SN 2024aecx: A double-peaked rapidly evolving Type IIb supernova at 11 Mpc

TL;DR

SN 2024aecx is a nearby, double-peaked Type IIb supernova in NGC 3521 that was discovered within a day of explosion. The authors present a densely sampled, multi-wavelength photometric and spectroscopic dataset, enabling a detailed view of the shock-cooling phase and the subsequent nickel-powered peak. Through shock-cooling modeling and pseudo-bolometric light-curve fitting, they infer an extended progenitor with envelope mass $M_{en}\sim0.03$–$0.24\ M_\odot$ and radius $R\sim169$–$200\ R_\odot$, and estimate a synthesized nickel mass $M_{Ni}\approx0.15\ M_\odot$, ejecta mass $M_{ej}\approx0.70\ M_\odot$, and kinetic energy $E_{K}\approx0.16\times10^{51}$ erg. These results place SN 2024aecx among luminous IIb events with an extended progenitor, contributing to the broader view of the IIb/Ib/c continuum and highlighting the importance of early, multi-band data for constraining progenitor structure and explosion physics.

Abstract

We present the results of low-resolution spectroscopic and densely sampled multi-band photometric follow-up of supernova (SN) 2024aecx. The SN was discovered in the spiral galaxy NGC 3521 (distance $\sim$11 Mpc) within a day after the explosion. The early spectra of SN 2024aecx show a weak signature of hydrogen lines, which disappeared in $\sim$30 days after the explosion. Light curves in all bands show a distinct feature of two peaks, and the first peak is likely due to the shock cooling emission. The early phase light curve evolution of SN 2024aecx has similarity with the typical Type IIb events, but the decay rate in different bands (e.g., $\rm Δm_{15}$ = 1.60 $\pm$ 0.05 mag, $g$-band) is significantly faster in the post-peak phase. It attained the secondary maximum in $\sim$19 days ($g$-band) with a peak absolute magnitude of M$_{g}$ = -17.94 $\pm$ 0.10 mag. SN 2024aecx colors trend redder in early epochs ($<$8 days), followed by a duration in which it grows bluer, then later gets redder again $>$20 days after explosion. The analytical model fitting to the light curves reveals an envelope mass and progenitor radii in the range of $\sim$0.03 - 0.24 $M_\odot$ and $\sim$169 - 200 $R_\odot$, respectively. Modeling of the pseudo-bolometric light curve suggests that synthesized $^{56}$Ni in the explosion was $\sim$0.15 M$_{\odot}$ with ejecta mass and kinetic energy of $\sim$0.7 M$_{\odot}$ and $\sim$0.16 $\times$ 10$^{51}$ erg, respectively. The observational properties and modeling indicate that the SN~2024aecx progenitor belongs to the extended progenitor category.

Figures (14)

• Figure 1: SN 2024aecx and the host galaxy NGC 3521. Mephisto's three bands ($u,g,r$) were used to create this RGB image. The zoomed central region of the host is shown in the inset, and SN is indicated with a cross-hair. The displayed image size is $\sim$10 $\times$ 10.
• Figure 2: Multi-band light curve evolution of SN 2024aecx with respect to the date of explosion. The 3$\sigma$ upper limits (observations taken before the SN explosion, see Section \ref{['obs_sp']}) in different bands at the SN location are displayed in the left panel. The template-subtracted SN apparent magnitudes are shown in the middle and right panels. Offsets (indicated in the legend) are applied for clarity. The epoch of explosion is shown with the dashed (red) line. The vertical line segments (black) at the bottom of the middle panel mark the epochs of spectroscopic observations. (The data used to create this figure are available in the online article.)
• Figure 3: The absolute magnitudes ($V$ or $g$ bands) of SNe Ib, Ic, and IIb are compared with SN 2024aecx in the top-left, top-right, and lower-left panels, respectively. The comparison sample is taken from 2018taddia and the literature (see Section \ref{['lc-prop']}). The $V/g$$\rm \Delta m_{15}$ parameter and absolute $V/g$ band magnitudes are plotted in the bottom-right panel.
• Figure 4: The color evolution ($u-g, g-r, g-i, v-r$ and $r-z$) of SN 2024aecx is displayed with different symbols. The colors are corrected for the total reddening $E(B-V )$ = 0.45 mag (see, Section \ref{['reddening']}).
• Figure 5: The BB temperature and radius evolution of SN 2024aecx (top panel), and the pseudobolometric luminosity (bottom panel).