Simultaneous optical and IR polarimetric observations of Blazars

Abstract

We present simultaneous UBVRIJHK photometry and polarimetry for a sample of 48 Blazars covering the period 1986Aug – 1988Feb. These data were obtained as part of a continuing monitoring program at UKIRT using the MkII Hatfield Polarimeter.

Analysis of the data shows that around ≈50% of objects have fluxes fitted by a power law over this wavelength range. Objects such as 3C345 which display polarization decreasing with frequency can be explained in terms of dilution of a synchrotron component by an unpolarized thermal component in the optical. Such a component cannot explain the ≈40% of observations where the polarization increases with frequency. No evidence is found to support any relation between the polarization and the degree of frequency dependence of polarization. In ≈30% of cases the position angle is frequency-dependent. Theoretically, wavelength-dependent behaviour proves to be a valuable tool, constraining conditions within the compact emission region of blazars and giving indications of an upper cutoff to the electron energy distribution. A frequency-dependent position angle may be evidence for emission from more than one region; the observed behaviour implies such regions must be inter-dependent.