Polarization observations of SNR G156.2+5.7 at λ6 cm

Context. G156.2+5.7 is a large supernova remnant (SNR) first discovered in the ROSAT X-ray survey due to its exceptional high X-ray brightness. Subsequently detailed X-ray, optical and Ha studies of this SNR were made, but radio observations are rare because of its low surface brightness and large size. Aims. Radio continuum and polarization images of the SNR G156.2+5.7 at lambda 6 cm are presented for the first time to be discussed in the context with available data to investigate its physical properties. Methods. The SNR G156.2+5.7 was observed with the Urumqi 25 m telescope in radio continuum and linear polarization. The results are analyzed together with 11 cm and 21 cm maps previously obtained with the Effelsberg 100 m telescope and compared with a broad-band X-ray ROSAT image, 100 mu m infrared radiation data and a Ha emission map. Results. We obtained an integrated flux density for G156.2+5.7 of S-6 cm = 2.5 +/- 0.5 Jy. The spectral index of the integrated emission is alpha = -0.48 +/- 0.08 (S similar to upsilon(alpha)) between lambda 74 cm and lambda 6 cm excluding compact radio sources. We also obtained a spectral index map, which shows little variation between the shells and the central area of the SNR. This is consistent with the constant integrated flux density spectrum. Highly polarized radio emission has been detected from the SNR shell, but also from a central patch, which probably originates in the front part of the spherical SNR shell. We derived the distribution of rotation measure from polarization data at lambda 11 cm and lambda 6 cm and found RM gradients of opposite direction in the SNR shell. Conclusions. The SNR G156.2+5.7 is unusual by its exceptionally high X-ray brightness and very low surface-brightness in the radio range. The magnetic field is very well ordered along the shell periphery as expected for a compressed ambient magnetic field. A toroidal magnetic field component is indicated by the RM distribution.