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arXiv · 2509.02813

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
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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.

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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. 2025-09-02. Near-Discovery SOAR Photometry of the Third Interstellar Object: 3I/ATLAS. https://doi.org/10.1093/mnras%2Fstaf1994

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