Radio astronomy and high-energy astrophysics

The early history of radio astronomy was recounted in Section 7.3; that story ended in the mid 1950s, by which time the Galactic and extragalactic nature of the discrete radio sources was established. From the point of view of astrophysics, the key realisation was that, in most cases, the radio emission was the synchrotron radiation of ultra-high-energy electrons gyrating in magnetic fields within the source regions. The synchrotron radiation process began to be applied to other astronomical objects in which there was evidence for high-energy astrophysical activity.

In 1942, Rudolph Minkowski showed that the emission of the supernova remnant known as the Crab Nebula consists of two components, the filaments, which form a network defining the outer boundary of the remnant, and diffuse continuum emission originating within the nebula, which contributes most of its optical luminosity (Minkowski, 1942). The continuum emission had a featureless spectrum and could not be accounted for by any form of thermal spectrum. In 1949, John Bolton and Gordon Stanley found that the flux density of the Crab Nebula at radio wavelengths was about 1000 times greater than in the optical waveband (Bolton and Stanley, 1949). To account for the continuum emission, Iosif Shklovsky (1916–1985) proposed in 1952 that both the radio and optical continuum was synchrotron radiation, the energies of the electrons radiating in the optical waveband being very much greater than those radiating in the radio waveband (Shklovsky, 1953).