A Wideband Polarization Study of Cygnus A with the JVLA. I: The
  Observations and Data
  We present results from deep, wideband, high spatial and spectral resolution
observations of the nearby luminous radio galaxy Cygnus A with the Jansky Very
Large Array. The high surface brightness of this source enables detailed
polarimetric imaging, providing images at $0.75\arcsec$, spanning 2 - 18 GHz,
and at 0.30$\arcsec$ (6 - 18 GHz). The fractional polarization from 2000
independent lines of sight across the lobes decreases strongly with decreasing
frequency, with the eastern lobe depolarizing at higher frequencies than the
western lobe. The depolarization shows considerable structure, varying from
monotonic to strongly oscillatory. The fractional polarization in general
increases with increasing resolution at a given frequency, as expected.
However, there are numerous lines of sight with more complicated behavior. We
have fitted the $0.3\arcsec$ images with a simple model incorporating random,
unresolved fluctuations in the cluster magnetic field to determine the high
resolution, high-frequency properties of the source and the cluster. From these
derived properties, we generate predicted polarization images of the source at
lower frequencies, convolved to 0.75$\arcsec$. These predictions are remarkably
consistent with the observed emission. The observations are consistent with the
lower-frequency depolarization being due to unresolved fluctuations on scales
$\gtrsim$ 300 - 700 pc in the magnetic field and/or electron density superposed
on a partially ordered field component. There is no indication in our data of
the location of the depolarizing screen or the large-scale field, either, or
both of which could be located throughout the cluster, or in a boundary region
between the lobes and the cluster.
