Blazar jet evolution revealed by multi-epoch broad-band radio polarimetry

Abstract

We investigate the previously proposed possibility that multi-epoch broad-band polarimetry could act as a complement or limited proxy for very long baseline interferometry observations of blazars, in that the number of polarized emission components in the jet, and some of their properties and those of the foreground environment, might be inferred from the object's time-varying 1D Faraday depth spectrum alone. We report on a pilot-scale experiment designed to establish the basic plausibility and utility of this idea. We analyse temporal changes in the complex polarization spectra of nine spatially unresolved (at arcsecond scales) blazars in two epochs separated by ∼5 yr, using data taken with the Australia Telescope Compact Array. The data allow for precise modelling, and we demonstrate that all objects in our sample show changes in their polarization spectrum that cannot be accounted for by uncertainties in calibration or observational effects. By associating polarized emission components across epochs, we infer changes in their number, intrinsic fractional polarization, intrinsic polarization angle, rotation measure, and depolarization characteristics. We attribute these changes to evolution in the structure of the blazar jets, most likely located at distances of up to tens of parsecs from the central active galactic nuclei. Our results suggest that continued work in this area is warranted; in particular, it will be important to determine the frequency ranges and temporal cadence most useful for scientifically exploiting the effects.