SN 2024aecx: a fast-evolving Type IIb supernova with a prominent shock-cooling peak

TL;DR

SN 2024aecx is a nearby, early-detected Type IIb supernova whose dense UV–optical photometry and spectroscopy from discovery to 158 days allowed precise constraints on its progenitor structure and explosion. A two-component model combining shock cooling of an extended, low-mass H-rich envelope with $^{56}$Ni heating reproduces the double-peaked bolometric light curve, yielding $M_e \approx 0.04 M_\odot$, $R_e \approx 67 R_\odot$, $M_{ej} \approx 1.55 M_\odot$, and $M_{Ni} \approx 0.09 M_\odot$, and an ejecta velocity around $6500$ km s$^{-1}$. Nebular spectra reveal a blueshifted, oxygen-rich clump and a redshifted bulk component, with an [O I]/[Ca II] flux ratio near 2, pointing to an intermediate-mass progenitor likely in a binary system and an asymmetric explosion. A TRGB distance to the host and near-solar metallicity in the explosion environment, combined with significant host extinction, provide a robust environmental context, making SN 2024aecx a benchmark for probing IIb progenitors and explosion mechanisms. The results highlight the importance of early, high-cadence observations and environmental context in constraining the physics of stripped-envelope supernovae.

Abstract

SN 2024aecx is a nearby ($\sim$11 Mpc) Type IIb SN discovered within $\sim$1 d after explosion. In this paper we report high-cadence photometric (typically 0.5$\sim$1 day) and spectroscopic follow-up observations, conducted from as early as 0.27 d post discovery out to the nebular phase at 158.4 d. We analyze the environment of SN 2024aecx and derive a new distance (11.3$\pm$1.1 Mpc), metallicity and host extinction. The light curve exhibits a hot and luminous shock-cooling peak at the first few days, followed by a main peak with very rapid post-maximum decline. The earliest spectra are blue and featureless, while from 2.3 d after discovery prominent P-Cygni profiles emerge. At nebular phase, the emission lines exhibit asymmetric and double-peaked profiles, indicating asphericity and/or early dust formation in the ejecta. Nebular spectral modelling indicates a blueshifted O-rich clump moving toward observer, and the $[\text{OI}]/[\text{CaII}]$ line ratio suggests an intermediate-mass progenitor. We simulated the progenitor and explosion using a two-component model of shock cooling and radioactive $^{56}$Ni heating; our model favors an extended, low-mass H-rich envelope with $M_{\mathrm{e}} = 0.04\pm{0.01} M_{\odot}$ and a low ejecta mass of$M_{\mathrm{ej}} = 1.55^{+0.18}_{-0.14} M_{\odot}$. And the nebular-phase spectra and light-curve modelling both suggest that it most likely originated from an intermediate-mass binary progenitor system. The comprehensive monitoring of SN 2024aecx, coupled with the detailed characterization of its local environment, establishes it as a benchmark event for probing the progenitors and explosion mechanisms of Type IIb SNe.

Figures (14)

• Figure 1: SN 2024aecx and its host galaxy NGC 3521. Shown is an RGB composite assembled from LCO $g$-, $r$-, and $i$-band exposures; the inset at upper right highlights the enlarged region around the galaxy’s nucleus, with SN 2024aecx marked by a cross. The yellow and cyan squares denote the HST/ACS and VLT/MUSE fields of view, respectively. North is up and East is to the left.
• Figure 2: Top: Color–magnitude diagram of resolved stars in the outer regions of NGC 3521 from HST/ACS imaging. Each gray point represents an individual star, plotted in F814W versus F606W–F814W. The red dashed line marks the measured TRGB at F814W$_0 = 26.28$ mag, and the shaded band denotes its $\pm1\sigma$ uncertainty. The black dash–dot line indicates the approximate 50% completeness limit. Bottom: F814W luminosity function of the observed stars (solid black histogram) together with its smoothed version (blue dashed line). The red vertical dashed line and shaded region again show the TRGB and its uncertainty. Overlaid in black (right-hand axis) is the Sobel edge–detection response, whose peak identifies the TRGB.
• Figure 3: Top: VLT/MUSE H$\alpha$ flux map of the environment of SN 2024aecx in NGC 3521, calculated from the 0-th moment of the datacube and shown on a logarithmic scale. The black cross mark the SN position and the circle outlines the local H II region, within which we extracted a spectrum for analysis (see text). North is up and east is left. Bottom: Continuum‐subtracted spectrum extracted from the aperture.
• Figure 4: Normalized GTC spectrum of SN 2024aecx obtained at $t=0.7$ d, showing prominent Na i D absorption features. Black circles are the observed data, and the red curves are Gaussian fits to each line. The left component arises from the Milky Way, while the right component is from the host galaxy.
• Figure 5: Multi-band light curves of SN 2024aecx. Each filter is shifted vertically for clarity. Error bars denote $1\sigma$ photometric uncertainties.