SN 2024aecx: Stripped-Envelope Supernova

• SN 2024aecx is a nearby stripped-envelope supernova characterized by a double-peaked light curve, early shock-cooling emission, and a rapid post-maximum decline.
• Its multiwavelength photometry and spectroscopy reveal classification ambiguities between Type IIb and Type Ic, highlighting transient hydrogen features and later infrared signatures.
• Modeling and infrared echo analysis indicate an extended, low-mass progenitor envelope with significant circumstellar dust, offering insights into explosion asymmetry and mass-loss history.

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 (Xi et al., 15 Sep 2025, Zou et al., 26 May 2025, Tinyanont et al., 2 Feb 2026). 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 (Zou et al., 26 May 2025). 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 (Xi et al., 15 Sep 2025).

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 (Xi et al., 15 Sep 2025). A separate optical study adopts $D = 11.40 \pm 0.56$ Mpc, or $\mu = 30.28 \pm 0.11$ mag (Zou et al., 26 May 2025). 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 (Tinyanont et al., 2 Feb 2026).

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] $\sim 1$0, and the O3N2 calibration of Marino et al. (2013),

$\sim 1$1

gives $\sim 1$2 dex, approximately solar (Xi et al., 15 Sep 2025). Extinction estimates are likewise study-dependent. One analysis adopts Milky Way foreground extinction $\sim 1$3 mag and internal extinction toward the supernova of $\sim 1$4 mag for $\sim 1$5, while noting that the Balmer decrement $\sim 1$6 would imply $\sim 1$7 mag for the ionized gas (Xi et al., 15 Sep 2025). Another adopts $\sim 1$8 mag with $\sim 1$9 mag and $F814W_0 = 26.28 \pm 0.21$0 mag (Zou et al., 26 May 2025), while the near-infrared analysis corrects optical colors using $F814W_0 = 26.28 \pm 0.21$1 mag, host $F814W_0 = 26.28 \pm 0.21$2 mag, and $F814W_0 = 26.28 \pm 0.21$3 (Tinyanont et al., 2 Feb 2026).

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 $F814W_0 = 26.28 \pm 0.21$4 days, followed by a broader radioactive $F814W_0 = 26.28 \pm 0.21$5Ni-powered peak at $F814W_0 = 26.28 \pm 0.21$6 d in $F814W_0 = 26.28 \pm 0.21$7 and $F814W_0 = 26.28 \pm 0.21$8 d in $F814W_0 = 26.28 \pm 0.21$9 (Xi et al., 15 Sep 2025). A second optical analysis gives secondary-peak epochs of $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$0 d, $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$1 d, $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$2 d, $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$3 d, and $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$4 d, and identifies shock-cooling minima ranging from 7.30 d in $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$5 to 3.78 d in $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$6 (Zou et al., 26 May 2025).

The early and main maxima are both luminous. Approximate apparent magnitudes for the initial flash include Swift $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$7 and ATLAS $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$8 at $$\mu = 5\log_{10}(D/10\,{\rm pc}) = 30.27 \pm 0.20 \ {\rm mag},$$9, while the main peak reaches $D = 11.3 \pm 1.1$0 mag, corresponding to $D = 11.3 \pm 1.1$1 mag using $D = 11.3 \pm 1.1$2 and $D = 11.3 \pm 1.1$3 mag (Xi et al., 15 Sep 2025). In the $D = 11.3 \pm 1.1$4 band, the secondary maximum is reported as $D = 11.3 \pm 1.1$5 mag, with $D = 11.3 \pm 1.1$6 mag as an $D = 11.3 \pm 1.1$7-band proxy for $D = 11.3 \pm 1.1$8 (Zou et al., 26 May 2025).

The post-maximum decline is unusually fast for a Type IIb interpretation. Over 40 d after maximum, the $D = 11.3 \pm 1.1$9-band decline is $D = 11.40 \pm 0.56$0 mag, or about $D = 11.40 \pm 0.56$1 mag d$D = 11.40 \pm 0.56$2 (Xi et al., 15 Sep 2025). Consistently, the optical study reports $D = 11.40 \pm 0.56$3, $D = 11.40 \pm 0.56$4, $D = 11.40 \pm 0.56$5, $D = 11.40 \pm 0.56$6, and $D = 11.40 \pm 0.56$7, and interprets these fast declines as implying a low ejecta mass and short photon-diffusion time (Zou et al., 26 May 2025).

Bolometric reconstructions retain the same two-component structure. The shock-cooling peak reaches $D = 11.40 \pm 0.56$8 erg s$D = 11.40 \pm 0.56$9 at $\mu = 30.28 \pm 0.11$0, while the main peak reaches $\mu = 30.28 \pm 0.11$1 erg s$\mu = 30.28 \pm 0.11$2 at $\mu = 30.28 \pm 0.11$3 d (Xi et al., 15 Sep 2025). A pseudobolometric construction from UV and optical data gives a shock-cooling minimum of $\mu = 30.28 \pm 0.11$4 erg s$\mu = 30.28 \pm 0.11$5 at $\mu = 30.28 \pm 0.11$6, declining to $\mu = 30.28 \pm 0.11$7 at $\mu = 30.28 \pm 0.11$8 d, and a radioactive peak at $\mu = 30.28 \pm 0.11$9 d with $13.8 \pm 2.7$0 erg s$13.8 \pm 2.7$1 (Zou et al., 26 May 2025).

Color evolution is also structured. One study identifies a red-blue-red sequence between days $13.8 \pm 2.7$2 and 40 (Zou et al., 26 May 2025). More specifically, Phase I (0–8 d) shows redward evolution as the ejecta cools from $13.8 \pm 2.7$3 kK to $13.8 \pm 2.7$4 kK; Phase II (8–18 d) shows a blueward turnaround in $13.8 \pm 2.7$5, $13.8 \pm 2.7$6, and $13.8 \pm 2.7$7; Phase III (18–40 d) becomes redward again; and Phase IV ($13.8 \pm 2.7$8 d) shows colors settling (Zou et al., 26 May 2025). 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 $13.8 \pm 2.7$9–1.2 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 (Xi et al., 15 Sep 2025). Low-resolution optical spectra obtained from $R \approx 80$0 d through $R \approx 80$1 d show a very blue continuum at early phases, with a blackbody temperature of $R \approx 80$2 K at $R \approx 80$3 d from SYNAPPS and weak broad absorptions in the H$R \approx 80$4/Si II 6355 Å region, He I 5876 Å, and Ca II H & K (Zou et al., 26 May 2025).

From $R \approx 80$5 d, prominent P-Cygni features emerge. Reported lines include H$R \approx 80$6 blended with Si II $R \approx 80$7, Ca II H&K, the Ca II infrared triplet, Mg II $R \approx 80$8, Fe II $R \approx 80$9, and He I $\lambda 5007/{\rm H}\beta$0 (Xi et al., 15 Sep 2025). The intermediate-phase optical sequence between $\lambda 5007/{\rm H}\beta$1 and $\lambda 5007/{\rm H}\beta$2 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 $\lambda 5007/{\rm H}\beta$3 d the H lines vanish and He and metal lines dominate (Zou et al., 26 May 2025).

Absorption-minimum velocities indicate fast early expansion and steady decline. The earliest $\lambda 5007/{\rm H}\beta$4 and $\lambda 5007/{\rm H}\beta$5 are $\lambda 5007/{\rm H}\beta$6 km s$\lambda 5007/{\rm H}\beta$7, while $\lambda 5007/{\rm H}\beta$8 and $\lambda 5007/{\rm H}\beta$9 are $\sim 1$00 km s$\sim 1$01 and $\sim 1$02 is $\sim 1$03 km s$\sim 1$04 (Xi et al., 15 Sep 2025). All lines decline roughly as $\sim 1$05 until $\sim 1$06 d, and Fe II $\sim 1$07 indicates $\sim 1$08 km s$\sim 1$09 near the main peak (Xi et al., 15 Sep 2025). A separate analysis reports $\sim 1$10 km s$\sim 1$11 at early times, with H$\sim 1$12 decreasing from $\sim 1$13 km s$\sim 1$14 from day 1 to peak to 30 d, He I 5876 Å from $\sim 1$15 km s$\sim 1$16, and Fe II from $\sim 1$17 km s$\sim 1$18 (Zou et al., 26 May 2025).

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 $\sim 1$19m with absorption minima up to $\sim 1$20 km s$\sim 1$21, alongside Ca II and Mg I but with no sign of unambiguous He I or H I lines (Tinyanont et al., 2 Feb 2026). 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] $\sim 1$22, [Ca II] $\sim 1$23, and the Ca II infrared triplet $\sim 1$24 (Xi et al., 15 Sep 2025). These profiles are asymmetric and clearly double-peaked (Xi et al., 15 Sep 2025).

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 $\sim 1$25Ni/oxygen distribution, following the class of interpretations discussed by Maeda et al. (2008) (Xi et al., 15 Sep 2025). 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) (Xi et al., 15 Sep 2025). 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 (Xi et al., 15 Sep 2025).

The bolometric evolution is consistent with this ambiguity. The late-time decline at $\sim 1$26 d is reported to fall faster than the pure radioactive model even when $\sim 1$27-ray leakage is allowed, and this is interpreted as hinting at early dust formation and/or multi-dimensional $\sim 1$28-ray escape (Xi et al., 15 Sep 2025). 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 $\sim 1$29Ni-heating component following the Arnett diffusion formalism (Xi et al., 15 Sep 2025). In this treatment, the shock-cooling luminosity is written as

$\sim 1$30

The fitted parameters are $\sim 1$31, $\sim 1$32, $\sim 1$33 erg, $\sim 1$34, and $\sim 1$35, with a diffusion timescale $\sim 1$36–15 d (Xi et al., 15 Sep 2025).

A separate analysis fits only the shock-cooling phase ($\sim 1$37 d) with the Sapir and Waxman (2017) polytropic model, testing both $\sim 1$38 and $\sim 1$39 envelope structures, and then models the pseudobolometric light curve at $\sim 1$40 d with an Arnett-style radioactive model (Zou et al., 26 May 2025). This yields, for $\sim 1$41, $\sim 1$42, $\sim 1$43, and $\sim 1$44 cm s$\sim 1$45; for $\sim 1$46, $\sim 1$47, $\sim 1$48, and $\sim 1$49 cm s$\sim 1$50 (Zou et al., 26 May 2025). Its radioactive model gives $\sim 1$51, $\sim 1$52, and $\sim 1$53 erg (Zou et al., 26 May 2025).

Quantity	Two-component MCMC	Sapir/Arnett analysis
Extended-envelope mass	$\sim 1$54	$\sim 1$55–$\sim 1$56
Radius	$\sim 1$57	$\sim 1$58–$\sim 1$59
Ejecta mass	$\sim 1$60	$\sim 1$61
Nickel mass	$\sim 1$62	$\sim 1$63

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 (Xi et al., 15 Sep 2025, Zou et al., 26 May 2025). 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 $\sim 1$64 to $\sim 1$65 d relative to peak, obtained with Gemini North/GNIRS, IRTF/SpeX, Keck II/NIRES, Keck I/MOSFIRE, and Gemini South/F2, show that by $\sim 1$66 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 (Tinyanont et al., 2 Feb 2026). The optical colors, however, remain consistent with normal SN Ic templates through $\sim 1$67 d after correction for the adopted Milky Way and host reddening (Tinyanont et al., 2 Feb 2026).

The NIR excess is fit with a single-temperature, optically thin dust model,

$\sim 1$68

assuming 0.1 $\sim 1$69m carbonaceous grains (Tinyanont et al., 2 Feb 2026). Best-fit parameters are $\sim 1$70 K with $\sim 1$71, $\sim 1$72 K with $\sim 1$73, and $\sim 1$74 K with $\sim 1$75 (Tinyanont et al., 2 Feb 2026). The temporal evolution is fit by

$\sim 1$76

and

$\sim 1$77

while the integrated IR luminosity remains roughly constant at

$\sim 1$78

The study argues against newly formed ejecta dust because the required dust mass, $\sim 1$79–$\sim 1$80, 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 (Tinyanont et al., 2 Feb 2026). Instead, it interprets the excess as an IR echo from pre-existing circumstellar dust. The prompt emergence at $\sim 1$81 d corresponds to a dust-paraboloid latus-rectum radius

$\sim 1$82

and only the main SN peak, with duration $\sim 1$83 d and luminosity $\sim 1$84 erg s$\sim 1$85, is considered capable of heating dust at these distances to $\sim 1$86 K while remaining optically thin (Tinyanont et al., 2 Feb 2026).

Among the geometries considered, a face-on, geometrically thick disk with half-opening angle $\sim 1$87 is favored. The onset of the NIR excess between $\sim 1$88 d and $\sim 1$89 d implies an inner edge

$\sim 1$90

and, assuming $\sim 1$91 cm s$\sim 1$92, the shock is predicted to reach this radius at

$\sim 1$93

The inferred gas density is $\sim 1$94 cm$\sim 1$95, and for a $\sim 1$96 km s$\sim 1$97 wind this implies $\sim 1$98 in the final $\sim 1$99 yr before explosion (Tinyanont et al., 2 Feb 2026). The study notes that such close-in circumstellar material is unexpected in simple case B binary stripping, which occurs $F814W_0 = 26.28 \pm 0.21$00–$F814W_0 = 26.28 \pm 0.21$01 yr before core collapse (Tinyanont et al., 2 Feb 2026).

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 $F814W_0 = 26.28 \pm 0.21$02 and $F814W_0 = 26.28 \pm 0.21$03 argue for a binary-stripped progenitor retaining only a dilute hydrogen skin, explicitly comparing it with SN 1993J, SN 2011dh, and SN 2016gkg (Xi et al., 15 Sep 2025). Another places the inferred progenitor radius in the range $F814W_0 = 26.28 \pm 0.21$04–200 $F814W_0 = 26.28 \pm 0.21$05 and the envelope mass in the range $F814W_0 = 26.28 \pm 0.21$06–0.24 $F814W_0 = 26.28 \pm 0.21$07, describing the object as an “extended” Type IIb and noting that the weak H$F814W_0 = 26.28 \pm 0.21$08 disappears by $F814W_0 = 26.28 \pm 0.21$09 d (Zou et al., 26 May 2025).

The event is singled out for its observational completeness. It is described as very nearby at 11.3 Mpc, discovered within $F814W_0 = 26.28 \pm 0.21$10 d of explosion, and showing one of the most luminous and swift shock-cooling peaks ever recorded (Xi et al., 15 Sep 2025). 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 (Xi et al., 15 Sep 2025). 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 $F814W_0 = 26.28 \pm 0.21$11 cm offering a new probe of the last centuries of mass loss in massive-star progenitors (Tinyanont et al., 2 Feb 2026).

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.