Astroparticle Physics

110 6 Primary Cosmic RaysAn important, so far unsolved problem of astroparticlephysics is the origin of cosmic rays (see also Sects. 6.1 and6.2). Investigations of charged primary cosmic rays are es-sentially unable to answer this question because chargedparticles have to pass through extended irregular magneticfields on their way from the source to Earth. This causesthem to be deflected in an uncontrolled fashion thereby‘forgetting’ their origin. Therefore, particle astronomy withcharged particles is only possible at extremely high energieswhen the particles are no longer significantly affectedby cosmic magnetic fields. This would require to go to energiesin excess of 10 19 eV which, however, creates anotherproblem because the flux of primary particles at these energiesis extremely low. Whatever the sources of cosmic raysare, they will also be able to emit energetic penetrating γrays which are not deflected by intergalactic or stellar magneticfields and therefore point back to the sources. It must,however, be kept in mind that also X and γ rays from distantsources might be subject to time dispersions. Astronomicalobjects in the line of sight of these sources can distort theirtrajectory by gravitational lensing thus making them lookblurred and causing time-of-flight dispersions in the arrivaltime also for electromagnetic radiation.charged-particle astronomy?arrival-time dispersionsmagnetic field linessources of γ rayselectronphotonFig. 6.32Production of synchrotronradiation by deflection of chargedparticles in a magnetic fieldPossible sources for cosmic rays and thereby also for γ raysare supernovae and their remnants, rapidly rotating objectslike pulsars and neutron stars, active galactic nuclei, andmatter-accreting black holes. In these sources γ rays can beproduced by different mechanisms.a) Synchrotron radiation:The deflection of charged particles in a magnetic fieldgives rise to an accelerated motion. An accelerated electricalcharge radiates electromagnetic waves (Fig. 6.32).This ‘bremsstrahlung’ of charged particles in magneticfields is called synchrotron radiation. In circular earth-bound accelerators the production of synchrotron radiationis generally considered as an undesired energy-lossmechanism. On the other hand, synchrotron radiationfrom accelerators is widely used for structure investigationsin atomic and solid state physics as well as inbiology and medicine.Synchrotron radiation produced in cosmic magnetic fieldsis predominantly emitted by the lightweight electrons.applicationsof synchrotron radiation6.3.2 Production Mechanisms for γ Rays