Relativistic jets as X-ray and gamma-ray sources

The radio-through-..gamma..-ray emission from beamed relativistic jets associated with bright, compact radio sources is considered and applied to the interpretation of extragalactic X-ray and ..gamma..-ray sources. It is argued that in sources which are viewed at small angles to their jet axes, the beamed emission component also contributes significantly to the observed X-ray and ..gamma..-ray radiation. Model synchrotron and inverse-Compton spectra for a resolved jet and for an unresolved inhomogeneous jet are calculated, taking into account the effect of synchrotron radiation losses. It is shown how the source nonuniformity in an unresolved jet gives rise to a spectrum which may be quite different from the local emission spectrum in both the optically thick and the optically thin regimes. On the basis of this model, it is pointed out that various observed features in the spectra of BL Lac objects could be attributed to emission from unresolved relativistic jets. The spectrum of 3C 273 is discussed, and it is concluded that the infrared-through-X-ray flux is probably unbeamed, but that the radio emission and perhaps also the high-energy ..gamma..-rays are beamed from the relativistic radio jet associated with this source. It is argued that if 3C 273 is a typical quasar, thenmore » a large fraction of the diffuse high-energy ..gamma..-ray background could be contributed by beamed sources. In addition, a synchrotron-radiation model is proposed for the resolved X-ray jet in Cen A.« less