Abstract
Williams and I1 have shown from analysis of the distribution of the radio spectral index α (defined by flux density S(ν) ∝ ν−α) within complete samples of sources selected at frequencies ν⩽1.4 GHz that for ν≲0.4 GHz some sources have spectral indices significantly less than those of the “main” low frequency population, for which the mean value of α is 0.725 ± 0.11 in samples selected near 1 GHz. Studies2–6 of sources at frequencies near 1.4 GHz have now provided further examples of such sources with low values of α, and a survey7 at 5 GHz has confirmed that these sources form a distinguishable spectral class, not merely resulting from random scatter in the distributions analysed by Williams and me. A sample of these sources has been observed at frequencies of 3.24, 6.63 and 10.63 GHz (wavelengths of 9.26, 4.52 and 2.82 cm) using the fully steerable 46 m paraboloid at the Algonquin Radio Observatory, Lake Traverse, Ontario, to determine their spectra at centimetre wavelengths.
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