Magnetic Fields in Cosmic Particle Acceleration Sources

Abstract

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 2 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.

The microphysics of collisionless shock waves

“…The maximum growth rate γ max = k max V a,0 is obtained at a wave number k max r * = ξ/2M 2 a,0 usually 1 (small scale perturbations). The instability grows much faster than the resonant streaming instability as demonstrated in a series of analytical [21,193,167,195,196,197,198,19] and numerical studies [17,129,199,200,201,26,202,203,204,205] (see §3.4). This process of fast amplification of short-scale modes proposed by [21] can be accompanied with amplification of the long-wavelength fluctuations that would allow the effective confinement and acceleration of higher energy particles [198,203,18,168].…”

“…[165] the resolution scale corresponds to about 100 TeV CR proton gyroradii. The energy containing scale of simulated spectra of CR-driven turbulence in the case of efficient CR acceleration is comparable with or larger than the gyroradius of the maximal energy proton [166,167,129,19,18,168]. This imply a possibility to study synchrotron structures associated with turbulent magnetic fields amplified by CR driven instabilities.…”

Collisionless Shocks in Partly Ionized Plasma with Cosmic Rays: Microphysics of Non-thermal Components

“…Here L turb (x, k) represents the dissipation of turbulence, the turbulence energy injection rate is S(x, k) = γ(x, k)W (x, k, t), and γ(x, k) is the rate of wave energy amplification by the CR-driven instabilities (see for a discussion Bykov et al 2012; Schure et al Fig. 3 The profile of a CR-modified shock simulated with Monte-Carlo nonlinear DSA model by Vladimirov et al (2008).…”

Fundamentals of collisionless shocks for astrophysical application, 2. Relativistic shocks

“…These are purely current driven non-resonant non-firehose modes, very low frequency magnetic oscillations driven by the cosmic-ray current of particles which on the unstable short scales are non-magnetised. Conditions under which this theory is applicable have been discussed in depth by Bykov et al (2011a).…”