JWST and Ground-based Observations of the Type Iax Supernovae SN 2024pxl and SN 2024vjm: Evidence for Weak Deflagration Explosions

TL;DR

This work presents the first MIR spectra of Type Iax supernovae, using JWST to obtain early panchromatic observations of SN 2024pxl and SN 2024vjm. The data reveal a rich mix of permitted and forbidden lines, with centrally peaked MIR emission indicating well-mixed interiors consistent with weak deflagration explosions and possible bound remnants. Radiative-transfer modeling with CMFGEN shows that SN 2024pxl aligns with a near-Chandrasekhar-mass single-ignition deflagration (N1def), while SN 2024vjm favors a low-energy deflagration (N5def_hybrid) possibly accompanied by remnant energy input to reproduce the IR continuum. The results demonstrate the diagnostic power of panchromatic, particularly MIR, spectroscopy for unveiling explosion physics and highlight the need for remnant-inclusive models to fully capture the diversity of SN Iax.

Abstract

We present panchromatic optical $+$ near-infrared (NIR) $+$ mid-infrared (MIR) observations of the intermediate-luminosity Type Iax supernova (SN Iax) 2024pxl and the extremely low-luminosity SN Iax 2024vjm. JWST observations provide unprecedented MIR spectroscopy of SN Iax, spanning from $+$11 to $+$42 days past maximum light. We detect forbidden emission lines in the MIR at these early times while the optical and NIR are dominated by permitted lines with an absorption component. Panchromatic spectra at early times can thus simultaneously show nebular and photospheric lines, probing both inner and outer layers of the ejecta. We identify spectral lines not seen before in SN Iax, including [Mg II] 4.76 $μ$m, [Mg II] 9.71 $μ$m, [Ne II] 12.81 $μ$m, and isolated O I 2.76 $μ$m that traces unburned material. Forbidden emission lines of all species are centrally peaked with similar kinematic distributions, indicating that the ejecta are well mixed in both SN 2024pxl and SN 2024vjm, a hallmark of pure deflagration explosion models. Radiative transfer modeling of SN 2024pxl shows good agreement with a weak deflagration of a near-Chandrasekhar-mass white dwarf, but additional IR flux is needed to match the observations, potentially attributable to a surviving remnant. Similarly, we find SN 2024vjm is also best explained by a weak deflagration model, despite the large difference in luminosity between the two supernovae. Future modeling should push to even weaker explosions and include the contribution of a bound remnant. Our observations demonstrate the diagnostic power of panchromatic spectroscopy for unveiling explosion physics in thermonuclear supernovae.

Figures (16)

• Figure 1: Panchromatic spectra of SN 2024pxl at $+$11, $+$21, $+$37, and $+$42 days post-B$_\mathrm{max}$ compared to SN 2024vjm at $+$12 days post-B$_\mathrm{max}$. Flux density is shown in an arcsinh scaling for display purposes over a large range, and each spectrum is offset for visual clarity. Each is a combination of ground-based optical and NIR spectra with NIR and MIR spectra from JWST at similar phases. Details are given in \ref{['tab:obs']}.
• Figure 2: Line identifications for SN 2024pxl from 1.0 to 20 $$m. Permitted transitions are marked by yellow dashed lines, semiforbidden transitions are marked by purple dashed-dotted lines, and forbidden transitions are marked by gray dotted lines. Only the most dominant lines contributing to each feature are labeled.
• Figure 3: Line identifications for SN 2024vjm from 1.0 to 20 $$m. Permitted transitions are marked by yellow dashed lines, semiforbidden transitions are marked by purple dashed-dotted lines, and forbidden transitions are marked by gray dotted lines. Only the most dominant lines contributing to each feature are labeled. For comparison, SN 2024pxl at $+$37 days is plotted in light teal.
• Figure 4: Comparison of the MIR spectra of normal SN Ia 2021aefx Kwok2023, 91bg-like SN Ia 2022xkq DerKacy2024, and the SN Iax 2024pxl and 2024vjm. Notably, the SN Iax IME profiles are centrally peaked indicating well-mixed ejecta, whereas SN 2021aefx and SN 2022xkq IME profiles (particularly [Ar ii] 6.89 $$m, and [Ar iii] 8.99 $$m) are flat-topped indicating stratified ejecta.
• Figure 5: Comparison of selected relatively isolated permitted lines, some of which display a clear P-Cygni profile, in SN 2024pxl at $+$37 days (teal) and SN 2024vjm (pink) at $+$12 days. The features that are confidently associated with the labeled are given in full opacity. Regions that may be contaminated by other lines are shown in low opacity. The spectra have not been scaled or offset.