Energy Extraction Via Large-scale Magnetic Fields And Quasi-Periodic Oscillations In Black Hole Systems

Rapid variations are usually found in the radiation from black hole (BH) systems and other astrophysical compact objects. Some X-ray variations are quasi-periodic, stationary and continuing, which are called Quasi-Periodic Oscillations (QPOs). QPOs have been observed in BH binaries, intermediate mass BHs and active galactic nuclei (AGNs). QPO is believed to be an effective approach to detect the accretion process near BHs since its time scale is equivalent to the dynamical time scale in the inner region of an accretion disk. Large-scale magnetic fields play an important role in high energy astrophysics. In this thesis, a model for 3:2 high frequency QPO (HFQPO) pairs in BH binaries and a unified model for QPOs in BH systems of different scales are established, which link QPO frequencies and emitted spectra based on energy extraction via large-scale magnetic fields.The epicyclic resonance models interpret the small integer ratio of QPO frequency pairs naturally, but there remain serious uncertainties as to whether epicyclic resonance could overcome the severe damping forces in accretion flows. The severe damping can be overcome by transferring energy and angular momentum from a spinning BH to the inner disk via magnetic connection (MC) between the hole and the disk. And the MC process is helpful to understand the link between QPO frequencies and spectra. In Chapter 2, we combine epicyclic resonance and the MC process to fit the 3:2 HFQPO pairs and the corresponding spectra of three BH banaries. It turns out that the 3:2 HFQPO pairs are associated with the steep power-law (SPL) states.Observations show similarity and unity among BH binaries, intermediate mass BHs and AGNs. In Chapter 3, we attempt to establish a unified model for QPOs in BH systems of different scales. The toroidal electric currents distributed in accretion disk around a Kerr BH will generate closed large-scale magnetic fields connecting the inner and outer regions of accretion disk. We fit QPOs with the corresponding X-ray spectra in BH systems of different scales based on the MC effects on disk-corona system with magnetic reconnection of the closed field lines.Complicated phenomena have been observed in BH X-ray binaries. In Chapter 4, we attempt to describe the state transitions based on the evolution of large-scale magnetic fields and energy extraction invoking of the BZ, BP and MC processes. The power index n indicating the concentration of magnetic fields on the disk is a key parameter in state transitions. Magnetic fields with BZ and BP processes disappear successively with the decreasing n, and the low hard (LH) state transferred into hard intermediate (HIM) state, and then into SPL state. We fit the main characteristics of these states and the time scale of the transition from SPL state to thermal dominated (TD) state is roughly estimated. The 'jet line'in HID can be interpreted naturally by the critical line in a*-n parameter space.The relativistic X-ray emission line is another effective approach to detect the inner region of an accretion disk. The lines become very broad when received by distant observers due to the relativistic effects and Fe Ka lines are the most obvious. The characteristics of inner disk region may probably be understood better if we use the methods of QPOs and Fe Ka lines together. In Chaper 5, we discuss the probable connection between 3:2 HFQPOs and broad Fe Ka lines in two BH binaries based on the MC process.3:2 HFQPOs are generated by the Kepler motion of inner and outer hotspots in the disk which are produced by the external torque exerted at the inner edge of the disk and screw instability of non-axisymmetric large-scale magnetic field respectively. On the other hand, the radiation of the inner disk is very concentrative and the emissivity decreases rapidly with the increasing radius due to the torque exerted at the inner edge of the disk and the MC process. The curves of broad Fe Ka lines calculated from the steep emissivity index are in accordance with observations.