Supernova 2011iw

Report the discovery of a supernova in unfiltered Catalina Sky Survey (CSS) images: SN 2011 UT R.A. (2000.0) Decl. Mag. 2011iw Nov. 29.09 23 34 48.20 +24 45 01.6 16.9 Nothing was visible at this position on a CSS image from Sept. 19.35 UT (limiting mag 19.5). Spectroscopic follow-up observations (range 400-800 nm) were taken with the Palomar 5-m telescope on Nov. 29.25. The spectrum exhibits strong, narrow Balmer emission lines corresponding to a redshift z = 0.023, which are superimposed on a blue continuum. A broad feature is observed near 610 nm that is likely due to He I. The object is thus identified as a likely type-IIn supernova. L. Tomasella, S. Benetti, A. Pastorello, S. Valenti, and F. Bufano, Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Padova; and G. Altavilla, INAF, Osservatorio Astronomico di Bologna, on behalf of a larger collaboration and the Gaia Science Alerts Follow-up Programme, report that spectrograms of 2011iw, obtained on Nov. 30.80 UT with the Asiago 1.82-m Copernico Telescope (+ AFOSC; range 350-820 nm, resolution 2.4 nm) by the Service Telescope Operating Team and with the Cassini 1.52-m telescope (+ BFOSC; range 490-800 nm; resolution 1.4 nm) on Nov. 30.81, show it to be a type-IIn supernova soon after explosion. The spectrum is dominated by a blue continuum and intense Balmer emissions with a composite profile. They consist of an unresolved component overimposed on a broader line (FWHM about 4000 km/s). A broad bump (FWHM about 4600 km/s) is also present close to the He I 587.6-nm transition. The redshift of the supernova (z about 0.0230) has been derived from the narrow emissions of the Balmer lines....