Modelling Relativistic Astrophysics at the Large and Small Scale

TL;DR

This thesis develops a new numerical approach for relativistic astrophysics, including a general relativistic magnetohydrodynamics code and insights into collisionless shock phenomena relevant to gamma-ray bursts and jets.

Contribution

It introduces a novel formulation of relativistic MHD equations and provides new understanding of magnetic field generation and electron acceleration in collisionless shocks.

Findings

Generated strong small-scale magnetic fields via two-stream instability.

Proposed a new local electron acceleration mechanism in shocks.

Linked plasma conditions to observed radiation signatures.

Abstract

In this thesis different numerical methods, as well as applications of the methods to a number of current problems in relativistic astrophysics, are presented. In the first part the theoretical foundation and numerical implementation of a new general relativistic magnetohydrodynamics code is discussed. A new form of the equations of motion using global coordinates, but evolving the dynamical variables from the point of view of a local observer is presented. No assumptions are made about the background metric and the design is ready to be coupled with methods solving the full Einstein equations. In the second part of the thesis important results concerning the understanding of collisionless shocks, obtained from experiments with a relativistic charged particle code, are presented. Relativistic collisionless shocks are important in a range of astrophysical objects; in particular in gamma ray burst afterglows and other relativistic jets. It is shown that a strong small scale, fluctuating, and predominantly transversal magnetic field is unavoidably generated by a two-stream instability. The magnetic energy density reaches a few percent of equipartition. A new acceleration mechanism for electrons in ion-electron collisionless shocks is proposed. The mechanism is capable of creating a powerlaw electron distribution in a collisionless shocked region. The non-thermal acceleration of the electrons is directly related to the ion current channels generated by the two-stream instability and is local in nature. Thus the observed radiation field may be tied directly to the local conditions of the plasma and could be a strong handle on the physical processes. (abridged)