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Near-Discovery SOAR Photometry of the Third Interstellar Object: 3I/ATLAS

Tessa T. Frincke, Atsuhiro Yaginuma, John W. Noonan, Henry H. Hsieh, Darryl Z. Seligman, Carrie E. Holt, Jay Strader, Thomas Do, Peter Craig, Isabella Molina

TL;DR

This study reports near-discovery photometry of the interstellar object 3I/ATLAS using the SOAR telescope and the Goodman HTS, augmenting the object’s composite light curve with 28 $r'$-band measurements across UT 2025 July 3, 9, and 10. The authors implement rigorous data reduction, aperture photometry, and contamination screening to distinguish genuine brightness changes from stellar blending, and they compile image stacks to assess coma presence. Their analysis finds no significant long-term variability and identifies apparent July 9–10 brightenings as artifacts from nearby stars rather than intrinsic activity, with mean magnitudes of 18.14, 17.55, and 17.54 on the three nights. They conclude that the observed activity is consistent with near-discovery levels and that rotation cannot be constrained given ongoing coma, highlighting the need for homogeneous, longer-baseline data and future precovery efforts to refine the secular light curve and constrain the nucleus’s properties.

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.

Near-Discovery SOAR Photometry of the Third Interstellar Object: 3I/ATLAS

TL;DR

This study reports near-discovery photometry of the interstellar object 3I/ATLAS using the SOAR telescope and the Goodman HTS, augmenting the object’s composite light curve with 28 -band measurements across UT 2025 July 3, 9, and 10. The authors implement rigorous data reduction, aperture photometry, and contamination screening to distinguish genuine brightness changes from stellar blending, and they compile image stacks to assess coma presence. Their analysis finds no significant long-term variability and identifies apparent July 9–10 brightenings as artifacts from nearby stars rather than intrinsic activity, with mean magnitudes of 18.14, 17.55, and 17.54 on the three nights. They conclude that the observed activity is consistent with near-discovery levels and that rotation cannot be constrained given ongoing coma, highlighting the need for homogeneous, longer-baseline data and future precovery efforts to refine the secular light curve and constrain the nucleus’s properties.

Abstract

3I/ATLAS was discovered on UT 2025 July 1 and joins a limited but growing population of detected 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 -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 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.

Paper Structure

This paper contains 13 sections, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. SOAR Observations of 3I/ATLAS
  3. Goodman HTS Imaging
  4. Imaging Data Reduction
  5. Photometry
  6. Aperture Experiments
  7. Stellar Contamination Search
  8. Background Subtraction
  9. Image Stacking
  10. Composite Light Curve
  11. Discussion
  12. Acknowledgments
  13. Data and Software Availability

Figures (8)

  • Figure 1: Reduced and rotated images from observations on the SOAR telescope with the Goodman HTS instrument of 3I/ATLAS on UT 2025 July 3. Each panel is a single 90 s exposure taken in r'-band and 3I/ATLAS is identified within the red circle. Arrows indicate directions of North (N) and East (E). Associated animation is available online.
  • Figure 2: Reduced and rotated images from observations of 3I/ATLAS obtained with the SOAR/Goodman HTS on UT 2025 July 9. Each panel is a single 30 s exposure taken in $r'$-band and 3I/ATLAS is identified within the red circle. Arrows indicate directions of North (N) and East (E). Associated animation is available online.
  • Figure 3: Reduced and rotated images from observations on the SOAR/Goodman HTS of 3I/ATLAS on UT 2025 July 10. Each panel is a single 30 s exposure taken in $r'$-band and 3I/ATLAS is identified within the red circle. Arrows indicate directions of North (N) and East (E). Associated animation is available online.
  • Figure 4: Series of zoomed in images from observations of 3I/ATLAS on UT 2025 July 9. Each panel is a single 30.0 s exposure taken in $r'$-band and 3I/ATLAS is identified within the red circle.
  • Figure 5: Comparison of the variance of the magnitudes measured as a function of the different aperture sizes used during photometry performed on each set of SOAR observation of 3I/ATLAS. The teal, purple, and orange vertical lines correspond to the aperture size chosen for final photometry of 3I/ATLAS on UT 2025 July 3, 9, and 10, respectively.
  • ...and 3 more figures