A Sino-German $\lambda$6\ cm polarization survey of the Galactic plane   VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3
  Supernova remnants (SNRs) were often discovered in radio surveys of the Galactic plane. Because of the surface-brightness limit of previous surveys, more faint or confused SNRs await discovery. The Sino-German $\lambda$6\ cm Galactic plane survey is a sensitive survey with the potential to detect new low surface-brightness SNRs. We want to identify new SNRs from the $\lambda$6\ cm survey map of the Galactic plane. We searched for new shell-like objects in the $\lambda$6\ cm survey maps, and studied their radio emission, polarization, and spectra using the $\lambda$6\ cm maps together with the $\lambda$11\ cm and $\lambda$21\ cm Effelsberg observations. Extended polarized objects with non-thermal spectra were identified as SNRs. We have discovered two new, large, faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure. G178.2-4.2 has a size of 72 arcmin x 62 arcmin with strongly polarized emission being detected along its northern shell. The spectrum of G178.2-4.2 is non-thermal, with an integrated spectral index of $\alpha = -0.48\pm0.13$. Its surface brightness is $\Sigma_{1 GHz} = 7.2 x 10^{-23}{Wm^{-2} Hz^{-1} sr^{-1}}$, which makes G178.2-4.2 the second faintest known Galactic SNR. G25.1-2.3 is revealed by its strong southern shell which has a size of 80 arcmin x 30\arcmin. It has a non-thermal radio spectrum with a spectral index of $\alpha = -0.49\pm0.13$. Two new large shell-type SNRs have been detected at $\lambda$6\ cm in an area of 2200 deg^2 along the the Galactic plane. This demonstrates that more large and faint SNRs exist, but are very difficult to detect. 
X. Y. Gao, X. H. Sun, J. L. Han, W. Reich, P. Reich, R. Wielebinski
astro-ph.GA astro-ph.HE