10  Introduction

Cosmic ray accelerated are typically surrounded by a radiation field and matter eventually interacting and producing secondary particles. The picture is similar to that of beam dumps in particle accelerators at Earth, where a beam of particles is aimed at a specific target. Among the secondary particles produced we have gamma-rays and neutrinos both of them will point to the accelerators.

A cosmic-ray source it is said to be opaque or optically thick if photons and protons cannot escape from the source without lossing all or most of their energy. Alternatively a transparent source it is said to be optically thin. A source that is very optically thick won’t be visible in any messenger except neutrinos. We call those source hidden sources.