SN 2024gy: Multi-epoch Spectroscopic Evidence for Delayed Detonation in a Type Ia Supernova

Abstract: We present photometric and spectroscopic observations of SN 2024gy, a Type Ia supernova (SN Ia) exhibiting high-velocity features (HVFs) in its early-time spectra. This SN reaches a peak $B$-band magnitude of $-19.25 \pm 0.28$ mag and subsequently declines by $\Delta m_{15}(B) \approx 1.12$ mag, consistent with the luminosity-width relation characteristic of normal SNe Ia. Based on the peak thermal luminosity of $(1.2 \pm 0.3) \times 10<sup>{43}$ erg s$<sup>{-1}$, we estimate that $0.57 \pm 0.14~\rm M_{\odot}$ of $<sup>{56}$Ni was synthesized during the explosion. Our dense early spectral monitoring revealed significant velocity disparities within the ejecta. Notably, absorption features from the \CaII\ near-infrared triplet were observed at velocities exceeding 25,000 km s$<sup>{-1}$, while the \SiII\, \ld 6355 line velocity at the same epoch was significantly lower at $\sim$ 16,000 km s$<sup>{-1}$. This velocity disparity likely reflects distinct ionization states of intermediate-mass elements in the outermost layers. The prominent \CaII\, HVFs may originate from ionization suppression within the highest-velocity ejecta, potentially indicative of minimal hydrogen mixing in a delayed-detonation explosion scenario. Additionally, the Ni/Fe ratio derived from the nebular spectrum of SN 2024gy provides further support for this model.