Magnetic Fields in Cosmic Particle Acceleration Sources

We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. In particular, we discuss a number of cosmic-ray (CR) driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. The finite size twinkling, intermittent structures—dots, clumps, and filaments—are most apparent in the cut-off region of the synchrotron spectrum. Even though these X-ray synchrotron structures result from turbulent magnetic fields, they could still be highly polarized providing an important diagnostic of the spectrum of the turbulence. We discuss both the thermal and non-thermal observational consequences of magnetic field amplification in supernova remnants and radio-galaxies. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.