The Exceptionally Luminous Type Ia Supernova 2007if
  SN 2007if was the third over-luminous SN Ia detected after 2003fg and 2006gz.
We present the photometric and spectroscopic observations of the supernova and
its host by ROTSE-III, HET and Keck. From the H_alpha line identified in the
host spectra, we determine a redshift of 0.0736. At this distance, the
supernova 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 solar masses) 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
solar masses. Alternatively, we investigate circumstellar interaction that may
contribute to the excess luminosity.
