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 simultaneous optical imaging ($ugi$ and $vrz$ bands) follow-up of supernova (SN) 2024aecx. The photometric data is supplemented with $Swift$/UVOT and ATLAS survey observations. The SN was discovered in the spiral galaxy NGC 3521 (distance $\sim$11 Mpc) within a day after the explosion. The early spectra of SN 2024aecx show a weak signature of hydrogen lines, which disappeared in $\sim$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 shock cooling emission. The early phase light curve evolution of SN 2024aecx has similarity with the typical Type IIb events, but the decay rate in different bands (e.g., $\rm \Delta m_{15}$ = 1.60 $\pm$ 0.05 mag, $g$-band) is significantly faster in the post-peak phase. It attained the secondary maximum in $\sim$19 days ($g$-band) with a peak absolute magnitude of M$<em>{g}$= -17.94 $\pm$ 0.10 mag. The color evolution of SN 2024aecx is displaying a red-blue-red trend between days $\sim$8 to 40. The analytical model fitting to the light curves reveals an envelope mass and progenitor radii in the range of $\sim$0.03 - 0.24 $M</em>\odot$ and $\sim$169 - 200 $R_\odot$, respectively. Modeling of the pseudo-bolometric light curve suggests that synthesized $<sup>{56}$Ni in the explosion was $\sim$0.15 M$<em>{\odot}$ with ejecta mass and kinetic energy of $\sim$0.7 M$</em>{\odot}$ and $\sim$0.16 x 10$<sup>{51}$ erg, respectively. The observational properties and modeling indicate that the SN 2024aecx progenitor belongs to the extended progenitor category.