SN 2018agk: A Prototypical Type Ia Supernova with a Smooth Power-law Rise in Kepler (K2)

TL;DR

This paper presents high-cadence Kepler and ground-based observations of SN 2018agk, showing a smooth power-law rise without early bumps, and compares its early color evolution and companion interaction models with other SNe Ia.

Contribution

It provides detailed early light curve data of SN 2018agk and analyzes its rise behavior and color evolution, offering insights into the nature of Type Ia supernovae without early excess flux.

Findings

SN 2018agk's early light curve is consistent with a single power-law rise.

SNe Ia without early excess flux have slowly-evolving early colors.

Companion-interaction models are inconsistent with a typical non-degenerate companion at small viewing angles.

Abstract

We present the 30-min cadence Kepler/K2 light curve of the Type Ia supernova (SN Ia) SN 2018agk, covering approximately one week before explosion, the full rise phase and the decline until 40 days after peak. We additionally present ground-based observations in multiple bands within the same time range, including the 1-day cadence DECam observations within the first $\sim$5 days after the first light. The Kepler early light curve is fully consistent with a single power-law rise, without evidence of any bump feature. We compare SN 2018agk with a sample of other SNe~Ia without early excess flux from the literature. We find that SNe Ia without excess flux have slowly-evolving early colors in a narrow range ($g-i\approx -0.20\pm0.20$ mag) within the first $\sim 10$ days. On the other hand, among SNe Ia detected with excess, SN 2017cbv and SN 2018oh tend to be bluer, while iPTF16abc's evolution is similar to normal SNe Ia without excess in $g-i$. We further compare the Kepler light curve of SN 2018agk with companion-interaction models, and rule out the existence of a typical non-degenerate companion undergoing Roche-lobe overflow at viewing angles smaller than $45^{\circ}$.