Physics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........26s&link_type=abstract
Thesis (PHD). GEORGE MASON UNIVERSITY , Source DAI-B 58/07, p. 3693, Jan 1998, 133 pages.
Physics
2
Scientific paper
There is a substantial amount of observational evidence for the presence of relativistic outflows from blazars and also from some galactic black hole candidates, hut there has been little, if any, work done to explain the origin of these jets from the underlying accretion disks. In particular, proton-initiated radiation processes in jets have been invoked recently (e.g., Mannheim 1993; Mannheim et al. 1996; Dar & Laor 1997) in order to account for TeV emission from blazars like Mrk 421 and Mrk 521. The origin of the energetic protons in the jet in such models is somewhat unclear, and the work done in this thesis makes a significant contribution in that direction. Specifically, this work is concerned with the general scenario of hot, two temperature accretion disks around black holes. Such accretion disks are attractive candidates for explaining high energy emission from active galactic nuclei that are believed to contain black holes. The two principal issues addressed here are: (1) The structure of the disk as determined by the microphysical viscosity mechanism; (2) The connection between the jets (relativistic outflows) and the physics of the underlying disk. The first issue is important from the point of view of understanding the physical processes governing the disk structure. Matter fed into such disks invariably has angular momentum associated with it, and a viscosity mechanism is essential for the removal of angular momentum so that matter can accrete onto the central object. While there has been some work done in the past on identifying physical processes that give rise to viscosity in accretion disks, none of the previous models give satisfactory results for conditions prevalent in hot accretion disks. There is evidence from simulations of the magnetic shearing instability by the groups of Hawley et al. (1995), Brandenburg et al. (1995) and Matsumoto & Tajima (1995) for the existence of turbulent, tangled magnetic fields embedded in accretion disks. In hot accretion disks, the interaction of energetic protons with kinks in the tangled magnetic field may provide the dominant viscosity mechanism. It also turns out that the collisions of protons with kinks in the magnetic field (which act as scattering centers embedded in the overall shear flow) results in second-order Fermi acceleration of the protons. The acceleration is effective in a tenuous corona above the disk midplane, and results in the formation of an acceleration-enhanced tail to the proton distribution. The pressure in the tail is sufficient to power a relativistic outflow that can attain bulk Lorentz factors between a few and ~10 at large distances from the central object.
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