Measurements of the Rate of Type Ia Supernovae at Redshift z < ~0.3 from
  the SDSS-II Supernova Survey
  We present a measurement of the volumetric Type Ia supernova (SN Ia) rate
based on data from the Sloan Digital Sky Survey II (SDSS-II) Supernova Survey.
The adopted sample of supernovae (SNe) includes 516 SNe Ia at redshift z
\lesssim 0.3, of which 270 (52%) are spectroscopically identified as SNe Ia.
The remaining 246 SNe Ia were identified through their light curves; 113 of
these objects have spectroscopic redshifts from spectra of their host galaxy,
and 133 have photometric redshifts estimated from the SN light curves. Based on
consideration of 87 spectroscopically confirmed non-Ia SNe discovered by the
SDSS-II SN Survey, we estimate that 2.04+1.61-0.95 % of the photometric SNe Ia
may be misidentified. The sample of SNe Ia used in this measurement represents
an order of magnitude increase in the statistics for SN Ia rate measurements in
the redshift range covered by the SDSS-II Supernova Survey. If we assume a SN
Ia rate that is constant at low redshift (z < 0.15), then the SN observations
can be used to infer a value of the SN rate of rV = (2.69+0.34+0.21-0.30-0.01)
x10^{-5} SNe yr^{-1} Mpc-3 (H0 /(70 km s^{-1} Mpc^{-1}))^{3} at a mean redshift
of ~ 0.12, based on 79 SNe Ia of which 72 are spectroscopically confirmed.
However, the large sample of SNe Ia included in this study allows us to place
constraints on the redshift dependence of the SN Ia rate based on the SDSS-II
Supernova Survey data alone. Fitting a power-law model of the SN rate
evolution, r_V(z) = A_p x ((1 + z)/(1 + z0))^{\nu}, over the redshift range 0.0
< z < 0.3 with z0 = 0.21, results in A_p = (3.43+0.15-0.15) x 10^{-5} SNe
yr^{-1} Mpc-3 (H0 /(70 km s^{-1} Mpc^{-1}))^{3} and \nu = 2.04+0.90-0.89.
