ATLAS Photometry of Interstellar Object 3I/ATLAS

TL;DR

This paper reports calibrated photometry of interstellar object 3I/ATLAS, revealing its color evolution and brightness changes as it approached the Sun, using the ATLAS robotic network across multiple bands.

Contribution

First detailed multi-band photometric study of 3I/ATLAS, capturing its color transition and brightness slope break during perihelion approach.

Findings

Color transitioned from red to near-solar as tail appeared.

Brightness slope changed near 3.3 au from -0.035 to -0.012 mag/day.

Coma cross section varied with heliocentric distance as r^{-3.9} to r^{-1.1}.

Abstract

We present calibrated ATLAS photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from 28 March through 29 Aug 2025, obtained with the five-site, robotic ATLAS network in the c (420-650~nm), o (560-820~nm), and Teide w (420-720~nm) bands. Stacked difference images yield reliable light curves measured in four fixed apertures that capture the evolving coma. We observe 3I/ATLAS transitioning in color from red (c-o)~0.7 before MJD 60860 to near-solar (c-o)~0.3 after MJD 60870, coincident with the appearance of a prominent anti-solar tail. The absolute magnitude curve H(t) shows a slope break near MJD 60890 at r~3.3 au from -0.035 to -0.012 mag/day, or in terms of coma cross section as a function of heliocentric distance, r^{-3.9} to r^{-1.1}.