The non-thermal spectra of many extragalactic jets and active galactic nuclei have a power law form over many orders of magnitudes. This kind of spectrum is assumed to be the result of shock acceleration of relativistic elcctrons at these sources.
As observed by Rieke et al. (1982) some of the A.G.N's, especially the BL Lac type, have an abrupt cutoff at IR-V frequencies. In some cases the intensity of the non-thermal radiation decreases by
more than two orders of magnitude over a frequency band of less than one order of magnitude.