Relativistic compact systems are potential sources of UHECRs and VHE neutrinos. This, of course, requires that a considerable fraction of the dissipation energy will be tapped for acceleration of baryons to ultra-high energies, which poses a great challenge to any model. Nonetheless, this possibility is strongly motivated by the detection of cosmic rays at energies up to ~1020 eV. While cosmic rays below the knee, at energies of <1015 eV or so, are commonly believed to be accelerated by supernovae blast waves, the origin of the cosmic-ray population above the knee and, in particular, the UHECRs, is yet a mystery. Scenarios in which UHECRs and VHE neutrinos are produced in relativistic outflows, particularly GRB or blazar jets, have important implications for the jet content, the dissipation mechanism and, possibly, the physics of inner engines. In contrast to electromagnetic emission that can have either leptonic or hadronic origin, VHE neutrino emission is a unique diagnostic of hadronic content. Hence, their detection will be an important step in our understanding of compact astrophysical systems. Furthermore, it may lead to a firm identification of the mysterious astronomical origin of UHECRs.

Ultra-high energy cosmic rays

Cosmic rays were first discovered by Austrian Victor Hess in his pioneering balloon experiments in 1912. For this discovery, he was awarded the Nobel Prize in Physics 1936.