Electronic Telegram No. 5284 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 P/2015 XG_422 (PANSTARRS) K. Ly (El Monte, CA, USA) writes that he has identified cometary activity of the presumed minor planet 2015 XG_422 (cf. MPS 1096514) in archival telescope imagery on numerous nights, beginning with two g-band 150-s CCD exposures taken on 2016 Mar. 5.56 UT with the "SkyMapper" 1.35-m f/4.8 reflector at Siding Spring, when there was a condensed "head" surrounded by a faint, 3"-diameter diffuse halo with no obvious tail; the distinctly fuzzy object had a total magnitude of g = 20.5 as measured in a 5".5 aperture. Earlier r- and g-band 100-s exposures taken with the same telescope on 2016 Feb. 28.53 show only asteroidal appearance at g = 21.0 and r = 20.5. Ly also identified cometary images of 2015 XG_422 in 150-s g-band CCD exposures taken on 2016 Mar. 7 and r-band exposures on Mar. 9 with the 8.2-m Subaru Telescope (+ Suprime-Cam) at Mauna Kea; the comet then showed a condensed coma of size 2".5 x 2".0 (with g = 20.2 and r = 19.7 as measured in a circular aperture of diameter 5".5) and a broad, 10"-long tail pointing toward p.a. 280-320 deg (the coma appeared elongated because it was streaked in the direction of the comet's on-sky motion in p.a. 315 deg). Ly also identified images of the comet in two V-band and two I-band 240-s exposures taken with the 1.0-m Las Cumbres Observatory telescope "B" at Cerro Tololo on 2016 Mar. 10.24, when there was a very faint, 9"-long straight tail pointing toward p.a. 280 deg (but visible in only the V-band exposures); the comet's head then appeared very condensed in all four exposures, with a size of 1".4 (full-width-at-half- maximum) in 1".2 seeing (apparent magnitude V = 19.7 as measured within a 5".5-diameter circular aperture). Ly further identified 2015 XG_422 in 30-s i-band exposures taken with the Cerro Tololo 4-m reflector (+ DECam) on 2016 Aug. 4.97, 5.97, 14.98, and 15.97. On Aug. 4.97, there appears a very condensed coma 3" in diameter (mag i = 19.4 in a 5".5-diameter aperture) with no tail in 1".2 seeing; on Aug. 5.97, the comet appears more diffuse with a 3"-diameter coma and a faded central condensation (i = 19.4), but still no tail, in 1".4 seeing. On Aug. 14.98, exposure there is a moderately-condensed coma 4" in diameter (i = 19.6), and again no tail, in 1".5 seeing. No clear cometary features appeared on Aug. 15.97, as the exposure was of poor quality. Ly adds that the brightness in 2016 August was some 1.5 mag brighter than would be expected from an asteroidal magnitude prediction. Ly also notes that he searched unsuccessfully for the comet in Sloan Digital Sky Survey images from Nov. 2001 to Oct. 2009 (limiting mag 22.5), adding that a single 40-s r-band exposure taken with the DECam on 2018 Aug. 12.17 showed no sign of the comet down to limiting magnitude r = 23. This object was discovered as an apparently asteroidal object on exposures taken on 2015 Dec. 13 with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala (and apparently not published until 2019 on MPS 1096514), with the discovery observations tabulated below. 2015 UT R.A. (2000) Decl. Mag. Dec. 13.63282 11 15 03.24 - 4 43 19.9 22.4 13.63797 11 15 03.44 - 4 43 20.4 21.9 13.64312 11 15 03.61 - 4 43 21.5 22.0 13.64828 11 15 03.80 - 4 43 22.5 22.0 Following a request from P. Veres (Minor Planet Center), R. Weryk, Department of Physics and Astronomy, University of Western Ontario, looked at Pan-STARRS1 images of 2015 XG_422 and found cometary appearance on 2016 Apr. 29.3 UT, in which four 45-s w-band survey images taken in 1".3 seeing show a very condensed head of size 1".7 (FWHM) and mag 20.2-20.3 and no clear tail, adding that four similar images taken on 2017 May 26 show a very condensed in 1".4 seeing show a very condensed coma of size 1".8 (FWHM) and a straight tail 4" long in p.a. 245 degrees. Weryk also identified the comet as apparently asteroidal in pre-discovery images from 2010 Sept. 13.5 (mag 21.8- 23.0), 2010 Sept. 15.5 (mag 22.6-23.2), 2010 Sept. 16.44-16.47 (at mag 21.6-23.4), and 2010 Oct. 3.4 (mag 22.1-22.2). The available astrometry appears on MPEC 2023-P35. The following linked orbital elements by S. Nakano (Central Bureau) are from 71 observations spanning 2010-2017 (mean residual 0".4). Nakano notes that the 2010 observations show residuals of +35" mainly in R.A. from an orbit on MPO 521860. The comet passed 0.34 AU from Jupiter on 2010 Nov. 12 UT. Epoch = 2008 Apr. 4.0 TT T = 2008 Mar. 27.18204 TT Peri. = 60.90568 e = 0.2869197 Node = 181.78845 2000.0 q = 3.3311264 AU Incl. = 19.86243 a = 4.6714603 AU n = 0.09761691 P = 10.10 years Epoch = 2016 Nov. 28.0 TT T = 2016 Dec. 2.09696 TT Peri. = 43.61668 e = 0.3097594 Node = 178.98612 2000.0 q = 2.9905394 AU Incl. = 20.19518 a = 4.3326042 AU n = 0.10929003 P = 9.02 years Epoch = 2025 Nov. 21.0 TT T = 2025 Dec. 8.37487 TT Peri. = 43.49257 e = 0.3081645 Node = 178.96553 2000.0 q = 3.0050032 AU Incl. = 20.17104 a = 4.3435226 AU n = 0.10887820 P = 9.05 years Epoch = 2034 Dec. 24.0 TT T = 2035 Jan. 9.48253 TT Peri. = 43.89997 e = 0.3071405 Node = 178.79511 2000.0 q = 3.0149044 AU Incl. = 20.15043 a = 4.3513938 AU n = 0.10858292 P = 9.08 years Epoch = 2044 Jan. 26.0 TT T = 2044 Jan. 26.20908 TT Peri. = 43.93665 e = 0.3089991 Node = 178.76231 2000.0 q = 2.9934309 AU Incl. = 20.17746 a = 4.3320219 AU n = 0.10931207 P = 9.02 years Using photometric power-law parameters H = 13.0 and 2.5n = 8, it might be expected that the comet will be near mag 19 while around opposition in the first half of 2025. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 August 8 (CBET 5284) Daniel W. E. Green