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spectral index

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(S or α)

A measure of the way in which the intensity of the continuum emission from a radio source varies with frequency. The intensity usually varies exponentially, and the exponent is called the spectral index. It typically takes positive values from 0 to 2 for thermal emission, while non-thermal emission, such as synchrotron radiation, leads to negative values of the spectral index ranging from about −0.5 to −1.5.

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