|Annu. Rev. Astron. Astrophys. 1981. 19:
Copyright © 1981 by . All rights reserved
As illustrated in Figure 1, the compact radio sources have flat, undulating, or inverted radio spectra. Although in some sources the radio flux density is nearly independent of frequency (i.e. ~ 0), more typically there are several local maxima and minima. In other sources the spectrum is inverted and the flux density increases toward the shorter wavelengths. The compact source spectra may be contrasted to the "normal" power-law spectra found in the extended sources, where the spectral index has a constant value, ~ -0.8, or gradually steepens at high frequencies.
Because the compact sources are generally variable, accurate spectra are not easy to measure. However, with simultaneous observations made over a wide range of frequency, or by interpolation of regular observations made at different times, reliable spectra have been obtained for a few hundred compact sources over a range of frequencies extending from several hundred MHz to 100 GHz, and in a few cases to 300 GHz. Although at frequencies greater than 20 or 30 GHz the flux density generally decreases with decreasing wavelength, in some sources the spectrum remains flat or inverted even up to several hundred GHz (Kellermann & Pauliny-Toth 1969, 1971, Owen & Mufson 1977, Owen et al. 1978, 1980, Kreysa et al. 1980, Jones et al. 1981).