More than one century after their discovery, the origin of cosmic rays remains one of the most important open questions in astrophysics. They are detected as a flux of charged particles, mainly protons and light nuclei, reaching extremely high energies (up to around 10^20 eV). The detection of cosmic rays implies the existence of efficient particle accelerators in the Universe. Direct observations of cosmic rays are complicated by their very nature of charged particles: as they travel from the source to Earth, magnetic fields can deflect them, preventing us from reconstructing their incoming direction. 
A powerful tool to study cosmic-ray sources is multi-messenger astronomy: cosmic rays are not produced alone, and are always accompanied by a flow of photons and neutrinos. These two messengers travel along straight lines and can be used to uniquely pinpoint the sites of cosmic ray acceleration in the Universe. 

The recent years have seen the dawn of neutrino astronomy, with neutrino observatories such as IceCube and ANTARES currently operational; the gamma-ray band is also undergoing a golden age, both in the GeV band (thanks to the Fermi satellite) and the TeV band, which is covered by ground-based Cherenkov telescopes such as H.E.S.S.. A milestone in multi-messenger astronomy has been reached in 2017, with the first evidence (at the 3 sigma level) of joint photon and neutrino emission from an active galactic nucleus (AGN), TXS 0506+056. This association, if confirmed with higher statistical significance from other events, can uniquely identify AGNs, the effect of accretion of matter onto supermassive black-holes, as cosmic-ray accelerators. 

The goal of the internship is to use data from the H.E.S.S. array of Cherenkov telescopes to search for gamma-ray counterparts of high-energy neutrino events, detected with IceCube or ANTARES. In addition, the candidate will perform multi-wavelength studies using archival online databases to identify potential lower-energy counterparts.