published_or_final_version / Physics / Master / Master of Philosophy

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Thesis (M. Phil.)--University of Hong Kong, 1999. / Includes bibliographical references (leaves 103-113).

Thesis (M. Phil.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 54-60).

Thesis (Ph. D.)--University of Chicago, Dept. of Astronomy and Astrophysics, June 2000. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph.D.) -- McMaster University, 2001. / Includes bibliographical references. Also available via World Wide Web.

No description available.

No description available.

We combine optical, X-ray (Einstein and ROSAT) and infrared (IRAS) data to investigate the evolution and spatial distribution of AGN with particular emphasis on the implications for the origin of the diffuse X-ray background. First, we derive the IRAS Seyfert luminosity function to test the continuity of properties between the Seyfert and the QSO population. The QSO luminosity function evolved back to z ~ 0, agrees well with the Seyfert luminosity function. In particular, the similarity of the faint parts of the two luminosity functions, suggests that the optical luminosity function is not severely affected by incompleteness due to reddening. We analyze the clustering properties of the IRAS Seyfert sample in order to probe the AGN clustering evolution. We detect clear clustering (5σ) at scales < 20 h(^-1). Comparing the Seyfert with the QSO clustering results at higher redshifts we find that a comoving model for AGN clustering evolution, where AGN clusters are expanding with the Hubble flow, is probably favoured by the data. Using new faint CCD observations we recalibrate the photometry of the Durham UVX catalogue of Boyle et al. (1990). We show that the luminosity function 'knee' feature claimed by Boyle et al. is not an artefact of photometric errors at faint magnitudes. We evaluate the contribution of Active Galactic Nuclei (AGN) to the X-ray background using this optical luminosity function and evolve it according to Pure Luminosity Evolution models. We estimate that AGN produce around half of the X-ray background at 2 keV. This contribution is consistent with the small scale (arcmin) fluctuations of the X-ray background for both the stable and comoving model of clustering evolution. If a large number of low luminosity AGN with high intrinsic absorption is missed by the optical surveys, AGN could produce all the 2 keV intensity. Although the uncertainty in the estimate of the AGN contribution is high, this work demonstrates, at least, that Pure Luminosity Evolution models are consistent with both the X-ray background intensity and anisotropy constraints. A recent deep ROSAT observation yields a high surface density of X-ray sources (> 100 deg(^-2). Spectroscopic follow up observations show that most of these sources are QSOs. The identified QSOs contribute ~ 30% at 1 keV and therefore this is the lower limit of the AGN contribution to the X-ray background. No other class of sources contributing substantially to the X-ray background has been yet identified.

Blazars are a subclass of active galactic nuclei whose observable characteristics are generated by relativistic jets of high-energy plasma with trajectories closely aligned to the line of sight. This orientation results in extreme observed phenomena, such as ultraluminous emission, high amplitudes of variability, and high degrees of optical linear polarization. Furthermore, blazars are the most common extragalactic sources of γ-ray photons and have been proposed as sources of high-energy neutrinos. Long-timescale monitoring of blazars has revealed relatively quiescent states interspersed with active states featuring dramatic brightening events, with timescales ranging from months to years. New, high-time-resolution observations of blazars are revealing dramatic variability on timescales as short as several minutes, the physical drivers of which are not well understood. In this dissertation, I focus on observational signatures that can potentially identify these mechanisms. To characterize this variability, I combine observations of blazars from many telescopes, obtaining a comprehensive view of events occurring in the jets. Using the Very Long Baseline Array, I investigate moving and quasi-stationary features in the parsec-scale jets of 38 blazars. I find that the flow speeds, orientation, brightness temperatures, and opening angles of the jets in flat-spectrum radio quasars (FSRQs), BL Lacertae type objects (BL Lac objects), and radio galaxies (RGs) are statistically different, with FSRQs having the highest speeds and smallest viewing angles. Focusing on a typical source of each subclass, I characterize the optical variability observed at 2-min cadence (with the Transiting Exoplanet Survey Satellite), at gamma-ray energies (Fermi Large Area Telescope), X-ray energies (NICER, NuSTAR, and Swift satellites), and optical (ground-based instruments, especially, the Perkins telescope), including linear polarization measurements. I find that the minimum timescale of RG variability is longer than those of the FSRQ and BL Lac object, and is likely caused by changes in the accretion disk. In contrast, the variability in the FSRQ and BL Lac object can be associated with shocks, magnetic reconnections, and turbulence in the jet. The unprecedented availability of data, which promises to expand in the future, represents a new phase of observational astronomy that provides valuable information on changes in the jets of blazars.