A Sino-German λ6 cm polarization survey of the Galactic plane

VI. Discovery of supernova remnants G178.2−4.2 and G25.1−2.3

¹ National Astronomical Observatories, CAS, Jia-20 Datun Road, Chaoyang District, Beijing 100012, PR China
e-mail: hjl@nao.cas.cn; bearwards@gmail.com
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 3 May 2011
Accepted: 28 June 2011

Abstract

Context. 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 λ6 cm Galactic plane survey is a sensitive survey with the potential to detect new low-surface-brightness SNRs.

Aims. We aim to identify new SNRs from the λ6 cm survey map of the Galactic plane.

Methods. We searched for new shell-like objects in the λ6 cm survey maps and studied their radio emission, polarization, and spectra using the λ6 cm maps together with the λ11 cm and λ21 cm Effelsberg observations. Extended polarized objects with non-thermal spectra can be identified as SNRs.

Results. 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′ × 62′ 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 α =  −0.48 ± 0.13. Its surface brightness is Σ_1   GHz = 7.2 × 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′ × 30′. It has a non-thermal radio spectrum with a spectral index of α =  −0.49 ± 0.13.

Conclusions. Two new large shell-type SNRs have been detected at λ6 cm in an area of 2200 deg² along the Galactic plane. This demonstrates that more large and faint SNRs exist, but are very difficult to detect.