2008CBET.1462....1Y

F. Yuan, University of Michigan; R. Quimby, California Institute of Technology; T. McKay, D. Chamarro, M. D. Sisson, and C. Akerlof, University of Michigan; and J. C. Wheeler, University of Texas, on behalf of the ROTSE collaboration, report the discovery of a supernova (mag about 18.1) in unfiltered CCD images taken on Apr. 26.23 UT with the 0.45-m ROTSE-IIIb telescope at McDonald Observatory; the object was at mag 17.8 on images taken during Apr. 28.21-May 6.23. The new object is located at R.A. = 11h56m49s.13, Decl. = +54o27'25".7 (equinox 2000.0; uncertainty < 1"). The transient is near a group of galaxies at z = 0.062 in Sloan Digital Sky Survey (SDSS) images, but no counterpart is visible at the position of 2008es; the closest SDSS source is a galaxy (magnitude r = 23.1) some 8" away. A finding chart of the object can be found at the following website URL: http:/www.rotse.net/rsvp/j115649.1+542726/j115649.1+542726.jpg. There are no known x-ray or radio sources catalogued via NED or SIMBAD at the location of the optical transient. Additional magnitudes for 2008es were obtained with the Palomar 1.5-m reflector beginning on May 2, summarized by the following data, which indicate a slow brightening trend in bands g, r, and i: May 2.27, g = 18.1, r = 18.1, i = 18.2; May 8.24, g = 17.7, r = 17.8, i = 17.8. Spectra of 2008es were obtained with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph; range 420-890 nm) – on May 1.24 by S. Odewahn and V. Rileyand, and on May 8.25 by S. Rostopchin and A. Westfall – and on May 2.29 by B. Cenko with the Palomar 5-m telescope (+ Double Beam Spectrograph; range 320-900 nm). The spectroscopic data from the three nights all show a mostly featureless, blue continuum (f_nu approximately nu**0.35, where f_nu is flux density and nu is the frequency); there is a broad emission feature at 565.0 nm, which is seen at all three epochs. This transient was initially reported as a possible background active galactic nucleus (http://www.astronomerstelegram.org/?read=1515) and was observed by several groups (ibid.,/?read=1524, 1576, 1578, 1593).