Electronic Telegram No. 4084 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 F3 (SWAN) R. Matson, Newport Coast, CA, USA, reported on March 21 another moving object in low-resolution public website hydrogen Lyman-alpha images obtained during Mar. 5-17 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/); there was a gap in images during Mar. 9-15, and there was a significant increase in the object's brightness between Mar. 8 and 16, with the object moving around 1.4 degrees/day toward the end of this interval. V. Bezugly, Dnepropetrovsk, Ukraine, also reported the object and provided approximate positions. Matson measured the following very approximate positions, noting that he assumes 12h UT for the composite images but adding that there is indication that the image times could be almost a day later than the SWAN website is reporting (based on Mar. 16 and 17 images): 2015 UT R.A. (2000) Decl. Mar. 5.5 23 42.6 +26 00 6.5 23 42.4 +26 50 7.5 23 44.1 +27 40 8.5 23 45.8 +28 30 16.5 23 48.6 +36 40 17.5 23 49.0 +37 50 18.5 23 49.2 +39 10 19.5 23 48.9 +40 00 Bezugly measured R.A. = 23h49m.0, Decl. = +22d52' for Mar. 3.5 UT; his positions for other dates tabulated above are generally within 10' of Matson's measures. Matson alerted numerous ground-based observers to search for a possible comet, and the object was posted on the Minor Planet Center's NEOCP and PCCP webpages on March 24. Numerous CCD astrometrists have reported obtaining images that confirm the object's cometary appearance. M. Jaeger, Vienna, Austria, reports that he imaged the comet with a 0.25-m f/4 reflector on Mar. 24.1, measuring a 2' coma of mag 10-11 and a 10' tail pointing northward. P. Birtwhistle (Great Shefford, Berkshire, England; 0.40-m f/6 Schmidt-Cassegrain reflector) found the comet to be very diffuse, with a poorly concentrated inner coma, an outer coma of diameter 50", and a thin, straight tail 1'.5 long in p.a. 0 degrees (possibly up to 3' long) on images taken during Mar. 24.85-24.87; his follow-up images on Mar. 25.2 show a coma diameter of 54" and a straight, thin tail 3'.3 long in p.a. 359 degrees. H. Sato, Tokyo, Japan, writes that seven stacked 60-s exposures taken with a 0.10-m f/5.0 astrograph (+ luminance filter) near Mayhill, NM, USA, on Mar. 25.5 show a strongly condensed coma of diameter 50" with magnitude 11.6 as measured within a circular aperture of radius 28".0. K. Kadota (Ageo, Japan; 0.25-m f/5 reflector) relates that the comet is diffuse with a central condensation and a coma of diameter 2'.6 and total magnitude 11.8, and showing a tail 1'.5 long in p.a. 3 deg on Mar. 25.8. A. Hale, Cloudcroft, NM, USA, found the comet visually on Mar. 26.47 with a 0.2-m reflector, estimating total mag 10.3 with a 2'.3 coma. The available ground-based precise astrometry, the following preliminary parabolic orbital elements by G. V. Williams from 46 observations spanning Mar. 24-26, and an ephemeris appear on MPEC 2015-F122. T = 2015 Mar. 9.5436 TT Peri. = 57.9230 Node = 31.6006 2000.0 q = 0.834871 AU Incl. = 73.3627 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2015 CBAT 2015 March 27 (CBET 4084) Daniel W. E. Green