New λ6 cm and λ11 cm observations of the supernova remnant CTA 1

¹ National Astronomical Observatories, CASJia-20 Datun Road, Chaoyang District, 100012 Beijing, PR China
e-mail: xhsun@nao.cas.cn
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: wreich@mpifr-bonn.mpg.de

Received: 11 July 2011
Accepted: 29 August 2011

Abstract

Aims. We attempt to study spatial variations in the spectrum and rotation measures (RMs) of the large-diameter, high-latitude supernova remnant (SNR) CTA 1.

Methods. We conducted new λ6 cm and λ11 cm 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.

Results. We obtained new total intensity and polarisation maps at λ6 cm and λ11 cm with angular resolutions of 9'.5and 4'.4, respectively. We derived a spectral index of α =  −0.63 ± 0.05 (S_ν ∝ ν^α) 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 Δα = 0.3. The RM map derived from polarisation data at λ6 cm and λ11 cm 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 m^-2. A RM structure function of CTA 1 indicates a very regular magnetic field within the Faraday screen, which is stronger than about 2.7 μG for a distance of 500 pc.

Conclusions. CTA 1 is a large-diameter shell-type SNR located out of the Galactic plane, which makes it an ideal object to study its properties without suffering confusion. The previous detection of the rare breakout phenomenon in CTA 1 is confirmed. We identify a Faraday screen partly covering CTA 1 with a regular magnetic field in the opposite direction to the interstellar magnetic field. The detection of Faraday screens in the Galactic plane is quite common, but is difficult at high latitudes where the polarisation angles of weak polarised background emission are rotated. Additional RMs from extragalactic sources are needed for this purpose, although the number density of the extragalactic RMs that have been measured remains small despite significant observational progress.