SN Ia Standardization on the Rise: Evidence for the Cosmological Importance of Pre-Maximum Measurements

We present SALT2X, an extension of the SALT2 model for SN Ia supernova light curves. SALT2X separates the light-curve-shape parameter x1 into an x1r and x1f for the rise and fall portions of the light curve. Using the Joint Lightcurve Analysis (JLA) SN sample, we assess the importance of the rising and falling portions of the light curve for cosmological standardization using a modified version of the Unified Nonlinear Inference for Type Ia cosmologY (UNITY) framework. We find strong evidence that x1r has a stronger correlation with peak magnitude than x1f. We see evidence that standardizing on the rise affects the color standardization relation, and reduces the size of the host-galaxy standardization and the unexplained ("intrinsic") luminosity dispersion. Since SNe Ia generally rise more quickly than they decline, a faster observing cadence in future surveys will be necessary to maximize the gain from this work, and to continue to explore the impacts of decoupling the rising and falling portions of SN Ia light curves.
Comment: Updated to match refereed version