Supernova 2013G in NGC 3947 = Psn J11532132+2044100

S. Howerton, Arkansas City, KS, U.S.A.; A. J. Drake, S. G. Djorgovski, A. Mahabal, M. J. Graham, and R. Williams, California Institute of Technology; J. L. Prieto, Princeton University; M. Catelan, Pontificia Universidad Catolica de Chile; R. H. McNaught, Australian National University; and E. Christensen and S. M. Larson, Lunar and Planetary Laboratory, University of Arizona, report the discovery of an apparent supernova in public images from the Catalina Sky Survey (CSS). SN 2013 UT R.A. (2000.0) Decl. Mag. Offset 2013G Jan. 5.43 11 53 21.32 +20 45 10.0 16.0 14".0 E, 3".8 N The variable was designated PSN J11532132+2044100 when it was posted at the Central Bureau's TOCP webpage (because the declination was posted there erroneously as +20d44') and is here designated SN 2013G based on the spectroscopic confirmation reported below. Additional unfiltered CCD magnitudes for 2013G: 2012 Dec. 3.44, [19.0 (CSS); Dec. 12, [18.5 (Simone Leonini, Siena, Italy; taken in the course of an automatic survey of the Italian Supernovae Search Project with a 0.53-m reflector; 2013 Jan. 4.179, 15.5 (Leonini, with P. Rosi and L. M. Tinjaca Ramirez; pre-discovery; limiting mag about 18.5; position end figures 21s.40, 07".9; offset 14" east, 4" south; image visible via website URL http://tinyurl.com/a6l7nqm); 8.175, 15.8 (Federica Luppi and Luca Buzzi, Italy; 0.38-m f/6.8 reflector; position end figures 21s.40, 07".8; reference stars from CMC-14 catalogue; image posted at website URL http://www.astrogeo.va.it/pub/TOCP/PSN_N3947.jpg); 9.399, 15.6 (Joseph Brimacombe, Cairns, Australia; remotely with a 51-cm RCOS telescope + luminance filter at the New Mexico Skies Observatory near Mayhill, NM, U.S.A.; position end figures 21s.41, 07".9; image posted at website URL http://www.flickr.com/photos/43846774@N02/8368274029/). SNe 1972C, 2001P, and 2006aa also appeared in NGC 3947. G. H. Marion and P. Challis, Harvard-Smithsonian Center for Astrophysics (CfA), on behalf of the CfA Supernova Group, report that a spectrogram (range 340-740 nm) of PSN J11532132+2044100 = SN 2013G was obtained on Jan. 6 UT by P. Berlind with the F. L. Whipple Observatory 1.5-m telescope (+ FAST). Cross-correlation with a library of supernova spectra using the "Supernova Identification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) shows that 2013G is a type-Ia supernova a few days before maximum light. A good fit is found to the template of the normal type-Ia supernova 2007A at two days before maximum. The velocity of the Si II 635.5-nm absorption feature is measured to be approximately 10700 km/s, using a redshift of z = 0.020671 for NGC 3947 (de Vaucouleurs et al. 1991, RC3.9C; via NED). Tianmeng Zhang, National Astronomical Observatory of China (NAOC); and Jun Mo and Xiaofeng Wang, Tsinghua Center for Astrophysics, Tsinghua University, report on an optical spectrogram (range 380-880 nm) of PSN J11532132+2044100 = SN 2013G that was obtained on Jan. 8.8 UT with the 2.16-m telescope (+ BFOSC) at the Xinglong Station of NAOC. The spectrum is consistent with a type-Ia supernova at about one week before maximum. Cross-correlation with a library of supernova spectra using the "Supernova Identification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) shows that the spectrum of 2013G matches with that of SN 1998aq at -9 days. Adopting a recession velocity of 6197 km/s for the host galaxy (NGC 3947; cf. Giovanelli et al. 1997, A.J. 113, 22), they measure the velocity of the Si II 635.5-nm absorption feature to be about 11800 km/s.