The Role of Electric Charge in Relativistic Accretion onto Compact Objects : an Analytical Approach

Abstract

This thesis is dedicated to the discussion of the role of electric charge in relativistic accretion onto compact objects. Many high-luminosity phenomena in the observed universe can be traced back to accretion processes, in which electromagnetic fields play an important role. These fields are either produced within the accreted matter or they enter the stage as external fields like interstellar magnetic fields or fields, produced by the accreting object. The first part of the thesis examines the accretion process of hydrogen plasma onto a weakly charged black hole. An analytical model describes a stationary accretion process of dilute collisionless plasma from a rotating spherical shell onto a rotating and charged black hole. Within the model the test-particle approach is used in order to describe the plasma particle motion. The model is applied to analyze the influence of a realistic black hole net charge on plasma accretion. It is shown, that even very small black hole charges may have a non-negligible effect on the accretion process, as long as the electromagnetic field, created by the plasma, is still negligible. The inner and outer edge of the forming accretion disc strongly depend on the particle charge in the plasma. Four possible accretion disc configurations are analyzed in detail. The second part of the thesis concerns charged thick accretion discs around spinning compact objects, which are affected by an external magnetic dipole field. The situation is described in an idealized way by the Kerr metric and a magnetic dipole a test-fielda in Kerr spacetime. Pressure equations describe the charged fluid structures. The self-consistency of the model and integrability conditions of the pressure equations lead to restrictions on the fluid conductivity and the charge distribution in the structures. Previous publications discussed the limiting case of a non-rotating central object. Therefore the discussion focuses on the influence of the central objecta s spin on the existence and locations of the charged structures. Frame dragging effects result in the existence of rigidly rotating polar clouds, which do not exist in the non-rotating case. Counter-rotating equatorial tori are preferred over co-rotating ones in the case of a rapidly spinning central object.