Coupled Shock Cooling and Radioactive Heating in the Type IIb Supernova SN 2024aecx: An Extended Envelope and Rapid Optical Decline

SN~2024aecx is a nearby, rapidly evolving stripped-envelope supernova with a prominent double-peaked ultraviolet–optical light curve. We model its multiband evolution with an extended version of $\mathtt{\text{TransFit}}$, in which the early shock-cooling emission and the subsequent radioactive heating are treated within a single time-dependent radiative diffusion calculation. To describe the stratified ejecta expected for a Type~IIb progenitor, we adopt a compact inner ejecta connected to a dilute extended outer envelope and fit the outer density slope directly from the early light curve. The model reproduces the short-lived first peak, the rise to the radioactive main peak, and the overall multiband evolution. We infer an effective outer radius of $R_0=109.6_{-3.5}^{+6.6}{R}_{\odot }$, an ejecta mass of $M_{\mathrm{e}\mathrm{j}}=2.14_{-0.19}^{+0.21}{M}_{\odot }$, a nickel mass of $M_{\mathrm{N}\mathrm{i}}=0.050\pm 0.002M_{\odot }$, and a steep outer density slope of $n_{\mathrm{o}\mathrm{u}\mathrm{t}}=13.33_{-0.12}^{+0.11}$. The steep outer profile favors a low-mass extended envelope, while the low ejecta mass explains the rapid evolution of the main peak. However, a control model with standard $\gamma$-ray leakage fades too slowly after maximum. We therefore introduce an effective optical-output factor to quantify the additional late-time suppression of the ultraviolet–optical luminosity. These results support the shock-cooling plus radioactive-heating interpretation of SN~2024aecx, but show that its rapid optical decline requires physics beyond the simplest radioactive-diffusion prescription.