COSMOLOGICAL CONSTRAINTS FROM MEASUREMENTS OF TYPE Ia SUPERNOVAE DISCOVERED DURING THE FIRST 1.5 yr OF THE Pan-STARRS1 SURVEY

We present griz P1 light curves of 146 spectroscopically confirmed Type Ia supernovae (SNe Ia; 0.03 < z < 0.65) discovered during the first 1.5 yr of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We find that the systematic uncertainties in the photometric system are currently 1.2% without accounting for the uncertainty in the Hubble Space Telescope Calspec definition of the AB system. A Hubble diagram is constructed with a subset of 113 out of 146 SNe Ia that pass our light curve quality cuts. The cosmological fit to 310 SNe Ia (113 PS1 SNe Ia + 222 light curves from 197 low-z SNe Ia), using only supernovae (SNe) and assuming a constant dark energy equation of state and flatness, yields w = -1.120 +0.360 -0.206 (Stat) +0.269 -0.291 (Sys). When combined with BAO+CMB(Planck)+H 0 , the analysis yields M = 0.280 +0.013 -0.012 and w = -1.166 +0.072 -0.069 including all identified systematics. The value of w is inconsistent with the cosmological constant value of -1 at the 2.3 level. Tension endures after removing either the baryon acoustic oscillation (BAO) or the H 0 constraint, though it is strongest when including the H 0 constraint. If we include WMAP9 cosmic microwave background (CMB) constraints instead of those from Planck, we find w = -1.124 +0.083 -0.065 , which diminishes the discord to <2 . We cannot conclude whether the tension with flat CDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 SN sample with three times as many SNe should provide more conclusive results.