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