G 21.5-0.9 - a supernova remnant resembling the crab nebula

The distributions of total intensity and linear polarization over the radio source G 21.5-0.9 have been mapped at lambda 6 cm with the Westerbork Synthesis Radio Telescope. The total intensity distribution is relatively smooth with the peak brightness near the geometric centre of the source. The spectrum of the integrated emission is found to be flat and may be well represented by a power law of spectral index of α = 0.01 +- 0.04 from 80 MHz to 10.7 GHz. The linear polarization at lambda 6 cm is quite uniform over the central regions of the source with typical values 10 - 15 %. Rotation measures consistent with the available polarization data are +358 and -1073 radians m2. Because it is a Galactic object and highly polarized, G21.5-0.9 is almost certainly a supernova remnant. It resembles the Crab Nebula and 3 C 58 in having a 'filled centre', rather than 'shell-like', brightness distribution and a very flat spectrum. Also, like the Crab Nebula (but unlike 3 C 58), it deviates from the Σ-D relation for supernova remnants. A detailed comparison of the observed and derived properties of the three sources is presented. (orig.)