The optical polarization of BL Lacertae objects.

Abstract

We have used the optical polarization properties of BL Lacertae objects to gain insights into the range and physical causes of their extreme and spectacular observed properties. This dissertation consists of three parts. In Part I we provide an introduction to BL Lacs and other active galactic nuclei. In Part II we present the result of an extensive monitoring program of the optical polarization of x-ray selected BL Lacs (XSBLs). In Part III we present the results of the most extensive optical polarization survey so far undertaken. Our study of the optical polarization properties of XSBLs confirms that the BL Lac candidates found in x-ray surveys like the Einstein Extended Medium Sensitivity Survey meet the requirements for membership in the class of BL Lacs. In addition to having featureless optical spectra, the majority of the XSBLs are also variable in their flux output and have intrinsic and variable polarized emission. Although x-ray selected BL Lac candidates have proven to be BL Lacs, the characteristics of their optical polarized emission are different from those of the classical radio selected BL Lacs. The XSBLs have lower maximum percent polarizations, a lower duty cycle, smaller variations in flux, and a greater tendency to have preferred angles of polarization than radio selected BL Lacs (RSBLs). We discuss the consequences of these differences and their consistency with the "beaming" model first proposed by Blandford and Rees (1978). Our optical polarization survey is the most extensive survey of its kind ever undertaken. The survey is complete to a B magnitude of 20 and covers 560 square degrees. Our sensitivity to polarized objects is a function of survey field. We did not find any confirmed BL Lacs or highly polarized quasars, but we are able to constrain the surface density of the various populations of polarized objects. For example we are able to rule out at the 90.0% confidence level the presence of a population of radio quiet BL Lacs or highly polarized quasars (with optical flux and polarization properties similar to radio selected BL Lacs) that would have a cumulative surface density of 0.03 per square degree down to a B magnitude of 20.