Black holes and accretion disks in active galactic nuclei : microlensing, caustics, and collisional stellar dynamics

Citation

Rauch, Kevin Patrick (1995) Black holes and accretion disks in active galactic nuclei : microlensing, caustics, and collisional stellar dynamics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cpyq-cr33. https://resolver.caltech.edu/CaltechETD:etd-10222007-142133

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Interactions between and the structure of black holes, accretion disks, and dense star clusters are investigated. Observed rapid gravitational microlensing variability in the quasar Q2237+0305 is used in conjunction with numerical simulations of microlensed quasar accretion disks to determine whether the observations constrain theoretical accretion disk models. It is found that blackbody disks are at least three times too large to account for the observed variability, and on that basis it is argued that the optical emission is either nonthermal or optically thin. Accurate, efficient, and general-purpose routines to compute geodesic trajectories in the Kerr spacetime describing rotating black holes are implemented and applied to several problems. The optical caustic structure of the Kerr metric describing rotating black holes is determined and its possible relevance to rapid X-ray variability in active galactic nuclei is discussed. It is found that the (primary) caustic is a small tube with an asteroid cross section which extends behind the black hole asymptotically parallel to the optic axis but displaced from it by an amount proportional to the spin of the hole, and that the angular magnification is unexpectedly high everywhere inside the caustic. Sample point source light curves and the appearance of thick accretion disks around Kerr black holes are calculated and the influence of caustics on them is assessed. The dynamical evolution of the core of a dense star cluster around a Kerr black hole and under the influence of star-disk interactions is examined. It is shown that there are astrophysically plausible regimes in which star-disk interactions can dominate all other dynamical processes. The effects of star-disk interactions on single orbits are illustrated. It is found that star-disk interactions steepen the initial density profile towards an equilibrium [...] profile and simultaneously increase the central density by up to two orders of magnitude. It is argued that this process could self limit when densities climb to such a level that collisions between stars become important. Simulations of the dynamical evolution of the density cusp of a star cluster around a massive black hole in a regime where stellar collisions dominate other dynamical processes are performed. The calculations are done using a discrete cluster of stars and a fully relativistic formalism. Versatile numerical methods are developed and applied to this problem. A modified form of Kepler's Equation asymptotically valid in the Kerr geometry is derived. It is found that collisions produce a constant density core which is mainly populated by stars on highly radial orbits, in contrast to previous Fokker-Planck analyses in which an [...] profile has been found. Collisional refilling of the loss cone is seen. Additional applications of the numerical algorithms are suggested.

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