mass stars (0.12--0.75~), while the companions are Saturn-mass planets (0.16--0.59 ) projected at separations of 1.1--7.8 au, placing them beyond the snowline of their hosts. These results demonstrate the capability of microlensing to detect and characterize cold giant planets around low-mass stars at kpc distances, populating the critical transition region between ice giants and gas giants.
Paper Structure (9 sections, 5 equations, 6 figures)

This paper contains 9 sections, 5 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Data
  3. Analysis
  4. KMT-2021-BLG-0852
  5. KMT-2024-BLG-2005
  6. KMT-2025-BLG-0481
  7. Source stars and Einstein radii
  8. Physical lens parameters
  9. Summary and Discussion

Figures (6)

  • Figure 1: Light curve of the microlensing event KMT-2021-BLG-0852. The bottom panel displays the full time-series of the event, while the top panel provides a magnified view of the anomalous region, including the best-fit model curves and residuals for both the inner and outer solutions. The preferred model (inner solution) is overlaid on the data points as a solid curve, and the outer solution is shown as a dotted curve for comparison. The two insets in the bottom panel illustrate the lens-system geometry. The right inset shows the wide-scale configuration, marking the positions of the binary lens components with black filled circles, the caustic structures in red, and the source trajectory with an arrowed line. The left inset shows a close-up of the caustic crossing, where the open magenta circle on the source trajectory indicates the source size scaled to the caustic size.
  • Figure 2: Light curve of KMT-2024-BLG-2005. The best-fit model (solution 1) is shown by the solid curve, and the alternative model (solution 2) by the dotted curve. Insets in the lower panel show the lens-system geometries: the two left insets correspond to the wide and enlarged views for solution 1, and the right inset to solution 2.
  • Figure 3: Light curve of the event KMT-2025-BLG-0481. Notations are similar to those in Fig. \ref{['fig:one']}.
  • Figure 4: Source positions within the instrumental color-magnitude diagrams (CMDs). The red dot represents the centroid of the red giant clump (RGC) used for the photometric calibration of the source color and magnitude. For KMT-2024-BLG-2005, the position of the blended light is also indicated. For KMT-2025-BLG-0481, the CMD is derived from the combination of KMTC (grey) and HST (brown) photometry.
  • Figure 5: Lomb--Scargle periodogram for the baseline flux of KMT-2021-BLG-0852.
  • ...and 1 more figures