A gaseous medium is essential for the propagation of a radio jet. Although hot gas is traced through its X-ray thermal bremsstrahlung and line radiation, the X-ray detection of gaseous environments around most radio sources has required the sensitivity and angular resolution available only in the last decade with ROSAT. While it is clear that an X-ray emitting medium must be present at some level for the well being of the radio source, major concerns from a radio-astronomy and jet-theory perspective are the extent to which the medium influences radio-jet fuelling, dynamics, propagation, and disruption. From an X-ray astronomy perspective, it is of interest to determine whether or not the X-ray environments of radio sources are special, and to use radio sources to indicate the presence of X-rays associated with large-scale structure.
The X-ray-emitting medium of radio sources is complicated to study because radio sources are multi-component X-ray emitters and include several possible regions of non-thermal emission. These non-thermal components are themselves an important probe of the physical conditions of the central engine and radio beams.
Since careful separation of the various X-ray components is essential before progress can be made, this paper first reviews non-gaseous emission components in radio galaxies, and presents examples of observational biases. The discussion of the X-ray emitting environments is broken down by radio-source power and morphology, and trends in external gas density and pressure near the extended radio structures are reviewed. There are many open issues, and I will identify the potential for resolving them using the new X-ray missions with their vastly improved resolution and sensitivity.
Because the central X-ray regions of radio galaxies experience anisotropic emission and absorption, assumptions concerning Unification color an approach to the subject and can be tested by the data. This paper makes the standard Unification assumptions that BL Lac objects, radio-loud quasars, and broad-line radio galaxies (BLRGs) are the unobscured, favorably-beamed counterparts of low-power FRI (Fanaroff & Riley 1974) radio galaxies, high-redshift FRII galaxies, and low-redshift FRIIs, respectively. Ho = 50 km s-1 Mpc-1 and qo = 0 are adopted throughout.