Radio Emission from the Cygnus Loop

Abstract

The Cygnus Loop, an expanding system of bright nebulosities about 3 of arc in extent, is thought to be the remnant of a supernova type II and probably represents the nearest of the Class I radio sources. The high resolution and sensitivity of the 250 ft paraboloid at 408 Mc/s made possible a detailed study of the radio emission within this type of object and its relation to the optical features. The radio emission was found to be most intense in the optically faint southern region, the position of maximum emission being 20 h 48 m 21 s , + 29° 40′ (1950). The radio source associated with the bright nebulosity NGC 6992–5 was resolved from the main body of emission and although very similar in size and shape to the optical feature it was displaced towards the centre of expansion by about 12′ of arc. An attempt was made to estimate the relative contributions of the thermal and non-thermal components of the radio emission by measuring the spectral index at several points in the Cygnus Loop by combining the 408 Mc/s survey with a lower resolution 158 Mc/s survey using the 250 ft paraboloid.