2008CBET.1308....1Y

F. Yuan, M. D. Sisson, D. Chamarro, and C. Akerlof, University of Michigan; R. Quimby, California Institute of Technology; and J. C. Wheeler, University of Texas, on behalf of the ROTSE collaboration, report the discovery of an apparent supernova located at R.A. = 12h51m11s.89, Decl. = +26o17'40".2 (equinox 2000.0; uncertainty < 1"), which is 0".6 east and 2".4 north of an extended object (magnitude r = 18.0) visible in the Sloan Digital Sky Survey. The new object, which appeared at mag 18.7 and 17.9 in unfiltered CCD images taken on Mar. 5.34 and 13.33 UT, respectively, with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory, was discovered by subtracting a reference image constructed from images taken in 2006, and 2008bg was not resolved from the extended background object (the magnitudes are unfiltered but calibrated to USNO-B1.0-catalogue R data). A finding chart for 2008bg can be found at the following website URL: http://rotse.net/rsvp/j125111.9+261740/j125111.9+261740.jpg. The object's lightcurve and location suggest it likely to be a type-Ia supernova. S. Blondin, Harvard-Smithsonian Center for Astrophysics (CfA), on behalf of the CfA Supernova Group, reports that a spectrum (range 350-740 nm) of 2008bg, obtained on Mar. 15.25 UT by W. Brown with the F. L. Whipple Observatory 1.5-m telescope (+ FAST), shows it to be a type-Ia supernova around maximum light. Cross-correlation with a library of supernova spectra using the "Supernova Identification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) indicates that 2008bg is most similar to the type-Ia supernova 1994ae at 2 days past maximum light, at a redshift cz = 19200 ± 1500 km/s, consistent with that determined from narrow lines in the host-galaxy spectrum (cz = 18976 ± 68 km/s from nine emission lines). Adopting this latter recession velocity for the supernova, the maximum absorption in the Si II line (rest 635.5 nm) is blueshifted by roughly 10300 km/s.