The Radio Frequency Spectra of Discrete Radio Sources

Abstract

Recent measurements of flux density made at the California Institute of Technology Radio Observatory, the Mullard Radio Astronomy Observatory and the Nuffield Radio Astronomy Laboratories, at nine frequencies, have been combined to determine the spectra of 160 sources. By using a number of moderately strong calibration sources, errors due to the uncertainty in calibration at each frequency have been minimized. In general it has been found that the spectrum of most of the sources could be represented at frequencies below 1420 Mc/s by a simple power law with a small dispersion about a median spectral index of 0.71. Among the more accurately determined spectra, however, over 50 per cent show deviations from a straight line, usually only above 1420 Mc/s. These spectra are almost invariably convex, i.e. the spectrum becomes steeper with increasing frequency. Those sources (about 10 per cent of the total number) which show deviations at frequencies below 1420 Mc/s generally have a high brightness temperature. Several extra-galactic objects, and one galactic object (3C 48) are of this type. In contrast, the spectra of supernova remnants show no evidence of curvature. No definite correlation was found between spectral features and any further properties of radio sources.