Apparent Outburst of Comet 40P/Väisälä
ATel #17621; Michael S. P. Kelley (Univ. Maryland), Quanzhi Ye (Univ. Maryland), Tim Lister (Las Cumbres Observatory), on behalf of the Zwicky Transient Facility Collaboration, and the LCO Outbursting Objects Key Project
on 21 Jan 2026; 20:04 UT
Credential Certification: Tim Lister (tlister@lco.global)
We report on Zwicky Transient Facility (ZTF; Bellm et al. 2019, PASP 131, a8002) observations of an apparent outburst of comet 40P/Väisälä. The comet brightened by â2.7 mag in a 5" radius aperture over the 24 hr period between 2026 Jan 19.567 and Jan 20.567 UTC, at a heliocentric distance of 1.95 au, a geocentric distance of 2.26 au, and a phase angle of 26 deg. In r-band images taken 2026 Jan 19.507 and 19.567 UTC, the comet was 17.05 and 17.04 mag, respectively, in a 5" radius aperture (PS1 magnitude system; Tonry et al. 2012, ApJ 750, 99). On the next night, at 2026 Jan 20.567 UTC, the comet had brightened to r=14.39 mag. The appearance also changed between the two nights, from a typical compact coma and tail, to a morphology strongly dominated by a compact source centered on the presumed comet nucleus. The compact source had a FWHM of 3.1" in 1.8" seeing.
Follow-up observations were scheduled using the Las Cumbres Observatory (LCO) network of 1-m telescopes as part of the LCO Outbursting Objects Key (LOOK) Project. Images taken at 2026 Jan 21.232 UTC with a LCO 1-m telescope at Teide Observatory (MPC site code Z24) confirmed the outburst: g=15.08 ± 0.03, r=14.54 ± 0.03 mag (5" radius, PS1 system). The ejecta was more extended than in the ZTF images, presumably due to the expansion of the ejecta, with an average FWHM of 4.3" in 2.6" seeing. A second set of LCO observations with a 1-m telescope at McDonald Observatory (MPC site code V37) taken 2026 Jan 21.483 showed the brightness in a 5" radius continued to decline: g=15.41 ± 0.02 and r=14.94 ± 0.01 mag. The ejecta appeared to have an average FWHM of 5.6â in poor seeing conditions of 3.5â.
Based on observations obtained with the Samuel Oschin Telescope 48-inch Telescope 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 makes use of observations from the Las Cumbres Observatory global telescope network.
