SN 2024aecx: Double-Peaked Light Curves and Rapid Evolution in a Nearby Type IIb Supernova

Abstract: SN 2024aecx is a nearby ($\sim$11 Mpc) Type IIb SN discovered within $\sim$1 d after explosion. In this paper we report high-cadence photometric and spectroscopic follow-up observations, conducted from as early as 0.27 d post discovery out to the nebular phase at 158.4 d. We analyze the environment of SN 2024aecx and derive a new distance, metallicity and host extinction. The light curve exhibits a hot and luminous shock-cooling peak at the first few days, followed by a main peak with very rapid post-maximum decline. The earliest spectra are blue and featureless, while from 2.3 d after discovery prominent P-Cygni profiles emerge. At nebular phase, the emission lines exhibit asymmetric and double-peaked profiles, indicating asphericity and/or early dust formation in the ejecta. We simulated the progenitor and explosion using a two-component model of shock cooling and radioactive $<sup>{56}$Ni heating; our model favors an extended, low-mass H-rich envelope with$$M_{\mathrm{e}} = 0.08^{{+0.02}_{-0.03}\,} M_{\odot}$$ and a low ejecta mass of $$M_{\mathrm{ej}} = 2.65^{{+1.21}_{-0.73}} \, M_{\odot}.$$The comprehensive monitoring of SN 2024aecx, coupled with the detailed characterization of its local environment, establishes it as a benchmark event for probing the progenitors and explosion mechanisms of Type IIb SNe.