Carnegie Supernova Project: kinky $i$-band light-curves of Type Ia supernovae

Abstract

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.

arXiv comment15 pages, 13 figures, accepted to be published on MNRAS

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