Variability of Radio Sources and Spectral Type

Abstract

VARIABLE radio sources are found among quasi-stellar sources, Seyfert galaxies, optical variable sources, sources with one or more compact components and sources with spectra which had an excess of radiation at centimetre wavelengths. The spectrum of a radio source is perhaps more readily determined than the other properties and is a characteristic of primary importance. In particular, observers have always assumed that variability is likely to occur in sources with centimetre excess spectra. We can now establish this relationship without doubt and in a quantitative form: between 90% and 100% of sources with centimetre excess spectra are variable. Furthermore, no source with a normal spectrum shows any evidence of variations. These conclusions are based on observations made during 5.5 yr at the Algonquin Radio Observatory of eighty-four suspected variable sources, each of which has one or more of the characteristics usually associated with variability. Only the association with spectral type has been found to be a dependable indicator of variability. The other characteristics do not show an exclusive relationship with either variability or non-variability.