New 6cm and 11cm observations of the supernova remnant CTA 1
  (Abridged) We conducted new 6cm and 11cm observations of CTA 1 using the
Urumqi 25-m and Effelsberg 100-m telescopes. Data at other wavelengths were
included to investigate the spectrum and polarisation properties. We obtained
new total intensity and polarisation maps at 6cm and 11cm with angular
resolutions of 9.5 arcmin and 4.4 arcmin, respectively. We derived a spectral
index of alpha=-0.63+/-0.05 based on the integrated flux densities at 408 MHz,
1420 MHz, 2639 MHz, and 4800 MHz. The spectral index map calculated from data
at the four frequencies shows a clear steepening of the spectrum from the
strong shell emission towards the north-western breakout region with weak
diffuse emission. The decrease of the spectral index is up to about 0.3. The RM
map derived from polarisation data at 6cm and 11cm shows a sharp transition
between positive RMs in the north-eastern and negative RMs in the south-western
part of the SNR. We note a corresponding RM pattern of extragalactic sources
and propose the existence of a large-diameter Faraday screen in front of CTA 1,
which covers the north-eastern part of the SNR. The RM of the Faraday screen is
estimated to be about +45 rad/m2. A RM structure function of CTA 1 indicates a
very regular magnetic field within the Faraday screen, which is larger than
about 2.7 microG in case of 500 pc distance.
