Electronic Telegram No. 879 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://cfa-www.harvard.edu/iau/cbat.html SUPERNOVA 1996cr IN ESO 97-G13 [with corrected sentence, re: CBET 876] F. Bauer, Columbia Astrophysics Laboratory, reports the discovery of an apparent type-IIn supernova in ESO 97-G13 (the "Circinus galaxy"), the position of 1996cr given as R.A. = 14h13m10s.01, Decl. = -65o20'44".4 (equinox 2000.0), which is about 23" south of the center of the galaxy. SN 1996cr was originally identified as a variable x-ray source but later discovered to be a young supernova candidate via archival optical and radio data. The x-ray object that led to the discovery of the optical images of 1996cr was designated CG X-2 in a paper by Sambruna et al. (2001, Ap.J. 546, L13) and later identified as a potential young supernova by Bauer et al. (2001, A.J. 122, 182); both of these papers rely on Chandra HETGS data taken on 2000 June 15. Smith and Wilson (2001, Ap.J. 557, 180) later identified the same source in earlier Chandra ACIS-S observations taken on 2000 Mar. 14. CG X-2 was not detected in any of the five previous ROSAT HRI observations (thus, it was at least 30 times fainter in the 0.5- to 2.0-keV band prior to 2000). Notably, the x-ray spectrum of CG X-2 was unlike most ultraluminous x-ray sources, showing a thermal spectrum of kT approximately 9-10 keV and a broad Fe emission complex at 6.6-7.0 keV with a high equivalent width (about 1 keV); such spectral features are more typical of type-II x-ray-emitting supernovae. The archival discovery CCD images were taken with the Anglo-Australian Telescope (+ TAURUS); the raw data were obtained through the AAT archive website. The observations that best capture the explosion date are several narrow-band-filtered images centered on typical emission lines of He II, [O III], and [S II] lines in the rest-frame of the Circinus galaxy taken in March 1996; photometric calibrations were derived by matching each AAT image against photometry measured by Bauer et al. 2001 (Ap.J. 122, 182) with band conversions calculated using SYNPHOT. Measured narrow-band and broadband magnitudes -- corresponding to the He II, [O III], H-alpha + [N II], and S[II] emission lines -- for 1996cr (including 2-sigma limiting magnitudes from earlier AAT images), are as follows: DATE UT FILTER (BANDWIDTH) MAGNITUDE 1995 Feb. 21.64 501 nm (3 nm) [22.0 Feb. 22.65 659 nm (4) [18.3 Feb. 28.8 662 nm (5) [20.3 Feb. 28.73 R [19.6 Feb. 28.74 B [22.1 Feb. 28.8 659 nm (4) [18.1 1996 Mar. 16.710 469 nm (3) 19.5 +/- 0.6 Mar. 16.728 673 nm (3) 15.8 +/- 0.5 Mar. 17.8 469 nm (3) 19.8 +/- 0.5 Mar. 19.73 501 nm (2) 18.7 +/- 0.4 Mar. 19.738 673 nm (3) 15.4 +/- 0.3 Mar. 19.755 469 nm (3) 19.7 +/- 0.3 The position of 1996cr coincides with a faint H II region. A flux-calibrated reduced spectrogram (range 300-850 nm; four 1200-s exposures), obtained of the region around 1996cr with the Very Large Telescope (+ FORS1) in 0".7 seeing on 2006 Jan. 26.30, shows narrow emission lines (most notably strong H_alpha) superimposed on classic broad, asymmetric [O III], [O I], and [Ca II] blended emission profiles -- characteristic features of late-time type-IIn supernovae. A deblended fit to the narrow H_alpha-and-[N II] emission blend gives a line width of about 700 km/s, with a small broader H_alpha base remaining unfitted; the various forbidden lines in the spectrum have similar line widths. Radio data published by Elmouttie et al. (1998, MNRAS 297, 1202), and additional radio images at 3 and 6 cm (comprised of data taken between 1994 and 1996; Elmouttie, private communication), confirm that 1996cr has a faint (about 1- to 2-mJy) radio counterpart. From a radio light curve constructed by Bauer from ATCA-archive data, 1996cr begins to peak up above the noise by 1996 Dec. 5 (about 1-2 mJy). From the available data, it is unclear whether the peak of the radio emission has been reached yet; as of 2004 Apr. 2, 1996cr had the following radio-flux densities: 8.6 GHz, 162 mJy; 4.8 GHz, 240 mJy; 2.4 GHz, 364 mJy; 1.4 GHz, 481 mJy. The explosion date for 1996cr is constrained by these data to lie between 1995 Feb. 28 and 1996 Mar. 16. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2007 CBAT 2007 March 12 (CBET 879) Daniel W. E. Green