Electronic Telegram No. 5631 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2025 V1 (BORISOV) Gennadii Vladimirovich Borisov, Nauchnij, Crimea, reports his discovery of another comet, which appeared with a diffuse coma of diameter about 25" with no tail, on CCD images obtained on Nov. 2 with a 0.50-m f/1.9 reflector located at his MARGO Observatory. He noted that the total green magnitude was 12.1 as measured inside a circular aperture of size 0'.5. The discovery astrometry is tabulated below. 2025 UT R.A. (2000) Decl. Mag. Observer Nov. 2.10833 12 06 21.52 - 5 35 30.7 12.0 Borisov 2.11762 12 06 26.83 - 5 34 58.6 12.1 " 2.12552 12 06 31.27 - 5 34 31.3 12.2 " S. Korotkiy (Observatory Ka-Dar), N. Dudin, and E. Shevtsova obtained nine confirming unfiltered 20-s CCD images at the Astrovert Astrofarm (Nizhny Arkhyz, Russia) on Nov. 3.09 UT using a Canon 135-mm-f.l. f/2 telephoto lens, at a scale of 14"/pixel; they report the comet at mag 11.7 (Tycho 2-catalogue V-magnitudes) with the coma's head being 126" x 84" in size, elongated in the direction of motion (p.a. 69 degrees), on nine stacked exposuers. After the comet was posted to the Minor Planet Center's PCCP webpage, other observers also reported on the cometary appearance. Hirohisa Sato writes that thirty stacked 30-s CCD exposures taken on Nov. 3.83 UT with a 0.25-m f/4 reflector at Sukagawa, Japan, show a strongly condensed coma about 1'.2 in diameter with no tail; the total mag was 11.6. K. Kadota (Ageo, Japan) reports that CCD images obtained with a 0.25-m f/5 reflector on Nov. 3.83 show a strong central condensation and a coma of diameter 2'.4 (total magnitude 11.4), with no tail. K. Yoshimoto (Yamaguchi, Japan) notes that thirty 15-s stacked CMOS exposures taken with a 0.3-m f/4 reflector on Nov. 3.84-3.85 show a condensed coma 3'.2 in diameter and no tail; the total magnitude was measured as 11.4. N. Dudin obtained twenty-five 20-s CCD images with a 0.20-m f/3 telescope located at the Astrovert Astrofarm (Nizhny Arkhyz, Russia) on Nov. 4.1 UT that show a coma of size 72". M. Jaeger reports that twenty-five stacked 20-s CMOS images obtained on Nov. 4.18 with a 0.28-m f/2.2 reflector (+ green filter) at Martinsberg, Oed, Austria, show a highly condensed coma of size 220" with a total mag of 10.4, and no visible tail. M. Masek (Institute of Physics, Czech Academy of Sciences) writes that multi-filter CCD images obtained on Nov. 4.25 with a 135-mm-f.l. f/2 telephoto lens at the Cherenkov Telescope Array site on La Palma while the comet was at altitude about 15 degrees show the magnitude to be V = 10.9 with a coma diameter of 4'.6 (and R = 12.5 with a 1'.8 coma using a red filter). Ten stacked 15-s CMOS exposures taken remotely by Hidetaka Sato (Bunkyo-ku, Tokyo, Japan) using a 0.25-m f/3.8 astrograph located at Rio Hurtado, Chile, on Nov. 4.35-4.37 show a moderately condensed comet with an outer coma 1'.2 in diameter and no tail; the magnitude was 12.3 as measured within a circular aperture of radius 38".1. CMOS exposures taken on Nov. 4.36 by T. Prystavski (Lviv, Ukraine) with an iTelescope 0.25-m f/3.8 Newtonian reflector located at Rio Hurtado Valley in twilight show a coma of diameter about 1'.7 of total mag about 11.2, with no tail. The available astrometry appears on MPEC 2025-V40 and includes pre- discovery observations obtained with the Palomar 1.2-m Schmidt telescope and reported by Q.-z. Ye after the discovery by Borisov; the magnitude was given as r = 17.5-17.7 on Oct. 29.55 UT and r = 15.4 on Nov. 1.56, suggesting a rapid rise in brightness. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 89 observations spanning 2025 Oct. 29-Nov. 5 (mean residual 0".7). T = 2025 Nov. 16.58451 TT Peri. = 47.59975 Node = 91.72849 2000.0 q = 0.4630433 AU Incl. = 112.71510 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 13.0 and 2.5n = 8. Given the apparent rapid rise in brightness, the predicted magnitudes should be considered highly uncertain. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 10 12 10 19.93 -16 53.1 1.405 0.945 42.2 45.2 13.5 2025 10 17 10 34.90 -15 19.0 1.264 0.856 42.4 51.8 13.0 2025 10 22 10 54.01 -13 15.3 1.120 0.767 42.0 60.3 12.3 2025 10 27 11 19.76 -10 27.4 0.975 0.681 40.5 71.2 11.6 2025 11 01 11 56.25 -06 36.7 0.840 0.602 37.2 85.3 10.9 2025 11 06 12 48.94 -01 33.5 0.734 0.533 31.8 101.8 10.1 2025 11 11 13 59.77 +03 51.7 0.688 0.484 26.5 114.2 9.7 2025 11 16 15 16.74 +07 19.4 0.727 0.463 26.0 110.4 9.6 2025 11 21 16 20.22 +07 37.6 0.842 0.476 28.8 92.9 10.0 2025 11 26 17 04.13 +05 56.9 0.998 0.519 30.3 73.7 10.7 2025 12 01 17 33.52 +03 37.8 1.166 0.584 30.0 57.7 11.5 2025 12 06 17 54.09 +01 19.1 1.332 0.662 28.5 45.3 12.2 2025 12 11 18 09.46 -00 46.8 1.489 0.747 26.3 35.7 12.8 2025 12 16 18 21.66 -02 37.2 1.635 0.835 23.7 28.3 13.4 2025 12 21 18 31.83 -04 13.1 1.771 0.924 21.1 22.6 14.0 2025 12 26 18 40.60 -05 36.6 1.895 1.013 18.6 18.0 14.4 2025 12 31 18 48.36 -06 49.4 2.009 1.102 16.4 14.6 14.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 November 6 (CBET 5631) Daniel W. E. Green