(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.
Subjects: Science and technology — Astronomy and Cosmology