The Galactic Spur

Abstract

New observations have been made of the radio emission from the galactic spur at lII = 30° at 237 Mc/s to investigate the detailed structure of the sharp edges and the distribution of emission over the declination range observable from Jodrell Bank. The weak radio emission at 1415 Mc/s was measured at three sections of the northern edge of the spur, and the results were also compared with the 38 Mc/s survey of the region obtained by Costain and Smith (1960). This data allows a rediscussion of the nature of the spur. The spur has a characteristic sharp outer edge which in places falls to half intensity in 0°.5. Also it has structure parallel to the edge which indicates that the source is a multiple shell like the Cygnus Loop which it resembles in brightness temperature, spectrum and structure. If its radius is taken as 20 pc its centre lies at a distance of 30 pc and is composed of shells each with a thickness of about 2 pc. The observed emission from both the Cygnus Loop and the galactic spur is rather more than expected from background cosmic ray electrons alone, emitting in the enhanced magnetic field of the expanding envelope, and suggests that some of the emission comes from cosmic rays generated within the shell sources. The polarization of the main ridge of the spur confirms that it is close to the Sun and also that it radiates by the synchrotron process. It is concluded that the galactic spur, like the Cygnus Loop, is a supernova remnant.