The Spectral Variability of the GHz-Peaked Spectrum Radio Source PKS
  1718-649 and a Comparison of Absorption Models
  Using the new wideband capabilities of the Australia Telescope Compact Array
(ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked
spectrum radio source. The observations, between approximately 1 and 10 GHz
over three epochs spanning approximately 21 months, reveal variability both
above the spectral peak at ~3 GHz and below the peak. The combination of the
low and high frequency variability cannot be easily explained using a single
absorption mechanism, such as free-free absorption or synchrotron
self-absorption. We find that the PKS 1718-649 spectrum and its variability are
best explained by variations in the free-free optical depth on our
line-of-sight to the radio source at low frequencies (below the spectral peak)
and the adiabatic expansion of the radio source itself at high frequencies
(above the spectral peak). The optical depth variations are found to be
plausible when X-ray continuum absorption variability seen in samples of Active
Galactic Nuclei is considered. We find that the cause of the peaked spectrum in
PKS 1718-649 is most likely due to free-free absorption. In agreement with
previous studies, we find that the spectrum at each epoch of observation is
best fit by a free-free absorption model characterised by a power-law
distribution of free-free absorbing clouds. This agreement is extended to
frequencies below the 1 GHz lower limit of the ATCA by considering new
observations with Parkes at 725 MHz and 199 MHz observations with the newly
operational Murchison Widefield Array. These lower frequency observations argue
against families of absorption models (both free-free and synchrotron
self-absorption) that are based on simple homogenous structures.
