Apparent Outbursts of Comet C/2025 K1 (ATLAS)

ATel #17482; Michael S. P. Kelley (Univ. Maryland), Carrie E. Holt (Las Cumbres Observatory), Quanzhi Ye (Univ. Maryland/Boston Univ.), Dennis Bodewits (Auburn Univ) on behalf of the LCO Outbursting Objects Key Project, and the Zwicky Transient Facility Collaboration
on 6 Nov 2025; 20:44 UT
Credential Certification: Carrie Holt (cholt@lco.global)

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We report on Las Cumbres Observatory (LCO) and Zwicky Transient Facility (ZTF; Bellm et al. 2019, PASP 131, a8002) observations of two apparent outbursts of comet C/2025 K1 (ATLAS). The larger of the two events had a relative strength of ~-0.9 mag, based on the comparison of photometry measured within a 5 arcsec radius. The comet had an apparent brightness of 11.95 ± 0.02 mag in ZTF r-band images obtained 2025 November 2 at 13:06 UTC in 2.2 arcsec seeing conditions. Independent images were obtained by an LCO 1-m telescope at McDonald Observatory as part of the LCO Outbursting Objects Key (LOOK) Project (Lister et al. 2022, PSJ 3, 173) 2025 November 4 at 12:16 UTC. Resulting photometry shows that the comet brightened to r = 11.08 ± 0.03 mag in 2.3 arcsec seeing. On November 4, the comet was 0.792 au from the Sun, 0.739 au from the Earth, and at a phase angle of 80.7 degrees.

A comparison of LCO images reveals the morphology of the comet varied. The November 4 image shows evidence for material distributed in a near-parabolic shape, with its vertex near the presumed nucleus of the comet. The material is readily seen up to 10 arcsec from the nucleus, indicating that our 5 arcsec photometric apertures do not encompass all of the ejecta. In contrast, an LCO image taken November 5 seems to lack this feature and has a brighter tail. Together, these observations are consistent with outburst material ejected from the nucleus over a broad opening angle that was quickly accelerated tailward.

In addition to the above observations, ZTF's Twilight Survey imaged the comet nightly from 2025 October 30 to 2025 November 2, with a subtle variation in its apparent brightness between October 31 and November 1, suggesting an outburst of relative strength ~-0.3 mag. However, scale and brightness variations due to the changing geometric circumstances need to be addressed before a confident morphological assessment of such a small event can be made. All photometry is reported in the following table using 5 arcsec radii and the PS1 magnitude system.