ATLAS Photometry of Interstellar Object 3I/ATLAS (2509.05562v2)

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 -0.014 mag/day, or in terms of coma cross section as a function of heliocentric distance, r^{-3.9} to r^{-1.2}. We interpret the evolution as a shift from dust lifted from a reddened surface to production of small, optically bright icy grains and changing coma optical depth. We release the aperture photometry with geometry and uncertainties to enable cross-instrument synthesis of 3I/ATLAS activity and color evolution.

• The paper demonstrates how precise ATLAS photometry detects a significant color transition in interstellar comet 3I/ATLAS.
• The paper employs a rigorous multi-band imaging and calibration process to track the evolution of coma brightness and morphology.
• The paper finds that the shift from dust-dominated to volatile-rich emissions corresponds with a marked change in the comet’s light curve slope.

ATLAS Photometry and Color Evolution of Interstellar Object 3I/ATLAS

Introduction and Context

The paper presents a comprehensive photometric analysis of the interstellar comet 3I/ATLAS (C/2025 N1), the third confirmed interstellar object to traverse the solar system. The paper leverages the high-cadence, all-sky ATLAS network, spanning five robotic observatories, to obtain calibrated photometry in the $c$ (420–650 nm), $o$ (560–820 nm), and Teide $w$ (420–720 nm) bands. The dataset covers the period from late March through late August 2025, capturing the object's approach to the Sun and the subsequent evolution of its coma and tail.

The discovery and initial characterization of 3I/ATLAS are notable for the object's hyperbolic trajectory and pronounced cometary activity, distinguishing it from the previously discovered interstellar objects 1I/'Oumuamua and 2I/Borisov. The ATLAS survey's systematic coverage and automated reduction pipeline enabled early detection and continuous monitoring, providing a unique temporal record of the object's photometric and morphological changes.

Figure 1: Discovery $o$ band image and stacked difference images of 3I/ATLAS, illustrating the detection and background subtraction methodology.

Observational Methodology and Data Reduction

The ATLAS network's robotic telescopes operate with a standardized survey cadence, typically acquiring four images per field per night. The reduction pipeline includes astrometric and photometric calibration, difference imaging against a static sky template, and detection classification via multiple algorithms and a convolutional neural network. The photometric measurements are reported in four concentric apertures, designed to capture the evolving coma brightness while mitigating contamination from background sources.

The color calibration is rigorously tied to the Pan-STARRS and Gaia-based Refcat2 catalog, with explicit transformations provided between ATLAS bands and standard photometric systems. The paper emphasizes the importance of well-characterized bandpasses, aperture definitions, and background estimation for reliable cross-instrument synthesis.

Figure 2: Temporal evolution of 3I/ATLAS in stacked $c$ and $o$ band images, showing the transition from a sunward "rubble fountain" to a prominent anti-solar tail.

Photometric Evolution and Color Transition

The light curve analysis reveals several distinct phases in the activity of 3I/ATLAS:

• Early Phase (MJD < 60860): The coma exhibits a red color, with $(c-o) \approx 0.7$, and a relatively flat absolute $H$ magnitude. This suggests dominance of dust lifted from a reddened surface, potentially analogous to the ejection of surface material observed on Bennu.
• Discovery and Rubble Fountain (MJD 60845–60860): The color remains red, but the $H$ magnitude brightens and then declines. This period coincides with the appearance of a sunward "rubble fountain," interpreted as ongoing ejection of surface material.
• Color Transition (MJD > 60870): The coma color shifts to near-solar, $(c-o) \approx 0.3$, indicating the emergence of optically bright icy grains. This transition is temporally correlated with the development of a prominent anti-solar tail.

The absolute magnitude curve $H(t)$ exhibits a marked slope change near MJD 60890 at $r \sim 3.3$ au, transitioning from $-0.035$ mag/day to $-0.014$ mag/day. In terms of coma cross section, this corresponds to a steep dependence on heliocentric distance ($r^{-3.9}$) prior to the break, flattening to $r^{-1.2}$ thereafter. The authors interpret this as a shift in the dominant coma constituents and optical depth effects, with small icy grains increasingly contributing to the observed brightness and being efficiently accelerated anti-sunward.

Figure 3: Light curve of 3I/ATLAS in a 6$\times$6\arcsec\ aperture, showing observed magnitudes (left) and absolute $H$ magnitudes corrected for geometry and phase (right).

Statistical Significance and Data Reliability

The early detections are statistically robust, with false alarm probabilities quantified via pixel brightness distributions in stacked images. The uncertainties reported for the photometry are reliable indicators of both measurement error and detection significance. The paper notes potential calibration issues for the Teide $w$ band data due to instrumental and reduction pipeline complexities, but provides these measurements for completeness and future refinement.

The authors highlight a discrepancy between their early $H$ magnitudes and those reported by ZTF, attributing this to differences in aperture definition and lack of uncertainty reporting in the latter. This underscores the necessity of standardized photometric practices for meaningful intercomparison across facilities.

Implications for Interstellar Object Populations

The photometric evolution of 3I/ATLAS provides insights into the physical processes governing interstellar comet activity. The observed color transition and slope break in the light curve suggest a compositional stratification, with surface-processed material giving way to volatile-rich grains as solar heating intensifies. The lack of detected interstellar objects in the size range between 1I/'Oumuamua and 2I/Borisov/3I/ATLAS remains an open question, with ATLAS's detection cross section implying that such objects should be observable if their population distribution mirrors that of solar system small bodies.

The release of well-calibrated, aperture-defined photometry enables synthesis with other datasets and facilitates the development of physical models for interstellar comet evolution. The ATLAS survey's systematic approach and public data availability set a benchmark for future interstellar object studies.

Conclusion

This paper delivers a detailed photometric record of 3I/ATLAS, capturing its color evolution, coma development, and activity transitions as it traversed the inner solar system. The robust methodology and transparent data release support cross-instrument analysis and physical modeling. The findings highlight the dynamic nature of interstellar cometary activity and provide constraints on the compositional and physical processes at play. Future work integrating ATLAS data with complementary observations will further elucidate the properties and origins of interstellar small bodies.