Supernova 2011gx in ESO 252-G10 = Tcp J05060576-4503003

Report the discovery of an apparent supernova in public images from the Siding Spring Survey (SSS). SN 2011 UT R.A. (2000.0) Decl. Mag. Offset 2011gx Oct. 3.69 5 06 05.76 -45 03 00.3 18.3 41".5 W, 10".7 S Nothing was visible at this position on a SSS image from Mar. 5.52 UT (limiting mag 19.5). The object was designated TCP J05060576-4503003 when it was posted on the Central Bureau's TOCP webpage and is here designated SN 2011gx based on the spectroscopic confirmation reported below. Infrared CCD magnitudes (filter bandpass > 700 nm) for 2011gx by Joseph Brimacombe, Cairns, Australia: Oct. 5.584, 16.8 (position end figures 05s.76, 00".7); 6.701, 16.6 (image posted at URL http://www.flickr.com/photos/43846774@N02/6218866501/); 8.780, 16.7; 9.638, 16.5; 10.769, 16.6; 12.675, 16.6 (image posted at website URL http://www.flickr.com/photos/43846774@N02/6238615803/). P. Ochner, S. Valenti, S. Benetti, A. Pastorello, and L. Tomasella, Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Padova; and F. Bufano, INAF, Osservatorio Astronomico di Catania, on behalf of a larger collaboration, report that a spectrum of TCP J05060576-4503003 = SN 2011gx, obtained on Oct. 18.17 UT with the New Technology Telescope (+ EFOSC2; range 360-930 nm; resolution 1.8 nm) shows it to be a type-II supernova. The best fit to this spectrum found by GELATO (Harutyunyan et al. 2008, A.Ap. 488, 383; publicly available at https://gelato.tng.iac.es) is with spectra of type- II-plateau supernovae at about one month post-explosion. Together with the classical P-Cyg Balmer features, prominent lines of Ca II, Fe II, and Na I D are detected. Adopting a recessional velocity of 9821 km/s for ESO 252-G10 (Lauberts and Valentijn, 1989, THE SURFACE PHOTOMETRY CATALOGUE OF THE ESO- UPPSALA GALAXIES, European Southern Obsrvatory) the expansion velocity deduced from the H_alpha absorption is about 8000 km/s....