Blazar jet evolution revealed by multi-epoch broadband radio polarimetry
  We investigate the previously proposed possibility that multi-epoch broadband
polarimetry could act as a complement or limited proxy for VLBI observations of
blazars, in that the number of polarised 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 (FDS) 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 polarisation spectra of nine spatially unresolved (at arcsecond scales)
blazars in two epochs separated by $\sim$5 years, 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 polarisation
spectrum that cannot be accounted for by uncertainties in calibration or
observational effects. By associating polarised emission components across
epochs, we infer changes in their number, intrinsic fractional polarisation,
intrinsic polarisation angle, rotation measure, and depolarisation
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.
