2025MNRAS.541.1048R

Stripped-envelope supernovae (SESNe) display a wide range of photometric and spectroscopic behaviours, often reflecting complex progenitor evolution. SN 2015ap is a type Ib event located in the nearby galaxy IC 1776, previously modelled as powered by radioactive decay and possibly a magnetar engine. In this work, we revisit its multiband photometry and spectroscopy, gathering all publicly available observational data for this source, to investigate the nature of its progenitor and power source. We use an innovative time analysis method based on Gaussian process, leveraging its ability to model both noise and periodic components in unevenly sampled data without requiring regular sampling. We detect significant periodic modulations in the post-peak light curve, with a characteristic time-scale of $\sim$8.4 d. These modulations are also seen in the H α line velocity, suggesting a structured circumstellar medium (CSM) shaped by binary interaction. We model the light curve with semi-analytical prescriptions (MOSFIT), including CSM and central engine components, and derive an ejecta mass of $\sim$2.2-2.4 $\mathrm{M}_\odot$, explosion energy of $\sim 3.4\times 10^{51}$ erg, and a $^{56}$Ni mass of $\sim$0.11 $\mathrm{M}_\odot$. The colour evolution indicates an additional energy injection, consistent with either prolonged breakout or delayed central powering. While the data are compatible with a weak magnetar contribution, the overall evidence favours a binary progenitor system, with non-conservative mass transfer shaping the observed CSM. SN 2015ap thus adds to the growing sample of SESNe where binarity plays a central role in driving both the explosion and its observables.