power-law spectrumA spectrum quantified in terms of a power law. A nonthermal continuum spectrum is usually expressed by the dependence of the flux density S on the frequency ν raised to a certain power, S ∝ ν–α, where α is the spectral index. The ubiquity of this power-law fit to the multiwavelength continuum spectrum of all active galactic nuclei is strong evidence for the emission to be powered by the same mechanism in all cases, such as an accreting black hole. Only such an engine is capable of scaling its energy output through several orders of magnitude in luminosity without changing its fundamental spectrum.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006