Apparent Outburst of Cometary Centaur C/2023 RS61 (PANSTARRS)
ATel #17360; Michael S. P. Kelley (Univ. Maryland), Helen Usher (The Open University), Megan E. Schwamb (Queen's University Belfast), Carrie E. Holt (Las Cumbres Observatory), on behalf of the Zwicky Transient Facility Collaboration, the LCO Outbursting Objects Key Project, and the Comet Chasers Project
on 26 Aug 2025; 19:59 UT
Credential Certification: Carrie Holt (cholt@lco.global)
We report on Zwicky Transient Facility (ZTF; Bellm et al. 2019, PASP 131, a8002) observations of an apparent outburst of cometary centaur C/2023 RS61 (PANSTARRS). The comet brightened by at least -2.2 mag in relative brightness over a period of 45 hours. It was initially undetected in two g-band images taken 2025 Aug 8.473 and 8.485 UTC with 5-sigma g-band lower limits of 20.6 mag. At 2025 Aug 10.370 UTC, the comet was detected with i = 17.90 ± 0.11 mag. In the next observations at 2025 Aug 11.352 and 11.471, the comet was slightly brighter at i=17.49 ± 0.11 mag and g=18.45 ± 0.07 mag, respectively. The comet's apparent brightness has remained near constant since, most recently measured to be g=18.41 ± 0.03 mag at 2025 Aug 25.478 UTC, and its morphology compact with a FWHM of 3.0 arcsec in 1.9 arcsec seeing. All photometry is reported for 5 arcsec radius apertures using the PS1 photometric system. At the time of the outburst, the comet was 8.98 au from the Sun and 9.06 au from the Earth, with a phase angle of 6.4 deg.
With a calculated color of g-i = 0.96 ± 0.13 mag from Aug 11, and g-r = 0.72 ± 0.04 mag measured on Aug 24, the object appears to be unusual for a cometary coma. Follow-up observations from a 1-m Las Cumbres Observatory telescope at McDonald Observatory and the 2-m Faulkes Telescope North confirm the colors: g-r = 0.73 ± 0.04 mag, r-i = 0.35 ± 0.04 mag (Aug 26.431 UTC); and g-r = 0.77 ± 0.03 mag, r-i = 0.36 ± 0.03 mag, i-z = 0.17 ± 0.03 mag (Aug 26.565), respectively.
In a study of 7300 color measurements of 225 comets with the ZTF, only two comets were measured to be redder than g-r = 0.65 mag (Kelley et al. 2024, 56th AAS Division for Planetary Sciences Meeting, #401.01). In addition, the outburst ejecta is redder than the comae of most active centaurs (e.g., Jewitt 2015, AJ 150, 201; Mazzotta Epifani et al. 2018, A&A 620, A93). Further characterization of this event and the object in general is encouraged. The next perihelion will occur in 2028 November.
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. This work is based on observations made with the MuSCAT3 instrument, developed by Astrobiology Center and under financial supports by JSPS KAKENHI (JP18H05439) and JST PRESTO (JPMJPR1775), at Faulkes Telescope North on Maui, HI, operated by the Las Cumbres Observatory.
The UK-based Comet Chasers project works with students and educators across the world, engaging and supporting them in making scientific observations (with the Las Cumbres Observatory global telescope network) of comets and other small bodies for researchers. The project is grateful for support from the Faulkes Telescope Project, the Open University, Cardiff University, the Royal Astronomical Society and the Science and Technology Facilities Council in the UK.
