Viscous Evolution of Black Hole Accretion Disks

Abstract

Accretion disks around black holes are at the heart of much of the activity ob- served in active galactic nuclei and galactic microquasars. In turn, the structure of accretion disks is governed by the micro-physical viscosity mechanism that enables nearby plasma to lose its angular momentum and go accrete onto the black hole. In this work, we focus on hot, two-temperature accretion disks where the ions are collisionless. We study two possible viscosity mechanisms. In the rst model, we assume a random magnetic eld present throughout the disk and characterize it using the ion inertial length as magnetic coherence length. In the second model, we use radial di usion of ions across the large-scale toroidal magnetic eld to characterize the coe cient of viscosity. Systems with an accretion disk also often harbor powerful jets that are often episodic. We attempt to understand the disk-jet connection using the second model to interpret observations of X-ray variations from active radio galaxy 3C 120. We compare the time scale of the observed variations to the viscous time scale associated with this particular disk model. We envisage a viscous instability which results in disk collapse and possible episodic ejections of blobs. Our work represents the rst attempt to quantify this scenario with a speci c viscosity model. xi