SN 2024gy: Multi-epoch Spectroscopic Features Suggestive of Delayed Detonation in a Type Ia Supernova

TL;DR

SN 2024gy is a normal-looking Type Ia supernova that nonetheless shows striking high-velocity Ca II HVFs in its early spectra, with $v_{Ca II} > 2.5\times 10^{4}$ km s$^{-1}$ and a relatively slower Si II photospheric velocity of $v_{Si II} \\approx 1.6\times 10^{4}$ km s$^{-1}$, indicating pronounced ejecta ionization stratification. From the peak luminosity $L_{ ext{peak}} = (1.2 \\pm 0.3) \ imes 10^{43}$ erg s$^{-1}$ and a peak magnitude $M_B \\approx -19.25 \\pm 0.29$ mag, the synthesized $^{56}$Ni mass is $M(^{56}$Ni$) = 0.57 \\pm 0.14\,M_$, and nebular-phase Ni/Fe ratios corroborate a delayed-detonation explosion with limited hydrogen mixing. Dense early-time spectroscopy revealing distinct HVFs and velocity stratification, together with consistent light-curve width and Ni mass estimates, supports a delayed-detonation explosion scenario for SN 2024gy and offers insights into the ionization and composition structure of outer ejecta in normal SNe Ia. These results refine our understanding of HVF origins and the diversity of SN Ia explosion channels, with implications for using SNe Ia as precise cosmological distance indicators.

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 $-19.25 \pm 0.29$ mag and subsequently declines by $Δm_{15}(B) \approx 1.12$ mag, consistent with the luminosity-width relation characteristic of normal SNe Ia. Based on the peak thermal luminosity of $(1.2 \pm 0.3) \times 10^{43}$ erg s$^{-1}$, we estimate that $0.57 \pm 0.14~\rm M_{\odot}$ of $^{56}$Ni was synthesized during the explosion. Our dense early spectral monitoring revealed significant velocity disparities within the ejecta. Notably, absorption features from the Ca II near-infrared triplet were observed at velocities exceeding 25,000 km s$^{-1}$, while the Si II $λ$6355 line velocity at the same epoch was significantly lower at $\sim$ 16,000 km s$^{-1}$. This velocity disparity likely reflects distinct ionization states of intermediate-mass elements in the outermost layers. The prominent Ca II 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.

Figures (2)

• Figure 1: Lijiang 2.4 m telescope color composite (B/g/r) image of SN 2024gy and its host galaxy NGC 4216.
• Figure 2: Optical light curves of SN 2024gy. Phase is relative to the time of $B$-band maximum light. Data in different filters are shown in different colors and shifted vertically for better display. Mephisto bands are marked with asterisks to distinguish them from SDSS bands. The dashed lines are the best fits of the SALT2 model. Regions with different colors are the 1$\sigma$ confidence interval for the best fits.