Résumé
While the youngest known supernova remnants, such as Cassiopeia A, have been proven to be able to accelerate cosmic rays only up to$\sim$ 10 $^{14}\,\mathrm{eV}$at their present evolutionary stages, recent studies have shown that particle energies larger than a few PeV ( $10^{15}\,\mathrm{eV}$ ) could be reached during the early stages of a core-collapse Supernova, when the high-velocity forward shock expands into the dense circumstellar medium shaped by the stellar progenitor wind. Such environments, in particular the type IIn SNe whose progenitors may exhibit mass-loss rates as high as$10^{-2}M_\odot\,\mathrm{yr}^{-1}$smith14, could thus lead to$\gamma$ -ray emission from$\pi^0$decay in hadronic interactions, potentially detectable with current Cherenkov telescopes at very-high energies. Such a detection would provide direct evidence for efficient acceleration of CR protons/nuclei in supernovae, and hence new insights on the long-standing issue of the origin of Galactic Cosmic Rays. In that context, the High Energy Stereoscopic System () has been carrying out a Target of Opportunity program since 2016 to search for such an early very-high-energy$\gamma$ -ray emission towards nearby core-collapse supernovae and supernova candidates (up to$\sim 10~\mathrm{Mpc}$ ), within a few weeks after discovery. After giving an overview of this \ Target of Opportunity program, we present the results obtained from the July 2019 observations towards the transient originally classified as a type IIn supernova, which occurred in the galaxy M74 at$\sim 9.8\,\mathrm{Mpc}$ . Although its nature still remains unclear, the derived \ constraints on this transient are placed in the general context of the expected VHE$\gamma$ -ray emission from core-collapse supernovae.