Early light curve excess in Type IIb supernovae observed with ATLAS

Context. Type IIb supernovae (SNe IIb) often exhibit an early light curve excess (EE) preceding the main peak powered by 56Ni decay. The physical origin of this early emission remains an open question. Among the proposed scenarios, shock cooling (SC) emission, resulting from the interaction of the shock wave with extended envelopes, is considered the most plausible mechanism. However, the occurrence rate of such events has yet to be reliably constrained. Aims. This study aims to quantify the frequency of EE in SNe IIb and investigate its physical origin by analysing optical light curves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, ultimately providing qualitative constraints on their progenitor systems. Methods. We identified 74 potential SNe IIb from 153 spectroscopically classified events reported in the Transient Name Server (TNS), observed by ATLAS with peak fluxes exceeding 150 μJy (18.46 mag) and explosion epoch uncertainties below six days. Using a spectral reclassification method, we selected a sample of 66 SNe IIb and a cleaned sample of 59 SNe IIb for analysis. We then applied light curve model fitting and outlier analysis to identify objects exhibiting EE emission and studied their photometric properties. Results. We identify 20 SNe IIb with EE, corresponding to a frequency of approximately 30.5% to 50%, the higher value being obtained under the most stringent observational data-quality cuts. The duration and colour evolution of the early excess support its interpretation as shock cooling in extended envelopes. We also find that EE SNe IIb exhibit faster post-peak declines than non-EE events, while both groups show similar peak absolute magnitudes and rise-time distributions. Conclusions. Our findings suggest that EE and non-EE SNe IIb likely share similar initial progenitor masses but differ in their ejecta properties, potentially due to varying degrees of binary interaction. This study constrains EE SNe frequency and photometric properties, paving the way for future theoretical work, such as hydrodynamical modelling of EE SNe light curves, which could corroborate these results and contribute to constraining the evolutionary pathways of SNe IIb progenitor systems.