SN 2024aecx: A Fast-evolving Type IIb Supernova with a Prominent Shock-cooling Peak

SN 2024aecx is a nearby (∼11 Mpc) Type IIb SN discovered within ∼1 day after explosion. In this paper we report high-cadence photometric (typically 0.5 ∼ 1 day) and spectroscopic follow-up observations, conducted from as early as 0.27 day post discovery out to the nebular phase at 158.4 days. We analyze the environment of SN 2024aecx and derive a new distance (11.3 ± 1.1 Mpc), 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 postmaximum decline. The earliest spectra are blue and featureless, while from 2.3 days 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. Nebular spectral modelling indicates a blueshifted O-rich clump moving toward observer, and the [O I]/[Ca II] line ratio suggests an intermediate-mass progenitor. We simulated the progenitor and explosion using a two-component model of shock cooling and radioactive ⁵⁶Ni heating; our model favors an extended, low-mass H-rich envelope with M_e = 0.04 ± 0.01 M_⊙ and a low ejecta mass of Mej=1.55−0.14+0.18M⊙. And the nebular-phase spectra and light-curve modelling both suggest that it most likely originated from an intermediate-mass binary progenitor system. 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.