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Arxiv: A JWST Medium Resolution MIRI Spectrum and Models of the Type Ia supernova 2021aefx at +415 d Published: 4/26/2024 12:08:28 AM Updated: 4/26/2024 12:08:28 AM

Paper abstract: We present a JWST MIRI/MRS spectrum (5-27 \mathrm{\mu}m) of the Type Iasupernova (SN Ia), SN 2021aefx at +415 days past B-band maximum. Thespectrum, which was obtained during the iron-dominated nebular phase, has beenanalyzed in combination with previous JWST observations of SN 2021aefx, toprovide the first JWST time series analysis of an SN Ia. We find the temporalevolution of the [Co III] 11.888 \mathrm{\mu}m feature directly traces thedecay of ^{56}Co. The spectra, line profiles, and their evolution areanalyzed with off-center delayed-detonation models. Best fits were obtainedwith White Dwarf (WD) central densities of \rho_c=0.9-1.1\times 10^9gcm^{-3}, a WD mass of M_{\mathrm{WD}}=1.33-1.35M_\odot, a WD magneticfield of ~10^6G, and an off-center deflagration-to-detonationtransition at ~ 0.5 M_\odot seen opposite to the line of sight of theobserver. The inner electron capture core is dominated by energy depositionfrom \gamma-rays whereas a broader region is dominated by positrondeposition, placing SN 2021aefx at +415 d in the transitional phase of theevolution to the positron-dominated regime. The formerly `flat-tilted' profileat 9 \mathrm{\mu}m now has significant contribution from [Ni IV], [Fe II],and [Fe III] and less from [Ar III], which alters the shape of the feature aspositrons excite mostly the low-velocity Ar. Overall, the strength of thestable Ni features in the spectrum is dominated by positron transport ratherthan the Ni mass. Based on multi-dimensional models, our analysis stronglysupports a single-spot, close-to-central ignition with an indication for apre-existing turbulent velocity field, and excludes a multiple-spot, off-centerignition.