Critical Kinetic Plasma Processes In Relativistic Astrophysics

Plasma astrophysics deals with collective plasma processes in
astrophysical scenarios. As observational astronomy pushes towards
unprecedented resolutions in space and time, the focus of
theoretical research necessarily ventures towards a description of
the plasma microphysics. On microphysical scales the plasma is
pervasively collisionless and the magnetohydrodynamic approximation
breaks down. Consequently theoretical concepts rely on a kinetic
plasma description as the most sophisticated plasma model. The
present work discusses some fundamental kinetic plasma processes in
relativistic astrophysics: Fast Magnetic Reconnection (FMR)
associated with discontinuities in the magnetic field topology, and
the Coupled Two-Stream-Weibel instability (CTW) in the wake of
collisionless shocks. Both processes are ubiquitous in
astrophysical sites, prevail over competing plasma modes because of
dominant growth rates, experience significant relativistic
modifications, and develop essential features solely in the highly
non-linear regime. The computational representation invokes the
entire 6D phase space. These characteristics distinguish FMR and
the CTW as distinctively critical processes. FMR and the CTW are
studied here in the framework of self-consistent, relativistic and
fully electromagnetic Particle-In-Cell (PIC) simulations. Typical
scenarios comprise ensembles of 10^9 particles and endure for
several 10^4 time steps. The computational task is challenging and
completely in the realm of the massively parallelized architectures
of state-of-the-art supercomputers. We present the first
self-consistent 3D simulations of FMR in relativistic pair plasma.
Focusing on the mechanism of particle acceleration we show that the
highly dynamic evolution of the current sheet in the non-linear
regime is the essential stage. Therein non-stationary acceleration
zones arise in the superposition of the relativistic tearing and
the relativistic drift kink mode as competing current sheet
instabilities. Though the topology of electromagnetic fields is
highly turbulent, the FMR process shows the remarkable quality to
generate smooth and stable power-laws in the particle distribution
function (PDF) out of an initial Maxwellian. The upper PDF cut-off
in relativistic energy is determined by the ratio of light to
Alfven velocity c/v_A. The power-law index assumes s~-1 within the
reconnection X-zone irrespective of parameter variations.
Intriguingly the power-law index appears as the universal
characteristic of the source process. The associated synchrotron
spectra provide a valid description of the extremely hard spectra
and rapid variabilities of `Flat Spectrum Radio Quasars'.
Conceptual Gamma-Ray Burst (GRB) synchrotron emission models depend
on a plasma process which ensures efficient magnetic field
generation. The CTW converts bulk-kinetic energy of
counter-streaming plasma shells into Weibel magnetic fields.
Pivoted by the linear analysis of the CTW, the PIC simulations
confirm the correspondence between saturation magnetic fields and
bulk-kinetic energy. Plasma shell collisions in GRBs are either
associated with internal or external shocks. As direct consequence
of the energy dependence the CTW evolves from a complex 3D topology
in internal collisions towards quasi-2D, Weibel-dominated
conformalizations at the higher external shock energies. The PIC
results prove that the Weibel fields are sufficiently strong to
sustain synchrotron emission scenarios, particularly in external
shocks. By determining the first lifetime limits we show that
Weibel fields are also sufficiently long-lived with respect to
typical synchrotron cooling times. We further identify the
stability-limiting diffusion process as of `Bohm'-type, i.e. the
diffusion coefficient exhibits the T/B-dependence and herewith
represents a conservative stability criterion. The CTW generates
stable power-law spectra in the magnetic fields implying power-law
shaped PDFs as self-similar solutions for diffusive particle
scattering. This suggests a universal power-law index as the
characteristic of the CTW process. Imposing a magnetic guide field
of well-defined strength suppresses the Weibel contributions of the
CTW in favour of the electrostatic Two-Stream instability (TSI).
The pulsar magnetosphere is the paradigmatic scenario in which we
discuss the mechanism of Coherent Collisionless Bremsstrahlung
(CCB) triggered by the TSI. The PIC simulations show that the CCB
mechanism provides a valid description of the phenomenon of `Giant
Radio Pulses' as recently observed from the Crab pulsar.