The last decade has seen massive progress in our understanding of the X-ray properties of extragalactic radio jets and their environments. Chandra's sub-arcsec spatial resolution has been of paramount importance in measuring resolved X-ray emission from kpc-scale jet structures, and in extending studies of X-ray nuclei to sources other than beamed quasars and BL Lac objects by separating the emission of weaker nuclei from that of the jets and X-ray emitting environments.
The assumption that radio structures roughly lie in a state of minimum energy between their relativistic particles and magnetic fields is broadly verified in a few tens of sources through combining X-ray inverse Compton with radio synchrotron data (Section 2.2). This is the assumption commonly adopted in the absence of other information, and so its verification is reassuring, although much sub-structure is likely to occur and there is no reason to expect minimum energy to hold in dynamical structures.
The increase in numbers of known resolved kpc-scale X-ray jets has been remarkable, from a handful to the several tens of sources that Chandra has mapped in detail. There are grounds to believe that there are X-rays from synchrotron radiation in sources both of FRI and FRII types (Section 5.1), requiring in-situ particle acceleration to TeV energies. The steepening in spectral slope which most commonly occurs at infra-red energies may be related more to acceleration processes than energy losses, but more multiwavelength observational work is required to characterize the acceleration sites and support a theoretical understanding. The fact that X-ray synchrotron emission with an X-ray to radio flux-density ratio, S1 keV / S5 GHz, between about 10-8 and 10-7 is so common in jets where the bulk flow is inferred to be relativistic implies that there will be many more X-ray jet detections with current instrumentation in sufficient exposure time.
The dominant X-ray emission mechanism in resolved quasar jets remains uncertain, but it is likely that beamed emission from scattering of CMB photons is dominant in jets at small angles to the line of sight. This requires that highly relativistic bulk flows exist far from the cores, contradicting earlier radio studies but possibly understandable in the context of transverse velocity profiles. The knotty appearance of these jets is then possibly a result of variable output from the nuclei. Much of the knotty X-ray appearance of FRI jets, on the other hand, likely arises from spatial variations in the strength of particle acceleration (Section 5.2).
Jet theory has had some pleasing successes, such as the agreement between X-ray pressure profiles and predictions from hydrodynamical models for low-power jets in the regions where they are believed to be slowed by entrainment of the external medium or stellar mass loss (Section 4).
We are still largely ignorant of jet composition, and this is a difficult problem to solve since jet dynamics are governed by the energy of the constituent particles rather than their mass. There is generally growing support for a strong presence of relativistic protons (Section 6).
The observation of bubbles and cavities in cluster gas produced dynamically by radio structures has renewed interest in the mechanisms by which active galaxies introduce heat into gaseous atmospheres. A few nearby bright systems have been the subject of intense study with Chandra (Section 7.2). Although the way in which energy is deposited on the large scale is still far from clear, information on morphology and temperature has been used to infer the underlying energetics of the structures.
An area where work is still in its infancy is that of understanding the triggering of radio sources, and the possible rôle played here by galaxy and cluster mergers in promoting or inhibiting radio-source development (Section 7.4). The emerging picture shows that very different accretion structures can host radio jets, with a tendency for quasar-type nuclei to be associated with more powerful jets. How jets are powered by these different accretion structures and gas infall, and the duration of a given mode relative to typical lifetimes of radio sources, remain to be better understood.
The future is bright. Chandra and XMM-Newton are now mature observatories. Operational experience is enabling both more ambitious and more speculative programs to be undertaken. For example, Chandra is completing sensitive exposures of all 3CRR radio sources within a redshift of 0.1, and a large shallow survey of quasar jets to study the X-ray-emission mechanism in a statistical sense and seek out more sources for deep, detailed study. Observations of a somewhat more speculative nature are also being made, such as observing radio sources of different inferred ages, and studying how galaxy and cluster mergers are impacting the radio-source structures and their influence on the surrounding atmospheres. These are just examples. At the same time, Suzaku is making spectral measurements of active-galaxy nuclei, and testing the spin characteristics of black holes hosting radio sources through searching for relativistic broadening in Fe lines. We can expect fantastic results from continuing X-ray work, and many surprises.
New facilities coming on line will enrich the X-ray results. Spitzer has measured dust, stars, and non-thermal cores in the centres of radio galaxies, placing constraints on the central structures. It has also detected a number of kpc-scale jets, helping to tie down the all-important breaks in the spectral distributions of the synchrotron radiation that are likely to be connected to the process of particle acceleration. Herschel will continue such work.
The characteristics of the non-thermal emission at energies higher than the X-ray provide a sensitive test of emission mechanisms and a probe of jet composition. The Fermi Gamma-ray Space Telescope is providing such data, particularly for the embedded small-scale jets of highly-beamed quasars and BL Lac objects, as are ground-based Cerenkov telescopes sensitive to TeV emission.
ALMA will probe the cool component of gas in active galaxies, and provide information on one possible component of accretion power. Radio measurements with e-MERLIN and EVLA will probe spatial scales intermediate between pc and kpc, important in the launching and collimation of jets. They will also provide improved information on transverse jet structure.
Extending polarimetry to the X-ray, as is under study in the community, will provide key tests of jet emission and acceleration mechanisms, just as such work with HST is starting to do in the optical. Most importantly, a future X-ray observatory that has the sensitivity and spectral resolution to probe gas dynamics associated with radio sources is crucial for confirming and extending source modelling that is currently in its infancy. Such capabilities will come with the launch of a new facility such as the International X-ray Observatory currently under study by ESA and NASA.
I am grateful to Mark Birkinshaw for his essential contributions to our collaborative work on radio sources since the early 1990s, when we first observed radio galaxies with ROSAT. More recently, colleagues and students too numerous to list have energetically worked on jet data, and helped feed my enthusiasm for the subject. I particularly thank all involved in making Chandra such a great success, permitting resolved X-ray emission to be seen so clearly in so many active-galaxy jets. The outline for this review has evolved from talks I gave at `6 Years with Chandra', Cambridge MA, November 2005, and `Observations from High Energy Astrophysics Satellites' at the Marcel Grossmann meeting, Berlin, July 2006, and I am grateful to the organizers of those meetings for their invitations. I thank Raffaella Morganti, Thierry Courvoisier and Mark Birkinshaw for suggestions that have improved the manuscript. The NASA Astrophysics Data System has greatly assisted me in constructing the bibliography for this review.