The rate of Type Ia supernovae at z ≈ 0.2 from SDSS-I overlapping fields

In the course of the Sloan Digital Sky Survey (SDSS-I), a large fraction of the surveyed area was observed more than once due to field tiling overlap, usually at different epochs. We utilize some of these data to perform a supernova (SN) survey at a mean redshift of z = 0.2. Our archival search, in ∼5 per cent of the SDSS-I overlap area, produces 29 SN candidates clearly associated with host galaxies. Using the Bayesian photometric classification algorithm of Poznanski et al., and correcting for classification bias, we find 17 of the 29 candidates are likely Type Ia SNe. Accounting for the detection efficiency of the survey and for host extinction, this implies a Type Ia SN rate of r_Ia = (14.0^+2.5+1.4_-2.5-1.1 ± 2.5) × 10^-14 h²₇₀ yr^-1 L ^-1_⊙,g, where the errors are Poisson error, systematic detection efficiency error and systematic classification error, respectively. The volumetric rate is R_Ia = (1.89^+0.42+0.18 _-0.34-0.15 ± 0.42) × 10^-5 yr^-1 h³₇₀ Mpc^-3. Our measurement is consistent with other rate measurements at low redshift. An order of magnitude increase in the number of SNe is possible by analysing the full SDSS-I data base.