Constraints on cosmic-ray acceleration ?by neutrino and gamma-ray observations ?of extragalactic targets – COCOA-NuGETs

The origin of cosmic rays represents a major missing block in our understanding of the Universe. The main challenge we face is that, being charged, they are deviated in their journey from their natural accelerator to the Earth. There is however an indirect way to study their acceleration sites: wherever a cosmic ray is accelerated to high energies, it unavoidably interacts with its environment, leading to the production of photons and neutrinos. These by-products can travel along geodesics and can thus directly point to the loci of particle acceleration in the Universe. While photon observations alone are often ambiguous, neutrinos represent a true smoking-gun for cosmic ray acceleration in astrophysical objects. This indirect window on natural particle accelerators has been opened at last with the development of neutrino astronomy and the first multi-messenger observing campaigns. We have now collected several evidences (at the 3 to 4-sigma levels) for neutrino emission from active galactic nuclei, the observational effect of accretion onto super-massive black-holes, but we still lack unambiguous detections above 5-sigmas. But the next-generation gamma-ray and neutrino telescopes are already under construction: they will drastically improve our observing capabilities, allowing us to fully enter into the multi-messenger era. The research project COCOA-NuGETs builds up and expands on the early multi-messenger results, and paves the way for the CTA era using HESS data and the first data from the first CTA telescopes. The project focuses on gamma-ray and multi-wavelength observations of neutrino counterparts and, at the same time, on developing new numerical tools for multi-wavelength/multi-messenger fitting. At the end of the four years we will have produced the first statistically sound constraints on cosmic-ray acceleration in extragalactic sources within an open source framework, and we will be ready, by 2027, to fully take advantage of the complete CTA arrays.