Carnegie Supernova Project: kinky \(i\)-band light-curves of Type Ia supernovae

We present detailed investigation of a specific \(i\)-band light-curve feature in Type Ia supernovae (SNe Ia) using the rapid cadence and high signal-to-noise ratio light-curves obtained by the Carnegie Supernova Project. The feature is present in most SNe Ia and emerges a few days after the \(i\)-band maximum. It is an abrupt change in curvature in the light-curve over a few days and appears as a flattening in mild cases and a strong downward concave shape, or a "kink", in the most extreme cases. We computed the second derivatives of Gaussian Process interpolations to study 54 rapid-cadence light-curves. From the second derivatives we measure: 1) the timing of the feature in days relative to \(i\)-band maximum; tdm\(_{2}\)(\(i\)) and 2) the strength and direction of the concavity in mag d\(^{-2}\) ; dm\(_{2}\)(\(i\)). 76\(\%\) of the SNe Ia show a negative dm\(_{2}\)(\(i\)), representing a downward concavity - either a mild flattening or a strong "kink". The tdm\(_{2}\)(\(i\)) parameter is shown to correlate with the color-stretch parameter s\(_{\mathrm{BV}}\), a SN Ia primary parameter. The dm\(_{2}\)(\(i\)) parameter shows no correlation with s\(_{\mathrm{BV}}\) and therefore provides independent information. It is also largely independent of the spectroscopic and environmental properties. Dividing the sample based on the strength of the light-curve feature as measured by dm\(_{2}\)(\(i\)), SNe Ia with strong features have a Hubble diagram dispersion of 0.107 mag, 0.075 mag smaller than the group with weak features. Although larger samples should be obtained to test this result, it potentially offers a new method for improving SN Ia distance determinations without shifting to more costly near-infrared or spectroscopic observations.