KMT-2023-BLG-1431Lb: A New Q < 10-4 Microlensing Planet from a Subtle Signature

The current studies of microlensing planets are limited by small number statistics. Follow-up observations of high-magnification microlensing events can efficiently form a statistical planetary sample. Since 2020, the Korea Microlensing Telescope Network (KMTNet) and the Las Cumbres Observatory (LCO) global network have been conducting a follow-up program for high-magnification KMTNet events. Here, we report the detection and analysis of a microlensing planetary event, KMT-2023-BLG-1431, for which the subtle (0.05 mag) and short-lived (5 hr) planetary signature was characterized by the follow-up from KMTNet and LCO. A binary-lens single-source (2L1S) analysis reveals a planet/host mass ratio of q = (0.72 ± 0.07) × 10−4, and the single-lens binary-source (1L2S) model is excluded by Δχ 2 = 80. A Bayesian analysis using a Galactic model yields estimates of the host star mass of M host = 0.57 − 0.29 + 0.33 M ⊙ , the planetary mass of M planet = 13.5 − 6.8 + 8.1 M ⊕ , and the lens distance of D L = 6.9 − 1.7 + 0.8 kpc. The projected planet-host separation of a ⊥ = 2.3 − 0.5 + 0.5 au or a ⊥ = 3.2 − 0.8 + 0.7 au, subject to the close/wide degeneracy. We also find that without the follow-up data, the survey-only data cannot break the degeneracy of central/resonant caustics and the degeneracy of 2L1S/1L2S models, showing the importance of follow-up observations for current microlensing surveys.