radio-source spectrumThe variation of the flux density of a radio source with frequency. The emissions from most radio sources are noise signals displaying continuous spectra that rise or fall with increasing frequency more or less smoothly (see spectral index). Sources of thermal and synchrotron emission both give spectra of this type, sometimes showing a low-frequency cut-off due to self-absorption (see synchrotron emission) or the Razin effect. The spectra of the radiation from maser sources or from clouds of interstellar molecules, such as some H I regions, have discrete lines of bright emission superimposed on a background continuum (see molecular-line radio astronomy). Extragalactic objects are often described as either steep-spectrum or flat-spectrum radio sources according to the size of the spectral index.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006