Near-infrared and Optical Nebular-phase Spectra of Type Ia Supernovae SN 2013aa and SN 2017cbv in NGC 5643

TL;DR

This study analyzes nebular-phase optical and near-infrared spectra of two Type Ia supernovae in NGC 5643, revealing insights into their explosion kinematics, progenitor scenarios, and the importance of NIR lines for velocity measurements.

Contribution

It demonstrates that NIR [Fe II] lines provide more reliable velocity measurements than optical lines and constrains progenitor models based on nebular spectra of two sibling SNe Ia.

Findings

NIR [Fe II] 1.644 μm lines yield consistent velocity shifts with optical lines but are less affected by fitting methods.

Low [Fe II] velocities suggest progenitor scenarios disfavoring close double-degenerate systems.

The spectral analysis indicates high progenitor white dwarf densities and possibly strong magnetic fields.

Abstract

We present multi-wavelength time-series spectroscopy of SN 2013aa and SN 2017cbv, two Type Ia supernovae (SNe Ia) on the outskirts of the same host galaxy, NGC 5643. This work utilizes new nebular-phase near-infrared (NIR) spectra obtained by the Carnegie Supernova Project-II, in addition to previously published optical and NIR spectra. By measuring nebular-phase [Fe II] lines in both the optical and NIR, we examine the explosion kinematics and test the efficacy of several emission line fitting techniques commonly used in the literature. The NIR [Fe II] 1.644 $\mu$m line provides the most robust velocity measurements against variations due to the choice of the fit method and line blending. The resulting effects on velocity measurements due to choosing different fit methods, initial fit parameters, continuum and line profile functions, and fit region boundaries were also investigated. The NIR [Fe II] velocities yield the same radial shift direction as velocities measured using the optical [Fe II] 7155 A line, but the sizes of the shifts are consistently and substantially lower, pointing to a potential issue in optical studies. The NIR [Fe II] 1.644 $\mu$m emission profile shows a lack of significant asymmetry in both SNe Ia, and the observed low velocities elevate the importance for correcting for any radial velocity contribution from the host galaxy's rotation. The low [Fe II] velocities measured in the NIR at nebular phases disfavors most progenitor scenarios in close double-degenerate systems for both SN 2013aa and SN 2017cbv. The time evolution of the NIR [Fe II] 1.644 $\mu$m line also indicates moderately high progenitor white dwarf central density and potentially high magnetic fields. These sibling SNe Ia were well observed at both early and late times, providing an excellent opportunity to study the intrinsic diversity of SNe Ia.