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 light curves. SALT2X separates the light-curve-shape parameter x(1) into an x(1)(r) and x(1)(f) for the rise and fall portions of the light curve. Using the Joint Lightcurve Analysis 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 x(1)(r) has a stronger correlation with peak magnitude than x(1)(f). 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.