Simultaneous Visible Spectrophotometry Of Interstellar Object 3I/ATLAS With Seimei/TriCCS

3I/ATLAS, also known as C/2025 N1 (ATLAS), is the third interstellar object (ISO) discovered in July 2025.

ISOs are particularly interesting because characterizing their physical properties helps us understand and test our knowledge of Solar System formation. 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 UTC July 15, 2025.

Our lightcurves show no significant variations during the 2.3 h observation, which is in good agreement with previous studies. Visible color indices of 3I/ATLAS, g−r=0.603±0.031, r−i=0.210±0.031, i−z=0.117±0.046, and r−z=0.327±0.035 suggest 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 October 2025 are desired to investigate its physical properties.

Jin Beniyama

Comments: 6 pages, 4 figures, accepted for publication in PASJ Letter. This is a pre-copyedited, author-produced version of an article accepted for publication in “Publications of the Astronomical Society of Japan” following peer review. The version of record is available online at https://apps.crossref.org/pendingpub/pendingpub.html?doi=10.1093%2Fpasj%2Fpsaf097

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2508.08829 [astro-ph.EP] (or arXiv:2508.08829v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.08829
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Related DOI:
https://doi.org/10.1093/pasj/psaf097
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Submission history
From: Jin Beniyama
[v1] Tue, 12 Aug 2025 10:37:07 UTC (1,646 KB)
https://arxiv.org/abs/2508.08829
Astrobiology,