Near-Discovery SOAR Photometry of the Third Interstellar Object: 3I/ATLAS (2509.02813v1)

Abstract: 3I/ATLAS was discovered on UT 2025 July 1 and joins a limited but growing population of detected $\sim10^2-10^3$ m scale interstellar objects. In this paper we report photometric observations of 3I/ATLAS from the nights of UT 2025 July 3, UT 2025 July 9, and UT 2025 July 10 obtained with the Southern Astrophysical Research Telescope (SOAR). The photometric observations are taken with the Goodman High Throughput Spectrograph (HTS) in the $r'$-band. These data provide 28 photometric data points to the rapidly growing composite light curve of 3I/ATLAS. They reveal that the object did not exhibit obvious long-term variability in its brightness when these observations were taken. These observations appear to have captured two moderate and independent brightening events on UT 2025 July 9, and UT 2025 July 10. However, we perform a series of stellar contamination, stacking, and aperture experiments that demonstrate that the increases in brightness by $\sim0.8$ magnitudes appear to be a result of poor seeing and stellar contamination by close-proximity field stars. We report the mean brightnesses of 3I/ATLAS on each night of magnitude 18.14, 17.55, and 17.54 for UT 2025 July 3, 9, and 10, respectively. Moreover, the presence of cometary activity in extant images obtained contemporaneously with these data precludes them from revealing insights into the rotation of the nucleus. We conclude that the activity of 3I/ATLAS on UT 2025 July 9 and UT July 10 was consistent with the near-discovery activity levels, with no obvious outburst activity.

• The paper presents 28 new photometric data points of 3I/ATLAS that confirm steady cometary activity during the early observation phase.
• The paper employs rigorous contamination mitigation via aperture photometry and radial profile analysis to ensure data accuracy under suboptimal conditions.
• The paper integrates SOAR data with broader observations, supporting the interpretation of sustained hypervolatile-driven activity at large heliocentric distances.

Photometric Characterization of 3I/ATLAS: Early SOAR Observations

Introduction

The discovery of 3I/ATLAS (C/2025 N1) by the ATLAS survey on UT 2025 July 1 marks the identification of the third macroscopic interstellar object traversing the Solar System. This work presents a detailed photometric analysis of 3I/ATLAS using data obtained with the Goodman High Throughput Spectrograph (HTS) on the SOAR telescope during the nights of UT 2025 July 3, 9, and 10. The paper addresses the object's brightness variability, the impact of stellar contamination, and the implications for understanding interstellar cometary activity. The observations fill a critical gap in the composite light curve of 3I/ATLAS, providing 28 new photometric data points during the near-discovery phase.

Observational Campaign and Data Reduction

Observations were conducted with the SOAR 4.1-meter telescope using the Goodman HTS Red Camera, with images binned $2 \times 2$ to achieve a pixel scale of $0.3''$/pixel. The campaign targeted 3I/ATLAS in the $r'$-band, with exposure times of 90 s on July 3 and 30 s on July 9 and 10. The observing conditions varied, with significant moonlight contamination on July 9 and 10 due to near-full moon phases.

Figure 1: Reduced and rotated $r'$-band images of 3I/ATLAS from SOAR on UT 2025 July 3, with the object identified in the red circle.

Figure 2: Reduced and rotated $r'$-band images of 3I/ATLAS from SOAR on UT 2025 July 9, highlighting the object's position.

Figure 3: Reduced and rotated $r'$-band images of 3I/ATLAS from SOAR on UT 2025 July 10, showing the target in the red circle.

Standard CCD reduction procedures were applied, including bias subtraction, flat-fielding, and trimming of overscan regions. Astrometry.net was used for WCS calibration, leveraging USNO-B, 2MASS, and Gaia DR2 catalogs for precise field registration.

Photometric Analysis and Contamination Mitigation

Aperture photometry was performed using Astropy's photutils, with aperture placement determined by JPL Horizons ephemerides. Zeropoint calibration utilized CALVIACat and the ATLAS-RefCat2 catalog. The analysis included systematic experiments with aperture sizes ranging from $1.2''$ to $3.0''$ to assess the impact of stellar contamination and optimize the signal-to-noise ratio.

Figure 4: Zoomed-in $r'$-band images of 3I/ATLAS from July 9, demonstrating the best seeing conditions and object identification.

Figure 5: Variance of measured magnitudes as a function of aperture size for each SOAR observation night, with vertical lines indicating the chosen aperture for final photometry.

The results indicate that aperture size did not significantly affect the scatter in measured brightness, justifying the use of the FWHM of the PSF for aperture selection. Visual inspection and radial profile analysis were employed to identify and reject contaminated photometric points, particularly those affected by background stars or poor seeing.

Light Curve Construction and Composite Analysis

The uncontaminated photometric data were used to construct a light curve for 3I/ATLAS across the three nights. The mean $r'$-band magnitudes were 18.14 (July 3), 17.55 (July 9), and 17.54 (July 10). The data revealed no significant long-term variability, and apparent brightening events were attributed to contamination rather than intrinsic activity.

Figure 6: Light curve of 3I/ATLAS from SOAR $r'$-band data, with markers indicating contamination by field stars and radial profile anomalies.

Composite image stacking was performed to enhance the detection of faint coma features and validate the identification of uncontaminated frames.

Figure 7: Rotated and stacked composite images of 3I/ATLAS and background stars from July 9 and 10, confirming the absence of contamination in selected frames.

The SOAR data were integrated into the broader composite light curve, which includes data from ATLAS, TRAPPIST, LCO, Faulkes, and other facilities.

Figure 8: Composite light curve of 3I/ATLAS, showing SOAR data alongside other ground-based observations and highlighting the consistency of brightness near discovery.

Discussion and Implications

The SOAR photometry confirms that 3I/ATLAS exhibited a flat light curve during the near-discovery phase, consistent with previous reports of sustained cometary activity and minimal brightness variation. The mean magnitudes align with those from other facilities, supporting the conclusion that the object's activity level remained stable through July 10. The data do not provide constraints on the nucleus's rotational period due to temporal sparseness and strong coma activity.

The secular light curve, extended by precovery observations from LSST and TESS, suggests that 3I/ATLAS's activity at heliocentric distances beyond 6 au is likely driven by hypervolatile ices. The SOAR data reinforce the hypothesis of sustained activity, with no evidence for outburst events during the observed window.

The paper highlights the importance of contamination mitigation in crowded fields and demonstrates robust photometric methodologies for faint, active interstellar objects. The results have implications for future surveys, such as LSST, which are expected to increase the detection rate of interstellar objects and enable more comprehensive characterization of their physical properties.

Future Prospects

Space-based follow-up, including potential in situ missions from Mars or Jupiter, could provide unprecedented insights into the composition and activity mechanisms of interstellar comets. The feasibility of such missions has been discussed in recent literature, with low $\Delta V$ trajectories identified for flybys of 3I/ATLAS. The continued development of all-sky surveys and rapid-response observational infrastructure will be critical for maximizing scientific return from future interstellar object discoveries.

Conclusion

The SOAR photometric campaign provides a valuable addition to the composite light curve of 3I/ATLAS, confirming stable activity and mean brightness consistent with other observations. The rigorous approach to contamination identification and photometric calibration sets a standard for future studies of faint, active interstellar objects. The results support the interpretation of sustained hypervolatile-driven activity and underscore the need for continued monitoring and rapid-response capabilities as the population of detected interstellar objects grows.