On the Rates of Type Ia Supernovae in Dwarf and Giant Hosts with
  ROTSE-IIIb
  We present a sample of 23 spectroscopically confirmed Type Ia supernovae that
were discovered in the background of galaxy clusters targeted by ROTSE-IIIb and
use up to 18 of these to determine the local (z = 0.05) volumetric rate. Since
our survey is flux limited and thus biased against fainter objects, the
pseudo-absolute magnitude distribution (pAMD) of SNeIa in a given volume is an
important concern, especially the relative frequency of high to low-luminosity
SNeIa. We find that the pAMD derived from the volume limited Lick Observatory
Supernova Search (LOSS) sample is incompatible with the distribution of SNeIa
in a volume limited (z<0.12) sub sample of the SDSS-II. The LOSS sample
requires far more low-luminosity SNeIa than the SDSS-II can accommodate. Even
though LOSS and SDSS-II have sampled different SNeIa populations, their
volumetric rates are surprisingly similar. Using the same model pAMD adopted in
the SDSS-II SNeIa rate calculation and excluding two high-luminosity SNeIa from
our sample, we derive a rate that is marginally higher than previous
low-redshift determinations. With our full sample and the LOSS pAMD our rate is
more than double the canonical value. We also find that 5 of our 18 SNeIa are
hosted by very low-luminosity (M_B > -16) galaxies, whereas only 1 out 79
nearby SDSS-II SNeIa have such faint hosts. It is possible that previous works
have under-counted either low luminosity SNeIa, SNeIa in low luminosity hosts,
or peculiar SNeIa (sometimes explicitly), and the total SNeIa rate may be
higher than the canonical value.
