Electronic Telegram No. 4091 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION CBAT Director: Daniel W. E. Green; 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/2015 F5 (SWAN-XINGMING) Several people noticed a moving object on low-resolution public website hydrogen Lyman-alpha images obtained during Mar. 29-Apr. 1 with the Solar Wind Anisotropies (SWAN) camera on the Solar and Heliospheric Observer (SOHO) spacecraft (see CBET 4068 and website URL http://swan.projet.latmos.ipsl.fr/), and the Central Bureau received two such reports during mid-day (UT) on Apr. 4 from Szymon Liwo (Swidnica, Poland) and from Worachate Boonplod (Samut Songkhram, Thailand), both of whom provided approximate positions. M. Mattiazzo, Swan Hill, Victoria, Australia, reports the following approximate positions for the object (uncertainty up to a degree, with an uncertain assumption of 12h UT for each image). 2015 UT R.A. (2000) Decl. Mar. 29.5 23 04.1 -00 53 30.5 23 06.4 +00 52 31.5 23 09.3 +02 56 Apr. 1.5 23 13.2 +04 58 For comparison, R. Matson (Newport Coast, CA, USA) provides his measurements from the same images: 2015 UT R.A. (2000) Decl. Mar. 29.5 23 06.1 -01 30 30.5 23 07.4 +01 05 31.5 23 09.6 +03 20 Apr. 1.5 23 13.5 +04 55 Early on Apr. 5 UT (apparently before any public posting of a possible new comet on SWAN images was made), a report was received from Xing Gao (XinJiang, China) of the independent discovery of an unknown comet with a 30" coma by Guoyou Sun on images taken in twilight on Apr. 4.9 UT (discovery observations tabulated below) by Gao with a 0.11-m f/5 refractor in the course of the Xingming sky survey; they imaged about 32 square degrees of neighboring area around the comet that morning. Sun then checked online SWAN images and noticed the above-mentioned four images moving with a motion and location consistent with the Xingming images. 2015 UT R.A. (2000) Decl. Mag. Observer Apr. 4.93769 23 27 58.07 +11 48 08.5 12.7 Gao 4.94028 23 27 58.29 +11 48 32.3 12.2 " 4.94288 23 27 59.40 +11 48 46.9 11.2 " Additional images obtained by Gao with the 0.11-m refractor (and measured by M.-T. Hui and G. Sun) show a coma diameter around 50" on Apr. 5.9 and around 1'.0 on Apr. 6.9 (with magnitude V = 11.4-11.7); follow-up images on Apr. 7.9 show a 1'.2 coma of red mag 11.4-12.5 with a 4'.5 tail in p.a. 300 deg. After the object was posted on the Minor Planet Center's PCCP webpage, other observations have also shown cometary appearance. Hui also reports that CCD images taken by Z.-Q. Guo with another 0.11-m refractor in China on Apr. 5.8 show a cyan color to the coma, whose diameter was around 50" with magnitude 11.3 and no tail in twilight. L. Buzzi (Varese, Italy, 0.60-m f/4.64 reflector) writes that stacked images taken in twilight and at low altitude (11 deg) on Apr. 8.15-8.16 show a 2' coma of red mag 14.0 without a strong central condensation and a thin, straight tail at least 6'.5 long in p.a. 307 deg; thirty stacked 10-s images taken in slightly better conditions at 10 deg altitude by Buzzi on Apr. 9.15 reveal a round coma 3' wide and a tail at least 6' long in p.a. 300 deg (after the first 4', the tail widens). Visual total-magnitude and coma-diameter estimates: Apr. 8.19 UT, 8.2, 1'.7 (J. J. Gonzalez, Leon, Spain; 20.3-cm reflector at Villaverde de Arcayos; altitude 8 degrees in twilight; condensed coma); 10.10, 7.9, 5' (M. Reszelski, Szamotuly-Galowo, Poland, 15x70 binoculars); 10.48, 9.6:, 2' (A. Hale, Cloudcroft, NM, 0.41-m reflector; low altitude; bright moonlight, twilight). The available ground-based astrometry, the following very preliminary parabolic orbital elements by G. V. Williams (from the poor-quality ground- based astrometry alone, spanning Apr. 4-10), and an ephemeris appear on MPEC 2015-G27. T = 2015 Mar. 28.4308 TT Peri. = 15.3704 Node = 288.5830 2000.0 q = 0.347798 AU Incl. = 148.8845 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2015 CBAT 2015 April 10 (CBET 4091) Daniel W. E. Green