THE EXCEPTIONALLY LUMINOUS TYPE Ia SUPERNOVA 2007if

SN 2007if was the third over-luminous Type Ia supernova (SN Ia) detected after 2003fg and 2006gz. We present the photometric and spectroscopic observations of the SN and its host by ROTSE-III, HET, and Keck. From the H_α line identified in the host spectra, we determine a redshift of 0.0736. At this distance, the SN reached an absolute magnitude of –20.4, brighter than any other SNe Ia ever observed. If the source of luminosity is radioactive decay, a large amount of radioactive nickel (~1.5 M_☉) is required to power the peak luminosity, more than can be produced realistically in a Chandrasekhar mass progenitor. Low expansion velocity, similar to that of 2003fg, is also measured around the maximum light. The observations may suggest that SN 2007if was from a massive white dwarf progenitor, plausibly exploding with mass well beyond 1.4 M_☉. Alternatively, we investigate circumstellar interaction that may contribute to the excess luminosity.