Electronic Telegram No. 3551 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 SUPERNOVA 2013dc IN NGC 6240 = PSN J16525897+0224255 Adam Block, Steward Observatory, reports the discovery of an apparent supernova (mag 18.7) on 900-s CCD exposures taken on Apr. 11 and 12 with a 0.8-m f/7 telescope (+ SBIG STX 16803 camera + clear filter with infrared block) at the Mount Lemmon SkyCenter. The new object is located at R.A. = 16h52m58s.97, Decl. = +2d24'25".2 (equinox 2000.0), which is 25" north of the nucleus of NGC 6240 along the plane of the galaxy itself. He notes the presence of an H II/star-forming region (but no star, as is now visible) in this position on Hubble Space Telescope images. The variable was designated PSN J16525897+0224255 when it was posted at the Central Bureau's TOCP webpage and is here designated SN 2013dc based on the spectroscopic confirmation reported below. The type-IIn supernova 2000bg also appeared in NGC 6240 (cf. IAUC 7392). Additional clear-filter magnitudes for 2013dc from Block: Apr. 13, 18.9; 14, 18.9; 19, 19.0; 20, 19.0; 21, 19.0; May 26, 20.3. Green-filter (broadband, similar to V) magnitudes for the variable from Block: Apr. 18, 18.8; 19, 18.9; 20, 18.9; 21, 18.9. Additional CCD magnitues for 2013dc: May 19.928, 18.9 (Gianluca Masi and Francesca Nocentini, remotely using the 43-cm robotic telescope at Ceccano, Italy; co-added images; position end figures: 59s.01, 25".8; image scale 1".2/pixel; image posted at website URL http://www.virtualtelescope.eu/2013/05/21/psn-j165258970224255-in-ngc-6240/); 20.231, 18.9 (Joseph Brimacombe, Cairns, Australia; remotely with a 51-cm telescope + luminance filter at the New Mexico Skies observatory near Mayhill, NM, U.S.A.; position end figures 58s.94, 24".5; image posted at website URL http://www.flickr.com/photos/43846774@N02/8764298096/); 20.392, [20.2 (L. Elenin, Lyubertsy, Russia; and I. Molotov, Moscow; three 150-s images remotely taken with a 0.45-m f/2.8 telescope at the ISON-NM Observatory near Mayhill; scale 2"/pixel); 21.239, 18.4 (Brimacombe; infrared filter with bandpass > 700 nm; position end figures 58s.91, 24".6; image posted at website URL http://www.flickr.com/photos/43846774@N02/8785083663/). L. Tomasella, S. Benetti, A. Pastorello, E. Cappellaro, P. Ochner, and M. Turatto, Osservatorio Astronomico di Padova, Istituto Nazionale di Astrofisica, report that an optical spectrogram of PSN J16525897+0224255 = SN 2013dc (range 340-820 nm; resolution 1.3 nm), obtained on June 7.05 UT with the Asiago 1.82-m Copernico Telescope (+ AFOSC), shows that this is a type-II supernova at redshift z about 0.0245 (Downes et al. 1993, Ap.J. 414, L13; via NED). A good match is found with normal type-II-P supernovae about two months after explosion. The expansion velocity, deduced from the H-alpha absorption, is about 6900 km/s. The Asiago classification spectra are posted at website URL http://graspa.oapd.inaf.it; classification was made via GELATO (Harutyunyan et al. 2008, A.Ap. 488, 383) and SNID (Blondin and Tonry 2007, Ap.J. 666, 1024). S. B. Cenko, Goddard Space Flight Center, NASA: W. Zheng, A. V. Filippenko, O. D. Fox, and K. I. Clubb, University of California, Berkeley; and S. D. Van Dyk, IPAC, California Institute of Technology, report that inspection of a CCD spectrogram (range 340-1000 nm), obtained on June 7.3 UT with the Shane 3-m reflector (+ Kast spectrograph) at Lick Observatory, shows that PSN J16525897+0224255 = SN 2013dc is a type-II supernova. Cross- correlation with a library of supernova spectra using the "SuperNova Identification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) results in best matches with a number of type-IIP supernovae when at least several weeks past maximum light. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2013 CBAT 2013 June 9 (CBET 3551) Daniel W. E. Green