| In this dissertation we make a detailed study of several Active Galactic Nuclei (AGN's) with particular interest on the {dollar}gamma{dollar}-ray emitting blazars: 3C 273, 3C 279 and Mrk 421.; Our photometric observations of 3C 273 during January 1995 with the EUVE Lexan/B Deep Survey (DS) instrument indicate strong evidence for variability, at a 99% confidence level, during the 12 day observation period. We utilized ROSAT PSPC soft X-ray power law models to correlate with the EUVE count rates. Besides variations in the normalization level between both observations, our EUV count rates are consistent with a simple power-law model with spectral energy index {dollar}alphasim1.77 (Fsb{lcub}nu{rcub}sim{lcub}sbnusp{lcub}-alpha{rcub}{rcub}){dollar} that can be extrapolated from the soft X-rays to the EUVE range. We found that a simple Synchrotron Self-Compton Model (SSC) is unable to predict the entire spectrum in 3C 273 and fails to reproduce the predicted correlations between {dollar}gamma{dollar}-rays and radio frequencies. However, the Proton Initiated Cascade (PIC) model is able to reproduce the entire spectrum of 3C 273 under the assumption that the Big Blue Bump (BBB) is a soft target for the accelerated protons.; The SSC model is able to reproduce the entire spectrum of 3C 279 during the low and high states. Variability and correlation analysis indicates good agreement between the basic theoretical predictions of SSC models but more detailed studies of the variations are required in order to discriminate between a self and external Compton (EC) source. The PIC model is able to reproduce the continuum in 3C 279 and its temporal predictions are similar to SSC model, however, it is less predictive in terms of degree of variations. Multifrequency observation of Mrk 421 indicate that the only model that is able to explain the entire spectrum, particularly the TeV emission, is the PIC model.; When comparing the Z-transformed Discrete Correlation Function (ZDCF) algorithms with traditional algorithms to reconstruct the correlation function, applied to several AGNs data sets, we found that the ZDCF provides a reliable and more stable calculation. Also, we found that the method can handle more efficiently some of the problems and statistical biases typically found in the analysis of data sets containing unevenly sampled data. Our comparative study seems to support the assumption that BL Lacs are a qualitatively different class of object in comparison with other {dollar}gamma{dollar}-ray emitting blazars like 3C 279 and 3C 273. |
