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) x 10(-4), and the single-lens binary-source (1L2S) model is excluded by Delta chi(2) = 80. A Bayesian analysis using a Galactic model yields estimates of the host star mass of M-host = 0.57 (+0.33)(-0.29) M-circle dot, the planetary mass of M-planet = 13.5(-6.8)(+8.1) M-circle plus, and the lens distance of D-L = 6.9(-1.7)(+0.8) kpc. The projected planet-host separation of a(perpendicular to) = 2.3(-0.5)(+0.5) au or a(perpendicular to) = 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.