Nonlinear Decline‐Rate Dependence and Intrinsic Variation of Type Ia Supernova Luminosities

Abstract

Published B and V fluxes from nearby Type Ia supernovae (SNe) are fitted to light-curve templates with four to six adjustable parameters. Separately, B magnitudes from the same sample are fitted to a linear dependence on B-V color within a postmaximum time window prescribed by the CMAGIC method. These fits yield two independent SN magnitude estimates Bmax and BBV. Their difference varies systematically with decline-rate Δm15 in a form that is compatible with a bilinear but not a linear dependence; a nonlinear form likely describes the decline-rate dependence of Bmax itself. A Hubble fit to the average of Bmax and BBV requires a systematic correction for observed B-V color that can be described by a linear coefficient R;=2.59+/-0.24, well below the coefficient RB~4.1 commonly used to characterize the effects of Milky Way dust. At 99.9% confidence the data reject a simple model in which no color correction is required for SNe that are clustered at the blue end of their observed color distribution. After systematic corrections are performed, Bmax and BBV exhibit mutual rms intrinsic variation equal to 0.074+/-0.019 mag, of which at least an equal share likely belongs to BBV. SN magnitudes measured using maximum luminosity or CMAGIC methods show comparable rms deviations of order ~0.14 mag from the Hubble line. The same fit also establishes a 95% confidence upper limit of 486 km s-1 on the rms peculiar velocity of nearby SNe relative to the Hubble flow.