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

Citations Over TimeTop 11% of 1992 papers

Abstract

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 <3% some of the time. 2. There is a highly significant correlation between percentage polarization and the dominance of a compact radio core as measured by R, the ratio of radio core flux density to flux density in the extended lobes. In order to include low-R objects and to increase the sample size, we have added data from the literature. We find that all 13 quasars (including 10 BL Lac objects) with log R> 2 are highly polarized while for log R < 0 this fraction is only 5%. If relativistic beaming is the explanation for the strong synchrotron emission of the radio core, then the strength of the polarized component of the optical continuum also has a strong angle dependence, suggesting that the optical flux is beamed as well. 3. The fraction of quasars with p> 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 < 1, 59% +/- 6% of the sample have p > 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 < 1. This has two immediate consequences. High optical polarization quasars and flat-spectrum core-dominant quasars are essentially the same objects. Therefore, the blazar unified scheme advocated by Blandford & Rees, and by Antonucci & Ulvestad, and the generalized unified scheme of Blandford & Konigl, Orr & Browne, and Wills & Browne, are approximately equivalent. Also, the apparent similarity in the radio and optical "beaming cones" suggests that the corresponding emission regions are closely related. The optical synchrotron radiation may originate in the radio core and in unresolved superluminal radio components producing the radio emission. There is a significantly lower incidence of high polarization among the intrinsically small compact steep-spectrum (CSS) and gigahertz peaked spectrum (GPS) sources - only two out of 23. Either their structure cannot be unified by relativistic beaming geometry, the core spectra are too steep for significant optical synchrotron emission, or the selection criteria for these sources eliminate the highly beamed examples. Finally, fewer than about 2% of the 3CR population of all FR II and compact sources are beamed in our direction, as indicated by the upper limit to the fraction of highly polarized quasars and core-dominant objects among those sources with S_ext_ > 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.

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