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Recent JWST observations of the Type Ia supernova (SN Ia) 2021aefx in thenebular phase have paved the way for late-time studies covering the fulloptical to mid-infrared (MIR) wavelength range, and with it the hope to betterconstrain SN Ia explosion mechanisms. We investigate whether public SN Iamodels covering a broad range of progenitor scenarios and explosion mechanismscan reproduce the full optical-MIR spectrum of SN 2021aefx at $\sim$270 dayspost explosion. We performed 1D steady-state non-LTE simulations with theradiative-transfer code CMFGEN, and compared the predicted spectra to SN2021aefx. The models can explain the main features of SN 2021aefx over the fullwavelength range. However, no single model, or mechanism, emerges as apreferred match. We discuss possible causes for the mismatch of the models,including ejecta asymmetries and ionisation effects. Our new calculations ofthe collisional strengths for Ni III have a major impact on the two prominentlines at 7.35 and 11.00 $\mu$m, and highlight the need for more accuratecollisional data for forbidden transitions. Using updated atomic data, weidentify a strong feature due to [Ca IV] 3.21 $\mu$m, attributed to [Ni I] inprevious studies. We also provide a tentative identification of a forbiddenline due to [Ne II] 12.81 $\mu$m, whose peaked profile indicates the presenceof neon all the way to the innermost region of the ejecta, as predicted forinstance in violent merger models. Contrary to previous claims, we show thatthe [Ar III] 8.99 $\mu$m line can be broader in sub-$M_\mathrm{Ch}$ modelscompared to near-$M_\mathrm{Ch}$ models. Our models suggest that key physicalingredients are missing from either the explosion models, or theradiative-transfer post-processing, or both. Nonetheless, they also show thepotential of the near- and MIR to uncover new spectroscopic diagnostics of SNIa explosion mechanisms.

Nebular spectra from Type Ia supernova explosion models compared to JWST observations of SN 2021aefx