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 (<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 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.