KMT-2021-BLG-2609Lb and KMT-2022-BLG-0303Lb: Microlensing planets identified through signals produced by major-image perturbations

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
² University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
³ Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
⁶ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁷ Center for Astrophysics | Harvard & Smithsonian 60 Garden St., Cambridge, MA 02138, USA
⁸ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, China
⁹ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea

Received: 25 May 2024
Accepted: 19 July 2024

Abstract

Aims. We investigate microlensing data collected by the Korea Microlensing Telescope Network (KMTNet) survey during the 2021 and 2022 seasons to identify planetary lensing events displaying a consistent anomalous pattern. Our investigation reveals that the light curves of two lensing events, KMT-2021-BLG-2609 and KMT-2022-BLG-0303, exhibit a similar anomaly, in which short-term positive deviations appear on the sides of the low-magnification lensing light curves.

Methods. To unravel the nature of these anomalies, we meticulously analyze each of the lensing events. Our investigations reveal that these anomalies stem from a shared channel, wherein the source passed near the planetary caustic induced by a planet with projected separations from the host star exceeding the Einstein radius. We find that interpreting the anomaly of KMT-2021-BLG-2609 is complicated by the “inner–outer” degeneracy, whereas for KMT-2022-BLG-0303, there is no such issue despite similar lens-system configurations. In addition to this degeneracy, interpreting the anomaly in KMT-2021-BLG-2609 involves an additional degeneracy between a pair of solutions, in which the source partially envelops the caustic and the other three solutions in which the source fully envelopes the caustic. As in an earlier case of this so-called von Schlieffen–Cannae degeneracy, the former solutions have substantially higher mass ratio.

Results. Through Bayesian analyses conducted based on the measured lensing observables of the event time scale and angular Einstein radius, the host of KMT-2021-BLG-2609L is determined to be a low-mass star with a mass ~0.2 M_⊙ in terms of a median posterior value, while the planet’s mass ranges from approximately 0.032 to 0.112 times that of Jupiter, depending on the solutions. For the planetary system KMT-2022-BLG-0303L, it features a planet with a mass of approximately 0.51 M_J and a host star with a mass of about 0.37 M_⊙. In both cases, the lenses are most likely situated in the bulge.