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ATLAS Photometry of Interstellar Object 3I/ATLAS

John Tonry, Larry Denneau, Miguel Alarcon, Alejandro Clocchiatti, Nicolas Erasmus, Alan Fitzsimmons, Javier Licandro, Karen Meech, Robert Siverd, Henry Weiland

TL;DR

This study provides calibrated ATLAS photometry of the interstellar object 3I/ATLAS across the $c$, $o$, and Teide $w$ bands, using a five-site network and stacked difference imaging to sample the evolving coma with four fixed apertures. It documents a pronounced color change from $(c-o) \approx 0.7$ to $(c-o) \approx 0.3$ that coincides with tail formation, and identifies a slope break in the absolute magnitude $H(t)$ near $MJD \approx 60890$ at $r \approx 3.3$ au, where the coma cross section transitions from $r^{-3.9}$ to $r^{-1.1}$. The results illuminate the onset and evolution of activity in an interstellar comet, demonstrating how multiband, high-cadence photometry from a global network can track coma development and tail morphology. By providing an open, velocity-aligned photometric dataset with carefully modeled backgrounds and fixed-aperture measurements, the work complements other facilities and informs models of coma physics in interstellar objects.

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}.

ATLAS Photometry of Interstellar Object 3I/ATLAS

TL;DR

This study provides calibrated ATLAS photometry of the interstellar object 3I/ATLAS across the , , and Teide bands, using a five-site network and stacked difference imaging to sample the evolving coma with four fixed apertures. It documents a pronounced color change from to that coincides with tail formation, and identifies a slope break in the absolute magnitude near at au, where the coma cross section transitions from to . The results illuminate the onset and evolution of activity in an interstellar comet, demonstrating how multiband, high-cadence photometry from a global network can track coma development and tail morphology. By providing an open, velocity-aligned photometric dataset with carefully modeled backgrounds and fixed-aperture measurements, the work complements other facilities and informs models of coma physics in interstellar objects.

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}.

Paper Structure

This paper contains 3 sections, 1 equation, 3 figures.

Table of Contents

  1. Introduction
  2. Observations and Data
  3. Discussion and Conclusions

Figures (3)

  • Figure 1: The discovery $o$ band image of 3I/ATLAS from MJD 60957 is shown on the left centered on 3I/ATLAS, and the four difference images from that night are stacked on the right. The 9.3 image on the left has north up, east left with the native 1.86 pixels; the 10 image on the right has 2 rebinned pixels; black indicates surface brightness brighter than 21 AB arcsec$^{-2}$.
  • Figure 2: These images illustrate the evolution of 3I/ATLAS. From left to right are the $c$ band stack from MJD 60818–60826 (28 May) the $o$ band stack from 60817–60828 (26 May), the epoch at 60884 (27 Jul) just before the $H$ band slope change described below, and the epoch at 60905 (17 Aug) when the eastward, anti-solar tail becomes prominent. The fields are 100 with 2 pixels, north up and east left, black indicates surface brightness brighter than 23.2 AB arcsec$^{-2}$.
  • Figure 3: Magnitudes of 3I/ATLAS in a 6$\times$6 aperture are shown on the left as a function of time. Red is $o$ band, blue is $c$ band, magenta is $w$ band from Teide. On the right the magnitudes are corrected for geometrical illumination and distance, $5\log(r\Delta)$, and the Schleicher HM phase function, i.e. an absolute $H$ magnitude in the observed bandpass that shows the physical evolution of 3I/ATLAS with time.