A survey for high optical polarization in quasars with core-dominant radio structure - Is there a beamed optical continuum?

We present 129 new optical linear polarization measurements of 84 quasars (defined as starlike on Sky Survey prints). We have discovered 23 new highly polarized objects (p> 3%), of which 14 are known to have broad emission lines. This represents a significant increase in the numbers of these objects that were known. With the addition of data from the literature, our measurements constitute well-defined samples of flat-spectrum core-dominant radio quasars brighter than about 18 mag, one selected from the list published by Perley (1982) and another from that by Kuhr et al. (1981). The results of this survey of 123 quasars and BL Lac objects are the following: 1. The fraction of objects that have been found to be highly polarized on at least one measurement is 45% +/- 5%. This is a lower limit to the total fraction of highly polarized quasars because the well-observed highly polarized objects of this class have p 2 are highly polarized while for log R 3% in a fixed observed passband apparently decreases with increasing redshift. Using the maximum observed values of p for each object, we find that for z 3%, compared with 24% +/- 6% for z > 1. A likely interpretation is that many of these highly polarized quasars show decreasing percentage polarization at shorter rest wavelengths where a very low polarization flat-spectrum "Big Blue Bump" component dominates over a highly polarized- steep-spectrum "synchrotron" component. Shorter rest wavelengths shift through our observing window with increasing quasar redshift. We present a simple spectrum model consisting of a steep polarized optical synchrotron component related to the radio core, and a flat, unpolarized Big Blue Bump, which explains results 1,2, and 3. The combined R and redshift dependence of polarization (or of fraction of highly polarized objects) results in a very high incidence of highly polarized objects, at least 74% +/- 8%, among those 36 objects with log R > 1.25 and z 10 Jy. If this fraction represents the radiation angle of a simple relativistic jet, the Lorentz factor, γ >= 5, is consistent with that deduced from other methods.