HATPI Preperihelion Time-series Photometry of the Interstellar Comet 3I/ATLAS

Hartman, JD; Bakos, GÁ; Jordán, A; Thiele, S; Csubry, Z; Talens, GJ; Bódi, A; Pigai, S; Domsa, I; Keyes, A; Suc, V; Gaitan, A; Thibault, A

Journal article

HATPI Preperihelion Time-series Photometry of the Interstellar Comet 3I/ATLAS

Abstract:: The Hungarian-made Automated Telescope PI Steradians (HATPI) is a recently commissioned time-domain facility at Las Campanas Observatory, Chile, that uses 64 wide-angle, 9.6 cm diameter lenses and back-illuminated CCDs, yielding a mosaic field-of-view of 7100 square arcdegrees, observing the night sky at a cadence of 45 s and a spatial scale of 19 .″ 7 pixel−1. In this paper, we present moving object time-series photometry with this facility, focusing on the interstellar comet 3I/ATLAS, which was first robustly recovered by HATPI on the night of 2025 July 2 (one night after its discovery) at a Gaia G-band magnitude of G = 17.796 ± 0.082 mag (±0.030 mag systematic uncertainty). The comet then increased in brightness to G = 14.071 ± 0.073 mag ± 0.030 mag by 2025 September 13, after which it became unobservable by HATPI as it approached perihelion. Before 3I/ATLAS achieved a brightness of G = 16.396 ± 0.029 mag ± 0.030 mag on 2025 August 6, it could be detected when stacking all HATPI observations from a single night, while after this date it is sufficiently bright to detect in individual 45 s exposures. We do not detect evidence for significant short-time-scale variations in the brightness of 3I/ATLAS after August 6. Compared to other light curves in the literature, the HATPI photometry exhibits a somewhat steeper rise in brightness with decreasing heliocentric distance, rH. The HATPI magnitudes are well-fit as a power law function of rH, with an exponential index of n = 5.167 ± 0.095, over the range 2.14 au

Publication status:: Published

Peer review status:: Peer reviewed

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APA Style

Hartman, J. D., Bakos, G. Á., Jordán, A., Thiele, S., Csubry, Z., Talens, G. J., Bódi, A., Pigai, S., Domsa, I., Keyes, A., Suc, V., Gaitan, A., & Thibault, A. (2026). HATPI Preperihelion Time-series Photometry of the Interstellar Comet 3I/ATLAS. The Astronomical Journal, 171(5), 270.

MLA Style

Hartman, JD, et al. “HATPI Preperihelion Time-Series Photometry of the Interstellar Comet 3I/ATLAS.” The Astronomical Journal, vol. 171, no. 5, 2026, p. 270.

Chicago Style

Hartman, JD, GÁ Bakos, A Jordán, et al. 2026. “HATPI Preperihelion Time-Series Photometry of the Interstellar Comet 3I/ATLAS.” The Astronomical Journal 171 (5): 270.
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