Optical-infrared synchrotron emission in luminous AGN

Abstract

We use high and variable linear polarization as a signature of synchrotron emission in the optical spectra of luminous AGN. Our new and other published measurements of a sample of 122 core-dominant radio quasars, including BL Lac objects (Wills, Antonucci, Barvainis, Breger and Wills 1988) show:(ί) more than half have a significant synchrotron component, with degree of polarization, p, > 3%, (ίί) there is a very significant decrease in the fraction of highly-polarized quasars with increasing redshift, simply explained by a decrease in p with decreasing rest wavelength in many core-dominant quasars. This result is readily explained by a quiescent, unpolarized quasar optical spectrum combined with a steep (α 1.5 to 2) optical synchrotron continuum, and (ίίί) there is a highly significant increase in p with increasing dominance of a compact radio core — if the radio core emission is relativistically beamed, then the optical synchrotron component is also beamed. This explanation of (ί), (ίί), and (ίίί) in terms of a two-component IR-UV continuum is borne out convincingly by measurement of the wavelength dependence of fluxdensity and polarization for individual core-dominant AGN at U, B, V, R, 1, J, H, K, (L′) (Wills, Hough, Bailey, Wills and Ballard 1988)—we have discovered steep ( α −1.8) IRoptical synchrotron components, distinct from the flatter, unpolarized Big Blue Bump (accretion disk?) and 3 pin μm components, in 6 of 18 “normal” quasars measured so far (including 3C 273).