2010CBET.2297....1Z

W. Zheng and F. Yuan, University of Michigan; J. Vinko, University of Szeged; R. Quimby, California Institute of Technology; M. D. Sisson, N. Whallon, A. Romadan, Y. Chen, C. Akerlof, and S. B. Pandey, University of Michigan; and J. C. Wheeler and E. Chatzopoulos, University of Texas, on behalf of the ROTSE collaboration, report the discovery of a new supernova (mag about 18.2) in unfiltered images taken on May 23.28 UT with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory. The new object, which was observed again on May 25.23 at mag about 17.7, is located at R.A. = 12:11:33.31, Decl. = +47:16:29.1 (equinox 2000.0; uncertainty about 1"), which is 0".1 west and 0".5 north of the center of the host galaxy (SDSS J121133.31+471628.5); a finding chart for the object can be found at website URL http://www.rotse.net/rsvp/j121133.3+471629/j121133.3+471629.jpg. A spectrum, obtained on May 28.23 UT with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph) by J. Caldwell, shows that 2010dm is a type-Ia supernova, a few days before maximum light. The spectrum contains the Si II 635.5-nm line and several other characteristic type-Ia- supernova features. The high-velocity Ca II infrared triplet feature is present, which supports the pre-maximum phase estimate. According to SNID (Blondin and Tonry 2007, Ap.J. 666, 1024), the spectrum most resembles that of SN 1990N at six days before maximum. The median redshift estimate is 0.032 ± 0.006, in good agreement with the photo-z estimate of the host galaxy from the Sloan Digital Sky Survey; adopting this redshift, the velocity (from the absorption minimum of the Si II 635.5-nm feature) is 11600 km/s.