2026ApJ...997...77Z

We present the results of low-resolution spectroscopic and densely sampled multiband photometric follow-up of supernova (SN) 2024aecx. The SN was discovered in the spiral galaxy NGC 3521 (distance ∼ 11 Mpc) within a day after the explosion. The early spectra of SN 2024aecx show a weak signature of hydrogen lines, which disappeared in ∼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 emission from shock cooling. The early-phase evolution of the light curve of SN 2024aecx is similar to the typical Type IIb events, but the decay rate in different bands (e.g., Δm₁₅ = 1.60 ± 0.05 mag, g band) is significantly faster in the postpeak phase. It attained the secondary maximum in ∼19 days (g band) with a peak absolute magnitude of M_g = -17.94 ± 0.10 mag. The colors of SN 2024aecx tend to be redder in early epochs (<8 days), then it grows bluer for a period, then redder again >20 days after explosion. The analytical model fitting to the light curves reveals an envelope mass and progenitor radius in the ranges ∼0.03-0.24 M_⊙ and ∼169-200 R_⊙, respectively. Modeling of the pseudobolometric light curve suggests that synthesized ⁵⁶Ni in the explosion was ∼0.15 M_⊙ with ejecta mass and kinetic energy of ∼0.7 M_⊙ and ∼0.16 × 10⁵¹ erg, respectively. The observational properties and modeling indicate that the progenitor of SN 2024aecx belongs to the category of extended progenitors.