SN 2024aecx: A Double-peaked Rapidly Evolving Type IIb Supernova at 11 Mpc

Abstract 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., Δ m 15 = 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 (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 56 Ni in the explosion was ∼0.15 M ⊙ with ejecta mass and kinetic energy of ∼0.7 M ⊙ and ∼0.16 × 10 51 erg, respectively. The observational properties and modeling indicate that the progenitor of SN 2024aecx belongs to the category of extended progenitors.