High-energy processes in hot plasmas: Applications to advection-dominated accretion flows around supermassive black holes

We investigate several high energy processes which occur in hot plasmas, and apply the results to advection-dominated accretion flows (ADAFs) around supermassive black holes. Chapter 1 provides a background to accretion flows in the context of supermassive black holes in Active Galactic Nuclei (AGN). We discuss theoretical models that use various high energy emission mechanisms to explain the observed spectra from these systems. Chapter 2 considers one such mechanism, cyclo-synchrotron radiation from electrons at mildly-relativistic and relativistic temperatures in a magnetic field. We present emission spectra for electron temperatures between {dollar}5times 10sp8 - 3times 10sp{lcub}10{rcub}{dollar} K, and present simple fitting formulae that give a fairly accurate approximation to the detailed results. These formulae are used in Chapter 3 to discuss general scaling laws for ADAFs. We calculate, primarily analytically, characteristic spectra from ADAFs, and use them to solve the problem, posed by Fabian & Canizares, of whether bright elliptical galaxies host dead quasars. Chapter 4 extends the theoretical treatment of emission spectra of ADAFs to include the production of gamma-rays. The gamma-rays are produced by the decay of neutral pions that are created in proton-proton collisions. The resulting spectrum is applied to an ADAF model of the Galactic Center, and a fairly good agreement with the observed gamma-ray spectrum is found. This agreement requires a non-thermal power law proton energy distribution. Chapter 5 discusses conditions under which the particle distributions are likely to be thermal or non-thermal. For all accretion rates, Coulomb collisions are too inefficient to thermalize the protons; the proton distribution function is therefore determined by the viscous heating mechanism, which is unknown. For accretion rates greater than 10{dollar}sp{lcub}-3{rcub}{dollar} of the Eddington accretion rate, the electrons are thermalized by both Coulomb collisions and the emission and absorption of synchrotron photons, throughout the flow. We discuss the possible consequences of these results for the spectrum from an ADAF. In Chapter 6 we conclude and propose future work.