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We report the analysis of additional multiband photometry and spectroscopyand new adaptive optics (AO) imaging of the nearby planetary microlensing eventTCP J05074264+2447555 (Kojima-1), which was discovered toward the Galacticanticenter in 2017 (Nucita et al.). We confirm the planetary nature of thelight-curve anomaly around the peak while finding no additional planetaryfeature in this event. We also confirm the presence of apparent blending fluxand the absence of significant parallax signal reported in the literature. TheAO image reveals no contaminating sources, making it most likely that theblending flux comes from the lens star. The measured multiband lens flux,combined with a constraint from the microlensing model, allows us to narrowdown the previously unresolved mass and distance of the lens system. We findthat the primary lens is a dwarf on the K/M boundary (0.581 \pm 0.033 M_sun)located at 505 \pm 47 pc and the companion (Kojima-1Lb) is a Neptune-massplanet (20.0 \pm 2.0 M_earth) with a semi-major axis of 1.08 ^{+0.62}_{-0.18}au. This orbit is a few times smaller than those of typical microlensingplanets and is comparable to the snow-line location at young ages. We calculatethat the a priori detection probability of Kojima-1Lb is only \sim 35%, whichmay imply that Neptunes are common around the snow line, as recently suggestedby the transit and radial velocity techniques. The host star is the brightestamong the microlensing planetary systems (Ks = 13.7), offering a greatopportunity to spectroscopically characterize this system, even with currentfacilities.

Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star