Radio properties of the low surface brightness SNR G65.2+5.7

Many physical properties of this SNR such as spectrum and polarization can only be investigated by radio observations. The $\lambda$11 cm and $\lambda$6\ cm continuum and polarization observations of SNR G65.2+5.7 were made with the Effelsberg 100-m and the Urumqi 25-m telescopes, respectively, to investigate the integrated spectrum, the spectral index distribution, and the magnetic field properties. $\lambda$21 cm archival data from the Effelsberg 100-m telescope have been also used. The integrated flux densities of G65.2+5.7 at $\lambda 11$ cm and $\lambda 6$ cm are $21.9\pm3.1$ Jy and 16.8$\pm$1.8 Jy, respectively. The power-law spectrum ($S\sim\nu^{\alpha}$) is well fitted by $\alpha = -0.58\pm0.07$ from 83 MHz to 4.8 GHz. Spatial spectral variations are small. Along the northern shell strong depolarizion is observed at both wavelengths. The southern filamentary shell of SNR G65.2+5.7 is polarized up to 54% at $\lambda 6$ cm. There is significant depolarization at $\lambda 11$ cm and confusion with diffuse polarized Galactic emission. Using equipartition principle, we estimated the magnetic field strength for the southern filamentary shell about 20 $\mu$G (filling factor 1) to 50 $\mu$G (filling factor 0.1). A faint HI shell may be associated with the SNR. Despite its unusual strong X-ray and optical emission and its very low surface brightness, the radio properties of SNR G65.2+5.7 are found to be typical for evolved shell type SNRs. SNR G65.2+5.7 may be expanding in a preblown cavity as indicated by a deficit of HI gas and a possible HI-shell.