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

Published B and V fluxes from nearby Type Ia supernovae are fitted tolight-curve templates with 4-6 adjustable parameters. Separately, B magnitudesfrom the same sample are fitted to a linear dependence on B-V color within apost-maximum time window prescribed by the CMAGIC method. These fits yield twoindependent SN magnitude estimates B_max and B_BV. Their difference variessystematically with decline rate Delta m_15 in a form that is compatible with abilinear but not a linear dependence; a nonlinear form likely describes thedecline-rate dependence of B_max itself. A Hubble fit to the average of B_maxand B_BV requires a systematic correction for observed B-V color that can bedescribed by a linear coefficient R = 2.59 +- 0.24, well below the coefficientR_B ~ 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 isrequired for SNe that are clustered at the blue end of their observed colordistribution. After systematic corrections are performed, B_max and B_BVexhibit mutual rms intrinsic variation equal to 0.074 +- 0.019 mag, of which atleast an equal share likely belongs to B_BV. SN magnitudes measured usingmaximum-luminosity or CMAGIC methods show comparable rms deviations of order ~0.14 mag from the Hubble line. The same fit also establishes a 95% confidenceupper limit of 486 km/s on the rms peculiar velocity of nearby SNe relative tothe Hubble flow.Comment: 21 pages, 11 figures, 10 tables, to appear in The Astrophysical Journal, uses emulateapj_051214.cl