Optical Observations on the Young Type Ia SN 2021fxy with Detached High Velocity Features

We present optical observations on the young type Ia supernova (SN Ia) SN 2021fxy obtained within a few days after the explosion, with a focus on its prominent high-velocity features (HVFs). It reached a B -band maximum of M max ( B ) = − 19.36 ± 0.31 mag, corresponding to a bolometric luminosity of ∼1.3 × 10 43 erg s −1 with a synthesized 56 Ni mass of 0.58 ± 0.14 M ⊙ . The early spectra exhibit strong HVFs of intermediate-mass elements that are significantly detached from the photospheric components. In particular, the velocity of the Si II λ 6355 HVFs follows a power-law evolution ( β ≈ 0.1), shallower than the expected photospheric velocity evolution expected for an assumed n = 10 density profile ( β ≈ 0.22) under homologous expansion. This behavior is consistent with the HVFs forming in intrinsic ejecta structures at least partially decoupled from the bulk outer ejecta, providing a possible constraint on the explosion physics of SNe Ia.