The origin of cosmic rays and TeV gamma-ray astronomy

Abstract

Cosmic rays are accelerated to high energies in Galactic and extragalactic objects like Supernova remnants (SNR) and active galactic nuclei (AGN). How these accelerators work and how e cient they accel- erate di erent types of particles to energies of 10 15 eV or beyond, is 100 years after the discovery of cosmic rays by Victor Hess, still unknown. Gamma rays trace cosmic rays at their site of acceleration and give crucial information on the nature and inner workings of these extreme objects. Gamma rays can be used to find the sources of cosmic rays and to determine their type, age and dynamics. We review in these proceedings the observational techniques and recent findings on gamma-ray emission from Supernova remnants. In 1912, the Austrian physicist Victor F. Hess discovered by means of several balloon ascents the particle popula- tion in the universe of highest energy: cosmic rays. Their energy spectrum can be described by a power law cov- ering 10 order of magnitudes in energy (up to 10 20 eV) and about 30 orders of magnitude in flux. Cosmic rays consist mainly of nuclei like protons, helium and heav- ier types up to iron. Supernova remnants (SNRs) have been the prime candidates as sources of galactic cosmic rays since the 1960s. The working hypothesis is that a 10% conversion of the blast wave's energy into relativis- tic particles during the lifetime of a SNR are enough to fill the Galaxy with cosmic rays up to about 10 15 eV. To- day, there is much unequivocal evidence for the accelera- tion of cosmic rays in SNRs from observations at di er- ent wavelengths: the expansion measurements in X-rays (1); observation of magnetic field amplification as byprod- uct of particle acceleration (2); dynamical measurements of shock positions and shock temperature (3) and non- thermal emission in gamma rays (e.g.(4)). The mechanism of di usive shock acceleration is a solid theoretical base for the production of cosmic rays by SNRs. Gamma rays can be used to trace and identify high- energy particles at or close to their acceleration site. The di erent production mechanisms for gamma rays depend on energy and type of the parent particle population and the surrounding photon, magnetic and matter fields. High- energy protons can produce gamma rays through proton- proton or proton-gamma interaction, where secondary neutral pions decay into high-energy photons. Long-living secondary particles in these interactions can initiate parti- cle cascades where photons can be produced through pair production or synchrotron radiation. High-energy leptons