A Novel Color Parameter as a Luminosity Calibrator for Type Ia Supernovae

Abstract

Type Ia supernovae (SNe Ia) provide us with a unique tool for measuring\nextragalactic distances and determining cosmological parameters. As a result,\nthe precise and effective calibration for peak luminosities of SNe Ia becomes\nextremely crucial and thus is critically scrutinized for cosmological\nexplorations. In this Letter, we reveal clear evidence for a tight linear\ncorrelation between peak luminosities of SNe Ia and their $B-V$ colors $\\sim\n12$ days after the $B$ maximum denoted by $\\Delta C_{12}$. By introducing such\na novel color parameter, $\\Delta C_{12}$, this empirical correlation allows us\nto uniformly standardize SNe Ia with decline rates $\\Delta m_{15}$ in the range\nof $0.8<\\Delta m_{15}<2.0$ and to reduce scatters in estimating their peak\nluminosities from $\\sim 0.5$ mag to the levels of 0.18 and 0.12 mag in the $V$\nand $I$ bands, respectively. For a sample of SNe Ia with insignificant\nreddenings of host galaxies [e.g., E$(B-V)_{host}\\lsim 0.06$ mag], the scatter\ndrops further to only 0.07 mag (or 3-4% in distance), which is comparable to\nobservational accuracies and is better than other calibrations for SNe Ia. This\nwould impact observational and theoretical studies of SNe Ia and cosmological\nscales and parameters.\n