Discovery of a new supernova remnant G21.8-3.0

Sensitive radio continuum surveys of the Galactic plane are ideal for discovering new supernova remnants (SNRs). From the Sino-German lambda 6-cm polarization survey of the Galactic plane, an extended shell-like structure has been found at l = 21 degrees.8, b = -3 degrees.0, which has a size of about 1 degrees. New observations were made with the Effelsberg 100-m radio telescope at lambda 11 cm to estimate the source spectrum, together with Urumqi lambda 6-cm and Effelsberg lambda 21-cm data. The spectral index of G21.8-3.0 was found to be alpha = -0.72 +/- 0.16. Polarized emission was mostly detected in the eastern half of G21.8-3.0 at both lambda 6 and lambda 11 cm. These properties, together with the Ha filament along its northern periphery and the lack of infrared emission, indicate that the emission is non-thermal, as is usual in shell-type SNRs.