Sn 2005ap:: A most brilliant explosion

We present unfiltered photometric observations with ROTSE- III and optical spectroscopic follow- up with HET and the Keck telescope of the most luminous supernova yet identified, SN 2005ap. The spectra taken about 3 days before and 6 days after maximum light show narrow emission lines ( likely originating in the dwarf host) and absorption lines at a redshift of z=0.2832 which puts the peak unfiltered magnitude at -22.7 +/- 0.1 absolute. Broad P Cygni features corresponding to H alpha, C III, N III, and O III are further detected with a photospheric velocity of similar to 20,000 km s(-1). Unlike other highly luminous supernovae such as 2006gy and 2006tf that show slow photometric evolution, the light curve of SN 2005ap indicates a 1 - 3 week rise to peak followed by a relatively rapid decay. The spectra also lack the distinct emission peaks from moderately broadened (FWHM similar to 2000 km s(-1)) Balmer lines seen in SN 2006gy and SN 2006tf. We briefly discuss the origin of the extraordinary luminosity from a strong interaction as may be expected from a pair instability eruption or a GRB- like engine encased in a H/ He envelope.