Large Apparent Outburst of Comet 220P/McNaught
ATel #17829; Michael S. P. Kelley (Univ. Maryland), Quanzhi Ye (Univ. Maryland), Carrie Holt (Las Cumbres Observatory), Josiah Purdum (Caltech Optical Observatories), Colin Snodgrass (Univ. Edinburgh), on behalf of the Zwicky Transient Facility Collaboration, and the LCO Outbursting Objects Key Project
on 3 Jun 2026; 16:59 UT
Credential Certification: Carrie Holt (cholt@lco.global)
We report the discovery of a large apparent outburst of comet 220P/McNaught with the comet near 1.6 au from the Sun. The event was first identified in two Zwicky Transient Facility (ZTF; Bellm et al. 2019, PASP 131, a8002) images taken 2026 May 31 11:18 UTC at Palomar Observatory. Both images were saturated in the core, but only a single pixel was saturated in the second image and we measured r=11.05+/-0.06 mag (PS1 magnitude system, 7" radius aperture, unless otherwise noted). The uncertainty is based on the calibration uncertainty and the image PSF, which suggests the saturated pixel contributes no more than ~10% to the total brightness. The comet was previously observed at May 26 11:22 UTC, measuring r=17.39+/-0.04 mag, yielding an outburst strength of -6.3 mag. Additional photometry is reported below. The ejecta was nearly symmetric, distributed out to 35" from the presumed nucleus location, with a total brightness of 9.11+/-0.04 mag.
A review of Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018, PASP 130, 064505) images taken from Mauna Loa, Hawaii (USA), Sutherland (South Africa), and Rio Hurtado (Chile) provide a more precise timing for the onset. In images obtained from the ATLAS Forced Photometry Service, the outburst started between May 30 14:05 and May 31 03:37 UTC. When compared to the ZTF data, the measurement on May 31, r=12.90+/-0.02 mag, indicates the brightening phase was ongoing at the time.
Targeted follow-up images taken with the Las Cumbres Observatory telescope network were obtained at Jun 02 04:14 UTC in the SDSS r' filter from a 1-m telescope at Sutherland, South Africa. Additional structures have developed within the ejecta, likely due to dust dynamical evolution, including a parabolic envelope and a linear feature along the anti-sunward direction.
Additional follow-up observations with the SED Machine low-resolution (R~100) integral field spectrograph at Palomar Observatory taken at Jun 2 11:05 UTC showed significant CN, C2, and C3 emissions in the visible wavelengths in a 5" radius aperture. Therefore, C2 gas contributed to the extended ejecta and total brightness above.
date source flt r unc
---------------- ------ --- ----- ----
2026-05-23 11:22 ZTF zr 17.36 0.04
2026-05-24 11:20 ZTF zr 17.33 0.04
2026-05-25 03:18 ATLAS o 17.36 0.08
2026-05-25 11:17 ZTF zr 17.39 0.04
2026-05-25 11:21 ZTF zr 17.40 0.04
2026-05-26 11:22 ZTF zr 17.39 0.04
2026-05-30 14:05 ATLAS o 17.38 0.02
2026-05-31 03:37 ATLAS o 12.90 0.02
2026-05-31 11:18 ZTF zr 11.05 0.06
2026-06-01 11:15 ZTF zr 10.99 0.03
--------------------------------------
flt: observed filter
r : PS1 r-band apparent magnitude
unc : uncertainty on r
Based on observations obtained with the Samuel Oschin Telescope 48-inch and 60-inch Telescopes at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, Weizmann Institute for Science, Oskar Klein Center at Stockholm University, University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, TANGO Consortium of Taiwan, University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.
This work uses data from the University of Hawaii's ATLAS project, funded through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575, with contributions from the Queen's University Belfast, STScI, the South African Astronomical Observatory, and the Millennium Institute of Astrophysics, Chile, the Instituto de Astrofisica de Canarias (Spain) and the University of Oxford.
This work makes use of observations from the Las Cumbres Observatory global telescope network.
