2023ApJ...959..132N

SN 2021aefx is a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first ∼ 2 days. We present analyses of this SN using our high-cadence KMTNet multiband photometry, spectroscopy, and publicly available data, including first measurements of its explosion epoch (MJD 59529.32 ± 0.16) and onset of power-law rise (t_PL = MJD 59529.85 ± 0.55; often called first light) associated with the main ejecta ⁵⁶Ni distribution. The first KMTNet detection of SN 2021aefx precedes t_PL by ∼ 0.5 hr, indicating presence of additional power sources. Our peak-spectrum confirms its intermediate Type Ia subclassification between core-normal and broad-Line, and we estimate an ejecta mass of ∼ 1.34 M_⊙. The spectral evolution identifies material reaching >40,000 km s^–1 (fastest ever observed in Type Ia SNe) and at least two split-velocity ejecta components expanding homologously: (1) a normal-velocity (∼ 12,400 km s^–1) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (∼ 23,500 km s^–1) secondary component visible during the first ∼ 3.6 days post-explosion, which locates the component within the outer <16% of the ejecta mass. Asymmetric subsonic explosion processes producing a nonspherical secondary photosphere provide an explanation for the simultaneous appearance of the two components, and may also explain the excess emission via a slight ⁵⁶Ni enrichment in the outer ∼ 0.5% of the ejecta mass. Our 300 days post-peak nebular-phase spectrum advances constraints against nondegenerate companions and further supports a near-Chandrasekhar-mass explosion origin. Off-center ignited delayed-detonations are likely responsible for the observed features of SN 2021aefx in some normal Type Ia SNe.