Multi-epoch spectrophotometric characterization of the mini-moon 2024 PT

Context. 2024 PT5 is a tiny (D ≤ 10 m) near-Earth asteroid (NEA) discovered in August 2024. 2024 PT5 was gravitationally bound to the Earth-Moon system from September to November 2024 and classified as a mini-moon. Several quick response observations suggest that 2024 PT5 has a lunar ejecta origin, while the rotation state and albedo, essential properties for investigating its origin, are not well constrained. Aims. We aim to characterize the spectrophotometric properties of 2024 PT5 by ground-based observations to test its taxonomic classification and origin. Methods. We performed visible to near-infrared multicolor photometry of 2024 PT5 from data taken using the TriColor CMOS Camera and Spectrograph (TriCCS) on the Seimei 3.8 m telescope during January 4–10, 2025. The Seimei/TriCCS observations of 2024 PT5 cover phase angles from 14 deg to 27 deg, and were obtained in the ɡ, r, i, and ɀ bands in the Pan-STARRS system. In addition, we analyzed Y, J, H, and K photometry taken with the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) on the Keck I 10-m telescope on January 16–17, 2025. Results. Our light curves show brightness variations over time periods of several tens of minutes. We infer that 2024 PT5 is in a tumbling state and has a light curve amplitude of about 0.3 mag. Visible and near-infrared color indices of 2024 PT5, ɡ – r = 0.567 ± 0.044, r – i = 0.155 ± 0.009, r – ɀ = 0.147 ± 0.066, Y – J = 0.557 ± 0.046, J – H = 0.672 ± 0.078, and H – Ks = 0.148 ± 0.098, indicate that 2024 PT5 is an S-complex asteroid, largely consistent with previous observations. Using the H–G model, we derived an absolute magnitude, HV,HG, of 27.72 ± 0.09 and a slope parameter, GV, of 0.223 ± 0.073 in the V band. A geometric albedo of 2024 PT5 was derived to be 0.26 ± 0.07 from the slope of its photometric phase curve. This albedo value is typical of the S- and Q-type NEAs. Conclusions. Using the albedo and absolute magnitude, the equivalent diameter of 2024 PT5 is estimated to be 7.4 ± 1.0 m. The color properties of 2024 PT5 derived from our observations match the ones of rock samples taken from the lunar surface, which agrees with previous studies.