Observational Aspects Of Black Hole Effects On The Propagation Of Electromagnetic Radiation

Einstein introduced the theory of general relativity in 1915. It unifies special rela-tivity and Newton's law of universal gravitation. As a geometric theory of gravitation, general relativity describes gravity as the curving of spacetime. Starting around 1960s, general relativity has been under extensive experimental scrutiny mainly in the solar system which is a weak field regime. Strong fields are associated only with compact objects such as black holes or neutron stars. In 1979, Hulse and Taylor discovered that the binary pulsar PSR B1913+16 (a pair of neutron stars in which one is detected as a pulsar) has a decrease in orbital period at a rate which is consistent with the gen-eral relativity prediction of gravitational wave energy loss. This discovery provided indirect evidence of gravitational waves. Though general relativity has been verified in a series of experiments, it has not been tested in the strong field regime with high precision. One of the central difficulties is that the strong field effects are always con-taminated by uncertainties and complexities of the astrophysical systems, while in the solar system tests, the weak field effects could, in most cases, be separated from other non-gravitational effects. However, with the advances of theoretical and experimental methods, more and more observational aspects of strong field effects have been inves-tigated. The aim of this thesis is to study the observational signatures of the black hole effects in two different astrophysical systems. The first system contains a supermassive black hole and a pulsar, the strong field leaves a footprint on the intensity and timing of the pulses that have passed close to the supermassive black hole (Ch.2 and Ch.3). The second system contains a stellar mass or supermassive black hole with a warped accretion disk orbiting around it. The strong field and the spin of the black hole have an influence on the profile of the emission line produced from the disk (Ch.4). For clarity, the present thesis is organized into five chapters. In chapter 1, a brief introduction is given to general relativity, neutron stars, black holes and their accretion disks, and trajectories of particles and photons in curved spacetime. These are the theoretical background for the work to be presented in later chapters.In chapter 2, we consider that a pulsar orbits a supermassive black hole, and we in-vestigate the strong gravitational field effects on the intensity and timing of pulses that have passed close to the black hole. In the case that the spin of the black hole can be ig-nored, we have shown that all strong field effects on the pulsar beam can be understood in terms of two "universal functions," one for the bending of photon trajectories and the other for the photon travel time on these trajectories. These functions are universal in that they depend only on a single parameter, the pulsar/black hole distance from which the beam is emitted. We apply this simple formalism to the case of a pulsar in circular orbit that beams its pulses into the orbital plane. In addition to the "primary" pulses that reach the receiver by a more-or-less direct path, we find that there are secondary and higher order pulses. These are usually much dimmer than the primary pulses, but they can be of comparable or even greater intensity if they are emitted when the pul-sar is on the side of the hole furthest from the receiver. We show that there is a phase relationship of the primary and secondary pulses that is a probe of the strongly curved spacetime geometry.Analogs of these phenomena are expected in more general configurations, in which a pulsar in orbit around a black hole emits pulses that are not confined to the orbital plane. In chapter 3, we apply a "universal functions" formalism to general pulsar-hole-observer geometries, with arbitrary alignment of the pulsar spin axis and arbitrary pulsar beam direction and angular width. We show that the analysis of the observational problem has two distinct elements:(i) the computation of the location and trajectory of an observer-dependent "keyhole" direction of emission in which a signal can be received by the observer; (ii) the determination of an annulus that represents the set of directions containing beam energy. Examples of each are given along with an example of a specific observational scenario.In chapter 4, we consider that a warped accretion disk orbits a rotating (Kerr) black hole. If the disk is illuminated by hard X-rays from a non-thermal corona, fluorescent iron lines will be emitted from the inner region of the accretion disk. The emission line profiles will show a variety of strong field effects, which may be used as a probe of the spin parameter of the Kerr black hole and the structure of the accretion disk. Here we generalize the previous relativistic line profile models by including, at the same time, black hole spin effects and the non-axisymmetries of warped accretion disks. Our results show different features from the conventional calculations for either a flat disk around a Kerr black hole or a warped disk around a Schwarzschild black hole by presenting multiple peaks, rather long red tails and possible time variations of line profiles with the precession of the disk. We show disk images as seen by a distant observer, which are distorted by the strong gravity. The calculation is general and is valid for any emission lines produced from a warped accretion disk around a Kerr black hole.In chapter 5, we summarize the observational aspects of black hole effects studied in the thesis and give the prospects for the possible extension of these studies of strong field effects.