Electronic Telegram No. 5619 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/2013 C2 (TENAGRA) This comet (cf. CBET 3417) has undergone at least one recent outburst in brightness at high heliocentric distance. B. Godson, Astronomy and Astrophysics Group, University of Warwick, first reported the discovery of an outburst in activity when C/2013 C2 was observed serendipitously by the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, MNRAS 511, 2405) as part of automated follow-up of the gamma-ray burst GRB 251005B (cf. website URL https://gcn.nasa.gov/circulars/events/grb-251005b) on 2025 Oct. 5.84 UT, when it appeared as a bright optical source (AB magnitude 18.6) that was initially believed to be related to the GRB. Subsequent investigation showed it to be coincident with the position of C/2013 C2, which along with two other detections later that night ruled out an association with the GRB and prompted us to a search of data from previous nights. This comet had no reported observations since CCD astrometry by W. Hasubick with his 0.44-m f/4.6 reflector at Buchloe, Germany, when the comet was measured at mag 19.1-19.6. The GOTO astrometry is tabulated below, and Godson notes that the comet appears point-like without evidence of tail or coma. The last non-detection at the comet's position made by GOTO was on 2025 Sept. 16, with a limiting AB magnitude of 19.5 (3-sigma). 2025 UT R.A. (2000) Decl. Mag. Oct. 3.90174 19 45 57.37 - 2 54 13.4 18.7 4.43608 19 45 57.32 - 2 54 44.6 18.6 4.89919 19 45 57.53 - 2 55 12.1 18.6 5.44873 19 45 57.65 - 2 55 45.2 18.8 5.84478 19 45 57.89 - 2 56 07.3 18.6 5.89220 19 45 57.96 - 2 56 09.6 18.6 5.93942 19 45 58.01 - 2 56 12.5 18.5 After being alerted by Godson about the recent outburst, S. Deen (Simi Valley, CA, USA) searched archival data, noting that C/2013 C2 is invisible on CCD images taken on 2025 Sept. 28.038 UT with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program (limiting r mag > 20.0), but that the comet is obvious in ATLAS images taken during 2025 Sept. 30.768-30.780 UT with a 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, at r = 14.882 AU with an apparent magnitude of about r = 19.2. Also, ATLAS images obtained at Teide during 2025 Sept. 28.915-28.996 and 29.842-29.934 (each with limiting w magnitudes 19.7-20.0 as reported by ATLAS) were unavailable to Deen. Additional searches by Deen yielded r-band images of the comet taken with the 8.1-m "Gemini South" telescope at Cerro Pachon, Chile, by C. Schambeau et al. on 2022 Apr. 2 (when r = 12.3 AU) and 2022 Oct. 29 (r = 12.75 AU) as part of a survey of active centaur-type minor planets; the comet was then at magnitudes 21.6 and 22.0, respectively (though the latter estimate is much less certain because it's very involved with a star of magnitude r = 17.4 in almost every image). Further searching by Deen revealed more observations of C/2013 C2 in 2021, with ATLAS images taken with a 0.5-m f/2 Schmidt reflector at Mauna Loa, Hawaii, on 2021 Aug. 22 showing the comet at magnitude o = 18.9. CCD images taken on 2021 Sept. 11 with the 1.2-m Schmidt telescope at Palomar, in the course of the "Zwicky Transient Facility" survey, show that the comet had faded to mag g = 20.7; all of the Sept. 2021 observations tabulated below are from Palomar. An image taken with the 8.1-m "Gemini North" telescope at Mauna Kea on 2021 Aug. 10 shows the comet at r magnitude about 21.0. Below are Deen's astrometric measures (including his own observations taken with a 0.43-m reflector at Rio Hurtado, Chile, on 2025 Oct. 7). 2021 UT R.A. (2000) Decl. Mag. Aug. 10.32496 17 51 10.39 -13 48 52.1 21.0 10.33269 17 51 10.32 -13 48 52.2 22.34480 17 50 04.56 -13 50 41.0 19.2 22.34896 17 50 04.56 -13 50 41.6 18.7 22.35586 17 50 04.59 -13 50 40.6 18.7 Sept. 3.17122 17 49 45.63 -13 53 33.9 20.7 5.17113 17 49 47.05 -13 54 07.2 20.7 7.14955 17 49 49.88 -13 54 41.7 21.1 9.14730 17 49 53.95 -13 55 16.8 20.5 11.14893 17 49 59.49 -13 55 53.3 20.8 2022 UT R.A. (2000) Decl. Mag. Apr. 2.33498 18 47 36.34 -11 59 13.3 21.6 2.35004 18 47 36.41 -11 59 12.0 Oct. 29.02460 18 30 38.17 -11 14 41.0 22.0 2025 UT R.A. (2000) Decl. Mag. Sep. 30.76779 19 45 58.22 - 2 51 04.5 18.9 30.77096 19 45 58.19 - 2 51 05.3 18.6 30.78053 19 45 58.26 - 2 51 05.0 18.3 Oct. 7.01107 19 45 58.80 - 2 57 14.0 18.8 7.01807 19 45 58.84 - 2 57 14.4 7.02507 19 45 58.80 - 2 57 14.6 F. D. Romanov (Yuzhno-Morskoy, Nakhodka, Russia) reports that eight 60-s stacked images obtained remotely of comet C/2013 C2 on 2025 Oct. 12.86 UT using the 2.0-m f/10 Ritchey-Chretien Liverpool Telescope at La Palma (Spain) show a condensed coma of size 0'.1, with Sloan-filter magnitudes r' = 18.9, g' = 19.6, i' = 18.7. The observations from 2021 to 2025 had residuals within 1" from the orbit published on NK 3833. The following orbital elements by S. Nakano (Central Bureau) are from 504 observations spanning 2013 Feb. 14-2025 Oct. 7 (mean residual 0".5). The comet passed 2.45 AU from Saturn in 1955 July. Epoch = 2015 Sept.15.0 TT T = 2015 Aug. 30.30384 TT Peri. = 308.79122 e = 0.4314479 Node = 247.52494 2000.0 q = 9.1309479 AU Incl. = 21.33908 a = 16.0600015 AU n = 0.01531390 P = 64.36 years Epoch = 2079 Feb. 11.0 TT T = 2079 Feb. 5.56690 TT Peri. = 309.14840 e = 0.4286476 Node = 247.49245 2000.0 q = 9.1333822 AU Incl. = 21.34486 a = 15.9855508 AU n = 0.01542100 P = 63.91 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 5.0 and 2.5n = 7.5, with the H being only about 1.5 mag brighter than a 2014 fit. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 10 12 19 46.11 -03 01.9 14.716 14.904 98.9 3.8 19.6 2025 10 22 19 46.71 -03 10.4 14.897 14.923 89.6 3.8 19.7 2025 11 01 19 47.74 -03 17.3 15.077 14.943 80.3 3.8 19.7 2025 11 11 19 49.17 -03 22.5 15.253 14.962 71.1 3.6 19.7 2025 11 21 19 50.98 -03 25.6 15.420 14.981 62.0 3.3 19.8 2025 12 01 19 53.12 -03 26.7 15.573 15.000 53.0 3.0 19.8 2025 12 11 19 55.54 -03 25.4 15.710 15.020 44.2 2.6 19.8 2026 03 21 20 22.50 -01 25.0 15.770 15.212 54.4 3.1 19.9 2026 03 31 20 24.23 -01 07.0 15.655 15.231 63.2 3.4 19.8 2026 04 10 20 25.60 -00 49.2 15.527 15.250 72.1 3.6 19.8 2026 04 20 20 26.57 -00 32.1 15.391 15.269 81.1 3.7 19.8 2026 04 30 20 27.14 -00 15.9 15.251 15.288 90.2 3.8 19.8 2026 05 10 20 27.29 -00 01.1 15.112 15.307 99.3 3.7 19.8 2026 05 20 20 27.03 +00 12.0 14.977 15.326 108.4 3.6 19.8 2026 05 30 20 26.37 +00 23.0 14.851 15.345 117.4 3.4 19.8 2026 06 09 20 25.34 +00 31.7 14.739 15.364 126.4 3.0 19.7 2026 06 19 20 23.98 +00 37.9 14.645 15.383 135.3 2.7 19.7 2026 06 29 20 22.35 +00 41.4 14.571 15.402 143.7 2.2 19.7 2026 07 09 20 20.50 +00 42.2 14.520 15.421 151.4 1.8 19.7 2026 07 19 20 18.53 +00 40.4 14.496 15.440 157.4 1.4 19.7 2026 07 29 20 16.50 +00 36.1 14.500 15.458 160.2 1.3 19.7 2026 08 08 20 14.50 +00 29.5 14.531 15.477 158.3 1.4 19.7 2026 08 18 20 12.63 +00 21.0 14.590 15.496 152.7 1.7 19.7 2026 08 28 20 10.94 +00 10.9 14.675 15.515 145.2 2.1 19.8 2026 09 07 20 09.53 -00 00.1 14.785 15.534 136.7 2.5 19.8 2026 09 17 20 08.44 -00 11.7 14.916 15.553 127.8 2.9 19.8 2026 09 27 20 07.72 -00 23.4 15.066 15.571 118.7 3.2 19.8 2026 10 07 20 07.40 -00 34.6 15.229 15.590 109.4 3.5 19.9 2026 10 17 20 07.50 -00 45.0 15.403 15.609 100.1 3.6 19.9 2026 10 27 20 08.02 -00 54.1 15.582 15.627 90.8 3.6 19.9 2026 11 06 20 08.96 -01 01.5 15.762 15.646 81.5 3.6 19.9 2026 11 16 20 10.28 -01 07.1 15.937 15.665 72.3 3.4 20.0 2026 11 26 20 11.97 -01 10.5 16.104 15.683 63.2 3.2 20.0 2026 12 06 20 13.97 -01 11.6 16.258 15.702 54.2 2.9 20.0 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 October 13 (CBET 5619) Daniel W. E. Green