Radio-Source Problems

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Abstract
The present paper is based on the theory first proposed by Ginzburg and Shklovsky that all non-thermal, non-stellar, cosmic radio emission arises from synchrotron radiation by electrons. The electrons will be regarded as secondaries from nuclear collisions of cosmic rays with ambient gas. Evidence for this point of view can be adduced from the observed radio frequency spectra. The consequences of cosmic rays existing (i) in galaxies, (ii) in clusters of galaxies, (iii) in space generally, are examined, on the assumption that the cosmic-ray energy density is always ∼ 1 eV cm −3 . Apart from the introduction, the paper is divided into three parts : I. Weak emission from galaxies, clusters, and intergalactic space. II. Strong emission from extragalactic sources. III. Strong emission from galactic sources. Halo emission from spiral galaxies—from M31 and the Galaxy in particular—is considered in I. The cases of Cygnus A and NGC 1275 are investigated in II—a new class of optically non-visible source being also proposed in this part. The cases of the Crab Nebula, Casseiopeia A, and IC 443 are discussed in III. So far as strong sources are concerned, the argument depends in an important way on the conditions that develop at the shock front of a cloud expanding at great speed into an ambient medium. The present situation differs in a curious way from ordinary gas dynamics, in that the momentum and energy terms of the Rankine–Hugoniot relations are carried by quite different components of the assembly—the momentum by the material of the gas cloud and the ambient medium, and the energy by a reservoir of relativistic particles and by a magnetic field. To overcome the difficulty that the detailed physical processes that take place within the shock front itself are not well understood, a rule is proposed that receives strong empirical support from a wide range of observed phenomena, including such diverse cases as the radio emission of Cygnus A and the optical emission of the Crab Nebula. When the magnetic field dominates in the energy terms, the rule yields results that differ markedly from those given by the de Hoffman–Teller equations. The field does not preserve a uniform structure at the front, as in the de Hoffman–Teller case, but develops a highly wrapped, contorted structure—such as is suggested by the observed optical filaments of Casseiopeia A. If the rule were also applicable to cases where the energy and momentum were carried by different components, the momentum by a comparatively high-density, low-temperature gas and the energy by a low-density, high-temperature non-relativistic gas (the gas considered in the present paper is relativistic) a new process would be available for producing very high temperatures.

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