Simultaneous visible spectrophotometry of interstellar object 3I/ATLAS with Seimei/TriCCS

Abstract 3I/ATLAS, also known as C/2025 N1 (ATLAS), is the third interstellar object discovered in 2025 July. Interstellar objects are particularly interesting because characterizing their physical properties helps us improve and use our knowledge of the formation of the Solar system. Several quick response observations of 3I/ATLAS were performed during the first few days after the discovery, and various results, such as reflectance spectra, have been reported. We performed simultaneous visible spectrophotometry of 3I/ATLAS from data taken using the TriColor CMOS camera and spectrograph (TriCCS) on the Seimei 3.8 m telescope. The Seimei/TriCCS observations of 3I/ALTAS were obtained in the g, r, i, and z bands in the Pan-STARRS system on 2025 July 15 UTC. Our light curves show no significant variations during the 2.3 hr observation, which is in good agreement with the results of previous studies. Visible color indices of 3I/ATLAS, $g-r = 0.603\pm0.031$, $r-i = 0.210\pm0.031$, $i-z=0.117\pm0.046$, and $r-z = 0.327\pm0.035$ suggest that it has a red surface similar to, or slightly redder than, that of D-type asteroids. Continuous observations of 3I/ATLAS before and after its perihelion passage in 2025 October are required in order to investigate its physical properties.