---
title: 'SN 2024aecx: Stripped-Envelope Supernova'
url: https://www.emergentmind.com/topics/sn-2024aecx
type: topic
---

# SN 2024aecx: Stripped-Envelope Supernova

SN 2024aecx is a nearby supernova in NGC 3521 with unusually early discovery, dense multiband photometric coverage, and spectroscopy extending from the first days after explosion to the nebular phase. The published literature summarized here is not taxonomically uniform: two optical studies describe it as a Type IIb event with weak hydrogen signatures, double-peaked light curves, and rapid post-maximum fading, whereas a later near-infrared study describes it as a hydrogen- and helium-poor Type Ic and interprets its late near-infrared continuum as an infrared echo from circumstellar dust [2509.12343] [2505.19831] [2602.02691]. Across these analyses, SN 2024aecx is treated as an observationally important stripped-envelope core-collapse supernova with unusually strong early shock-cooling emission, rapid evolution, and evidence for asymmetry and dust-related phenomena.

## 1. Discovery, host galaxy, and local environment

SN 2024aecx was discovered on 2024 Dec 16 in the spiral galaxy NGC 3521. One study adopts an explosion epoch of JD 2460659.95, taken as the midpoint between the last non-detection at MJD 60659.35 and the discovery at MJD 60660.56, and reports the discovery magnitude as $m_o = 14.68 \pm 0.01$ mag in the ATLAS orange band [2505.19831]. Another study states that the supernova was discovered within $\sim 1$ d after explosion and followed from as early as 0.27 d post discovery to 158.4 d [2509.12343].

Distance estimates differ modestly across the literature. A tip-of-the-red-giant-branch measurement gives $F814W_0 = 26.28 \pm 0.21$ mag and, adopting the Rizzi et al. (2007) zero point, a distance modulus
$$
\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},
$$
corresponding to $D = 11.3 \pm 1.1$ Mpc [2509.12343]. A separate optical study adopts $D = 11.40 \pm 0.56$ Mpc, or $\mu = 30.28 \pm 0.11$ mag [2505.19831]. The near-infrared study instead quotes a distance of $13.8 \pm 2.7$ Mpc and uses pre-explosion ZTF non-detections to constrain the explosion to MJD 60660.0 [2602.02691].

The local environment has also been characterized in detail. A VLT/MUSE spectrum of an H II region at $R \approx 80$ pc yields line ratios [O III] $\lambda 5007/{\rm H}\beta$ and [N II] $\lambda 6584/{\rm H}\alpha$, and the O3N2 calibration of Marino et al. (2013),
$$
12 + \log({\rm O/H}) = 8.533 - 0.214 \times {\rm O3N2},
$$
gives $8.49 \pm 0.18$ dex, approximately solar [2509.12343]. Extinction estimates are likewise study-dependent. One analysis adopts Milky Way foreground extinction $A_V^{\rm MW} = 0.144$ mag and internal extinction toward the supernova of $A_V^{\rm host} \simeq 1.2$ mag for $R_V = 3.1$, while noting that the Balmer decrement ${\rm H}\alpha/{\rm H}\beta = 5.33$ would imply $A_V \simeq 1.95$ mag for the ionized gas [2509.12343]. Another adopts $E(B-V)_{\rm total} = 0.45 \pm 0.10$ mag with $E(B-V)_{\rm MW} = 0.05$ mag and $E(B-V)_{\rm host} \simeq 0.40$ mag [2505.19831], while the near-infrared analysis corrects optical colors using $E(B-V)_{\rm MW}=0.05$ mag, host $E(B-V)=0.32\pm0.06$ mag, and $R_V=1.4\pm0.2$ [2602.02691].

## 2. Double-peaked photometric evolution

The defining photometric property of SN 2024aecx is a distinct double-peaked light-curve morphology. One study describes a hot shock-cooling flash within the first $\simeq 4$ days, followed by a broader radioactive $^{56}$Ni-powered peak at $t_{\rm peak}\simeq 15$ d in $u$ and $\simeq 20$ d in $r,i,z$ [2509.12343]. A second optical analysis gives secondary-peak epochs of $t_p(u)=16.72$ d, $t_p(g)=19.34$ d, $t_p(r)=20.30$ d, $t_p(i)=20.75$ d, and $t_p(z)=22.56$ d, and identifies shock-cooling minima ranging from 7.30 d in $u$ to 3.78 d in $z$ [2505.19831].

The early and main maxima are both luminous. Approximate apparent magnitudes for the initial flash include Swift $U \simeq 13.98$ and ATLAS $o \simeq 14.71$ at $t=0$, while the main peak reaches $m_r \simeq 14.4$ mag, corresponding to $M_r \simeq -16.9$ mag using $\mu$ and $A_V^{\rm tot}=1.34$ mag [2509.12343]. In the $g$ band, the secondary maximum is reported as $M_g({\rm max})=-17.94 \pm 0.10$ mag, with $M_o({\rm max})=-17.56 \pm 0.10$ mag as an $o$-band proxy for $r/i$ [2505.19831].

The post-maximum decline is unusually fast for a Type IIb interpretation. Over 40 d after maximum, the $r$-band decline is $\Delta m_r \simeq +4$ mag, or about $0.10$ mag d$^{-1}$ [2509.12343]. Consistently, the optical study reports $\Delta m_{15}(g)=1.60 \pm 0.05$, $\Delta m_{15}(r)=1.16 \pm 0.04$, $\Delta m_{15}(i)=1.00 \pm 0.05$, $\Delta m_{15}(z)=1.14 \pm 0.04$, and $\Delta m_{15}(o)=1.40 \pm 0.02$, and interprets these fast declines as implying a low ejecta mass and short photon-diffusion time [2505.19831].

Bolometric reconstructions retain the same two-component structure. The shock-cooling peak reaches $L_{\rm bol} \gtrsim 10^{42.5}$ erg s$^{-1}$ at $t\approx 0$, while the main peak reaches $L_{\rm bol} \simeq 10^{42.2}$ erg s$^{-1}$ at $t\approx 20$ d [2509.12343]. A pseudobolometric construction from UV and optical data gives a shock-cooling minimum of $\log L = 42.60 \pm 0.03$ erg s$^{-1}$ at $t=0$, declining to $\log L = 41.65 \pm 0.02$ at $\sim 5$ d, and a radioactive peak at $t = 20.75 \pm 0.30$ d with $\log L_{\rm peak}=42.39 \pm 0.02$ erg s$^{-1}$ [2505.19831].

Color evolution is also structured. One study identifies a red-blue-red sequence between days $\sim 8$ and 40 [2505.19831]. More specifically, Phase I (0–8 d) shows redward evolution as the ejecta cools from $\sim 14$ kK to $\sim 6.5$ kK; Phase II (8–18 d) shows a blueward turnaround in $g-r$, $g-i$, and $v-r$; Phase III (18–40 d) becomes redward again; and Phase IV ($>40$ d) shows colors settling [2505.19831]. This suggests a direct interplay between envelope cooling and radioactive reheating during the transition from the first to the second peak.

## 3. Spectroscopic evolution and classification ambiguity

Early optical spectroscopy shows a very blue continuum with weak or absent strong line structure. At $t=0.3$–1.2 d, the spectrum is described as featureless and blue, showing only interstellar Na I D absorption with Galactic and host components and no narrow flash lines [2509.12343]. Low-resolution optical spectra obtained from $+1.9$ d through $+54$ d show a very blue continuum at early phases, with a blackbody temperature of $T \simeq 14{,}600$ K at $+1.9$ d from SYNAPPS and weak broad absorptions in the H$\alpha$/Si II 6355 Å region, He I 5876 Å, and Ca II H \& K [2505.19831].

From $t\ge 2.3$ d, prominent P-Cygni features emerge. Reported lines include H$\alpha$ blended with Si II $\lambda 6355$, Ca II H\&K, the Ca II infrared triplet, Mg II $\lambda 4481$, Fe II $\lambda\lambda 4924,5018,5169$, and He I $\lambda 5876$ [2509.12343]. The intermediate-phase optical sequence between $+8$ and $+22$ d also shows progressive development of He I, Fe II, Sc II, Ti II, and the Ca II NIR triplet, while at phases later than $\sim 30$ d the H lines vanish and He and metal lines dominate [2505.19831].

Absorption-minimum velocities indicate fast early expansion and steady decline. The earliest $v({\rm H}\alpha)$ and $v({\rm Ca\,II\,IR})$ are $\simeq 12{,}000$ km s$^{-1}$, while $v({\rm He\,I}\,\lambda 5876)$ and $v({\rm Fe\,II})$ are $\simeq 10{,}000$ km s$^{-1}$ and $v({\rm Mg\,II})$ is $\simeq 7{,}000$ km s$^{-1}$ [2509.12343]. All lines decline roughly as $v \propto t^{-0.2}$ until $\simeq 20$ d, and Fe II $\lambda 5169$ indicates $v_{\rm ph} \simeq 6{,}500$ km s$^{-1}$ near the main peak [2509.12343]. A separate analysis reports $v_{\rm ph}=14{,}000$ km s$^{-1}$ at early times, with H$\alpha$ decreasing from $14{,}000 \rightarrow 10{,}000 \rightarrow 7{,}000$ km s$^{-1}$ from day 1 to peak to 30 d, He I 5876 Å from $12{,}000 \rightarrow 5{,}000$ km s$^{-1}$, and Fe II from $7{,}000 \rightarrow 5{,}000$ km s$^{-1}$ [2505.19831].

The near-infrared study introduces a different taxonomic description. It presents SN 2024aecx as a hydrogen- and helium-poor Type Ic, reports that the early NIR spectra are typical for a SN Ic, and emphasizes deep neutral carbon absorption at 0.909, 0.962, 1.069, 1.133, 1.261, 1.316, and 1.689 $\mu$m with absorption minima up to $v\sim 15{,}000$ km s$^{-1}$, alongside Ca II and Mg I but with no sign of unambiguous He I or H I lines [2602.02691]. A plausible implication is that the classification of SN 2024aecx depends strongly on phase, wavelength coverage, and the weighting assigned to weak, rapidly disappearing hydrogen and helium signatures.

## 4. Nebular asymmetry and dust-related signatures

Nebular spectroscopy extends the phenomenology beyond the early stripped-envelope classification problem. Spectra at 71.5, 123.5, and 158.4 d show strong forbidden and semi-forbidden emission from [O I] $\lambda\lambda 6300,6364$, [Ca II] $\lambda\lambda 7291,7324$, and the Ca II infrared triplet $\lambda\lambda 8498,8542,8662$ [2509.12343]. These profiles are asymmetric and clearly double-peaked [2509.12343].

The interpretation advanced for the nebular line shapes is explicitly non-unique. One possibility is large-scale ejecta asphericity, such as a bipolar or toroidal $^{56}$Ni/oxygen distribution, following the class of interpretations discussed by Maeda et al. (2008) [2509.12343]. An alternative is differential internal extinction by newly formed dust in the receding hemisphere, following the type of mechanism discussed by Bevan and Barlow (2016) [2509.12343]. No single analytic fitting formula is applied in that work, although standard two-component Gaussian fits are referenced as the usual means of quantifying peak separations and asymmetry [2509.12343].

The bolometric evolution is consistent with this ambiguity. The late-time decline at $t \gtrsim 40$ d is reported to fall faster than the pure radioactive model even when $\gamma$-ray leakage is allowed, and this is interpreted as hinting at early dust formation and/or multi-dimensional $\gamma$-ray escape [2509.12343]. The coexistence of double-peaked nebular profiles and a late decline steeper than simple radioactive expectations places geometry and dust on equal footing in the interpretation.

## 5. Explosion and progenitor modeling

Two optical analyses model the light curves with different semi-analytic frameworks and obtain substantially different progenitor parameters. One adopts a two-component MCMC fit to the bolometric light curve consisting of a shock-cooling component based on Piro et al. (2021) and a radioactive $^{56}$Ni-heating component following the Arnett diffusion formalism [2509.12343]. In this treatment, the shock-cooling luminosity is written as
$$
L_{\rm shock}(t) \simeq 2\times 10^{42}
\left(\frac{R_e}{10^{13}\,{\rm cm}}\right)
E_{51}^{0.85}
M_{10}^{-0.69}
\left(\frac{t}{1\,{\rm d}}\right)^{-0.35}
\ {\rm erg\,s^{-1}}.
$$
The fitted parameters are $R_e = 36.8^{+50.6}_{-13.2}\,R_\odot$, $M_e = 0.08^{+0.02}_{-0.03}\,M_\odot$, $E_e = 2.65^{+2.49}_{-1.93}\times10^{49}$ erg, $M_{\rm ej} = 2.65^{+1.21}_{-0.73}\,M_\odot$, and $M_{\rm Ni} = 0.13^{+0.04}_{-0.03}\,M_\odot$, with a diffusion timescale $\tau_m \simeq 10$–15 d [2509.12343].

A separate analysis fits only the shock-cooling phase ($<5$ d) with the Sapir and Waxman (2017) polytropic model, testing both $n=3$ and $n=3/2$ envelope structures, and then models the pseudobolometric light curve at $t>5$ d with an Arnett-style radioactive model [2505.19831]. This yields, for $n=3$, $R_{\rm prog}=169.15^{+1.23}_{-1.27}\,R_\odot$, $M_{\rm env}=0.24^{+0.0003}_{-0.0004}\,M_\odot$, and $v_{\rm shock}=2.35^{+0.003}_{-0.003}\times10^9$ cm s$^{-1}$; for $n=3/2$, $R_{\rm prog}=200.35^{+0.73}_{-0.70}\,R_\odot$, $M_{\rm env}=0.03^{+0.00003}_{-0.00003}\,M_\odot$, and $v_{\rm shock}=1.55^{+0.002}_{-0.002}\times10^9$ cm s$^{-1}$ [2505.19831]. Its radioactive model gives $M_{\rm Ni}=0.15\pm0.06\,M_\odot$, $M_{\rm ej}=0.70^{+0.18}_{-0.16}\,M_\odot$, and $E_k=(0.16\pm0.05)\times10^{51}$ erg [2505.19831].

| Quantity | Two-component MCMC | Sapir/Arnett analysis |
|---|---:|---:|
| Extended-envelope mass | $0.08^{+0.02}_{-0.03}\,M_\odot$ | $0.03$–$0.24\,M_\odot$ |
| Radius | $36.8^{+50.6}_{-13.2}\,R_\odot$ | $169.15$–$200.35\,R_\odot$ |
| Ejecta mass | $2.65^{+1.21}_{-0.73}\,M_\odot$ | $0.70^{+0.18}_{-0.16}\,M_\odot$ |
| Nickel mass | $0.13^{+0.04}_{-0.03}\,M_\odot$ | $0.15\pm0.06\,M_\odot$ |

Despite these differences, the physical picture is similar in outline. Both studies favor an extended, low-mass hydrogen-rich envelope and relatively low diffusion mass, and both connect the rapid rise and rapid decline to efficient diffusion in a stripped-envelope progenitor [2509.12343] [2505.19831]. This suggests that the main uncertainties concern the quantitative scale of the envelope radius and ejecta mass rather than the existence of an extended, low-mass outer layer.

## 6. Circumstellar dust echo and mass-loss history

A later near-infrared campaign adds a distinct circumstellar component to the interpretation of SN 2024aecx. Spectra from $-18$ to $+60$ d relative to peak, obtained with Gemini North/GNIRS, IRTF/SpeX, Keck II/NIRES, Keck I/MOSFIRE, and Gemini South/F2, show that by $+32$ d the supernova developed a strong broad NIR continuum, described as a strong NIR excess emerging 32 days post peak and unprecedented in normal SNe Ic [2602.02691]. The optical colors, however, remain consistent with normal SN Ic templates through $+60$ d after correction for the adopted Milky Way and host reddening [2602.02691].

The NIR excess is fit with a single-temperature, optically thin dust model,
$$
L_d(t)=4\pi\,M_d(t)\int_0^\infty \kappa_\nu B_\nu[T_d(t)]\,d\nu,
$$
assuming 0.1 $\mu$m carbonaceous grains [2602.02691]. Best-fit parameters are $T_d(32\,{\rm d}) = 1127.0\pm0.1$ K with $M_d(32\,{\rm d})=(0.66\pm0.06)\times10^{-4}\,M_\odot$, $T_d(43\,{\rm d}) = 1024.0\pm0.2$ K with $M_d(43\,{\rm d})=(1.30\pm0.17)\times10^{-4}\,M_\odot$, and $T_d(60\,{\rm d}) = 869.8\pm0.4$ K with $M_d(60\,{\rm d})=(2.60\pm1.10)\times10^{-4}\,M_\odot$ [2602.02691]. The temporal evolution is fit by
$$
T_d(t)\propto (t-t_{\rm peak})^\alpha,\quad \alpha \simeq -0.6\pm0.1,
$$
and
$$
M_d(t)\propto (t-t_{\rm peak})^\beta,\quad \beta \simeq 2.0\pm0.3,
$$
while the integrated IR luminosity remains roughly constant at
$$
L_d \approx (1.2\mbox{--}1.6)\times10^{41}\ {\rm erg\,s^{-1}}.
$$

The study argues against newly formed ejecta dust because the required dust mass, $10^{-4}$–$10^{-3}\,M_\odot$, appears within 1–2 months post peak, which is stated to be far too early for condensation in the cooling ejecta in the absence of very strong shock interaction, and because an optically thick dust shell would obscure the optical SN, which is not observed [2602.02691]. Instead, it interprets the excess as an IR echo from pre-existing circumstellar dust. The prompt emergence at $+32$ d corresponds to a dust-paraboloid latus-rectum radius
$$
r_{\rm echo}\simeq c\,(t-t_{\rm peak})\approx 5\times10^{16}\ {\rm cm},
$$
and only the main SN peak, with duration $\sim 10$ d and luminosity $L\sim2.5\times10^{42}$ erg s$^{-1}$, is considered capable of heating dust at these distances to $\sim 1000$ K while remaining optically thin [2602.02691].

Among the geometries considered, a face-on, geometrically thick disk with half-opening angle $\sim 20^\circ$ is favored. The onset of the NIR excess between $+12$ d and $+32$ d implies an inner edge
$$
R_{\rm in}\simeq (3\mbox{--}8)\times10^{16}\ {\rm cm}
\quad\longrightarrow\quad
R_{\rm in}=(5.7\pm2.6)\times10^{16}\ {\rm cm},
$$
and, assuming $v_{\rm shock}\approx 1.5\times10^9$ cm s$^{-1}$, the shock is predicted to reach this radius at
$$
t_{\rm int}=(440\pm200)\ {\rm days\ after\ explosion}.
$$
The inferred gas density is $\rho_{\rm gas}\sim 5\times10^3$ cm$^{-3}$, and for a $\sim 100$ km s$^{-1}$ wind this implies $\dot M\sim10^{-4}\,M_\odot\,{\rm yr}^{-1}$ in the final $\sim 10^2$ yr before explosion [2602.02691]. The study notes that such close-in circumstellar material is unexpected in simple case B binary stripping, which occurs $10^4$–$10^5$ yr before core collapse [2602.02691].

## 7. Position within stripped-envelope supernova studies

Within the Type IIb literature, SN 2024aecx is interpreted as an extended progenitor event with a tiny or low-mass hydrogen envelope. One analysis states that the envelope parameters $M_e \simeq 0.08\,M_\odot$ and $R_e \simeq 37\,R_\odot$ argue for a binary-stripped progenitor retaining only a dilute hydrogen skin, explicitly comparing it with SN 1993J, SN 2011dh, and SN 2016gkg [2509.12343]. Another places the inferred progenitor radius in the range $\sim 170$–200 $R_\odot$ and the envelope mass in the range $\sim 0.03$–0.24 $M_\odot$, describing the object as an “extended” Type IIb and noting that the weak H$\alpha$ disappears by $\sim 30$ d [2505.19831].

The event is singled out for its observational completeness. It is described as very nearby at 11.3 Mpc, discovered within $\sim 1$ d of explosion, and showing one of the most luminous and swift shock-cooling peaks ever recorded [2509.12343]. The dense early coverage, multicolor photometry, and nebular-phase asymmetries are said to establish it as a benchmark event for probing progenitors and explosion mechanisms of Type IIb supernovae [2509.12343]. In the near-infrared study, by contrast, it is presented as the first Type Ic with a prompt NIR echo, with the detection of circumstellar dust at $\sim 5\times10^{16}$ cm offering a new probe of the last centuries of mass loss in massive-star progenitors [2602.02691].

The combined record therefore supports two complementary uses of SN 2024aecx in current research. First, it is a high-cadence case study of double-peaked stripped-envelope light curves, rapid diffusion, and early spectral evolution. Second, it is a laboratory for late-time asymmetry, dust-related radiative transfer, and circumstellar structure. The unresolved tension between the Type IIb and Type Ic descriptions is itself informative: a plausible implication is that SN 2024aecx occupies a boundary region in stripped-envelope phenomenology where the apparent subtype is sensitive to the temporal and spectral window of observation rather than to a single unambiguous spectral snapshot.

Source: https://www.emergentmind.com/topics/sn-2024aecx