Qiang Xi, Ning-Chen Sun, David Aguado, Ismael Pérez-Fournon, F. Poidevin, Junjie Jin, Yiming Mao, Zexi Niu, Beichuan Wang, Yu Zhang, Kuntal Misra, D. K. Janghel, Justyn R. Maund, Amit Kumar, Samaporn Tinyanont, Liang-Duan Liu, Yu-Hao Zhang, Bhavya Ailawadhi, Monalisa Dubey, Zhen Guo, Anshika Gupta, Min He, Dhruv Jain, Debalina Kar, Wenxiong Li, Joe Lyman, Haiyang Mu, Kumar Pranshu, X. -J. Sun, Lingzhi Wang, Sarvesh Kumar Yadav, Yi-Han Zhao, Jie Zheng, Yinan Zhu, David López Fernández-Nespral, Alicia López Oramas, Yanan Wang, Klaas Wiersema, Jifeng Liu
Abstract 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 56 Ni 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 M ej = 1.5 5 − 0.14 + 0.18 M ⊙ . 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.