Strong Carbon Features and a Red Early Color in the Underluminous Type Ia SN 2022xkq

TL;DR

SN 2022xkq, an underluminous Type Ia supernova, exhibits unique early red color and flux excess, with persistent unburnt carbon features, challenging existing explosion models and providing new insights into faint SNe Ia.

Contribution

This study provides comprehensive multi-wavelength observations of SN 2022xkq, revealing novel early color and spectral features that challenge current explosion models for faint Type Ia supernovae.

Findings

First observation of red early color and flux excess in a transitional/91bg-like SN Ia.

Detection of persistent unburnt carbon features in optical and NIR spectra.

No existing explosion model fully explains the observed data.

Abstract

We present optical, infrared, ultraviolet, and radio observations of SN 2022xkq, an underluminous fast-declining type Ia supernova (SN Ia) in NGC 1784 ($\mathrm{D}\approx31$ Mpc), from $<1$ to 180 days after explosion. The high-cadence observations of SN 2022xkq, a photometrically transitional and spectroscopically 91bg-like SN Ia, cover the first days and weeks following explosion which are critical to distinguishing between explosion scenarios. The early light curve of SN 2022xkq has a red early color and exhibits a flux excess which is more prominent in redder bands; this is the first time such a feature has been seen in a transitional/91bg-like SN Ia. We also present 92 optical and 19 near-infrared (NIR) spectra, beginning 0.4 days after explosion in the optical and 2.6 days after explosion in the NIR. SN 2022xkq exhibits a long-lived C I 1.0693 $\mu$m feature which persists until 5 days post-maximum. We also detect C II $\lambda$6580 in the pre-maximum optical spectra. These lines are evidence for unburnt carbon that is difficult to reconcile with the double detonation of a sub-Chandrasekhar mass white dwarf. No existing explosion model can fully explain the photometric and spectroscopic dataset of SN 2022xkq, but the considerable breadth of the observations is ideal for furthering our understanding of the processes which produce faint SNe Ia.