The Origin and Relationship of the Radio, Infrared and Visual Radiation from Compact Nonthermal Sources

by Jeffrey John Puschell

Under the supervision of Professor Thomas W. Jones


The results of a study of the flux variability and polarization of compact extragalactic sources are presented. The studied set of 17 objects includes two class 1 Seyfert galaxies (3C 120 and NGC 4151), on class 2 Seyfert (NGC 1068), one broad line radio galaxy (3C 382), two QSO' (III Zw 2 and 3C 273) an 11 BL Lac objects (0109+224, AO 0235+164, 0300+471, Pks 0735+178, OI 090.4, Oj 287, Mk 421, 1308+326, 1418+546, Mk 501 and BL Lac). As BL Lac objects show the most extreme forms of both flux variability and polarization - important characteristics of presumed nonthermal activity - the attention of this dissertation is directed primarily at these sources in the hope they provide the best indications of the intrinsic nature of compact extragalactic nonthermal sources.
The spectral-flux distributions of the observed sources are generally flat or peaked in the radio portion of the spectrum with a steep decline into the infrared region. For most of the objects observed, this decline continues into the visual region. But, for some of the sources, notably III Zw 2 and 3C 373, the visual spectral-flux distribution cannot be joined smoothly to the infrared, suggesting different physical mechanisms are responsible for the observed emission in each region.
The variations at infrared wavelengths were clearly in phase with variations at visual wavelengths, for the BL Lac objects in the sample, implying a common physical origin for the fluxes in both spectral regions. But, for some objects such as 3C 120, which show clear evidence of thermal components in the form of emission lines and associated galactic nebulosity in the visual-infrared, the variations were not in phase and are consistent with a thermally re-radiating dust origin for the near-infrared fluxes.
Variations in the visual-infrared spectral shape were observed during changes in brightness for six sources. The most dramatic changes in the visual spectral index occurred for AO 0235+164 and Pks 0735+178, the only BL Lac objects for which strong absorption lines have been observed. The possibility that these changes may be explained by varying dust extinction in the form of a dust cloud passing through or condensing in the line of sight is explored. The changes in the infrared spectral index found for 1308+326 could be due to opacity effects occurring during outbursts.
A high degree of polarization was observed in the visual, infrared and frequently in the millimeter wavelength regions of the BL Lac objects in this study, indicating a high degree of ordering in the emission regions due to a nonisotropic magnetic field. The degree of polarization generally increased from the radio to visual. For some sources, the position angle of the plane of polarization was nearly the same at the highest radio frequencies as at visual frequencies, implying a direct relationship between the mechanisms responsible for the emission over this wide frequency range.
Episodes of wavelength dependent polarization and/or short time scale (-10^3 seconds) variations may occur during transitions in the polarized flux - especially if the degree of polarization changes appears to rule out static isotropically radiating synchrotron model. The observations are consistent with a relativistic motion of emission regions toward the observer. Observed rotations of the position angle may be explained by episodes of Faraday rotation occurring during outbursts in a relativistically moving source.