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University of Maryland Department of Physics 1117 John S. Toll Physics Building College Park , MD 20742



xxxxxxxxxxxxSeptember 2006 - Issue 49


David Garofalo---Relativistic Astrophysics

If we look into the distant universe, we find evidence for various phenomena such as star-forming regions in galaxies, creation of elements in supernovae, rapid pulses of high energy radiation coming from neutron stars and more. We also find evidence for small regions of space within which the largest amounts of energy are released anywhere in the universe. Although the source of this energy still eludes us in a fundamental sense, theory and observation appear to be converging on a paradigm whose primary components are rotating black holes surrounded by strong magnetic fields. But, within this paradigm, a fundamental and up to now unanswered question arises. What process carries the large magnetic field toward the black hole?

Two complications must be overcome. First, the horizon cannot support currents to generate magnetic fields in the sense of the surface of a star. Any astrophysical process occurring on the horizon is outside of causal contact with the outside universe thereby making it impossible for currents on the horizon to generate magnetic fields outside of it. This means that the currents generating the fields must be maintained near but outside of the horizon. The only place where such currents could be maintained is an accretion disk. Accretion disks seem to be ubiquitous and even our solar system displays the signature of such systems with all planets rotating in one plane in the same direction. In its most simple form, an accretion disk is a collection of stuff in rotation in a plane about a central object, usually ionized gas, susceptible therefore to the formation of currents and thus of magnetic fields. As the gas loses angular momentum and accretes toward the black hole, currents migrate along with the gas, thereby dragging the magnetic field toward the hole.

This is the basic physics behind the origin of magnetic fields around black holes. Unfortunately, basic magnetohydrodynamics, or the study of Maxwell's equations in an astrophysical environment, indicates that such a dragging of the magnetic field toward the black hole is subject to a rather strong diffusion process, thereby making it impossible to reach the required strengths around the black hole. My advisor, Chris Reynolds, a high energy astrophysicist in the Astronomy department, developed a model that uses certain previously ignored relativistic features of black hole accretion to overcome the large diffusion of the magnetic field. My job is to extend this model to the relativistic regime, specifically to the Kerr metric. The goal of this project, therefore, is to see if we can answer the most fundamental question about the paradigm that explains the largest energy emission in the universe in terms of a highly relativistic interaction between a black hole and a magnetic field.