Published in The Astronomy and Astrophysics Review, Vol. 17, p. 1-46, 2009.
astro-ph/0812.3401

For a PDF version of the article, click here.

Abstract. Jet physics is again flourishing as a result of Chandra's ability to resolve high-energy emission from the radio-emitting structures of active galaxies and separate it from the X-ray-emitting thermal environments of the jets. These enhanced capabilities have coincided with an increasing interest in the link between the growth of super-massive black holes and galaxies, and an appreciation of the likely importance of jets in feedback processes. I review the progress that has been made using Chandra and XMM-Newton observations of jets and the medium in which they propagate, addressing several important questions, including: Are the radio structures in a state of minimum energy? Do powerful large-scale jets have fast spinal speeds? What keeps jets collimated? Where and how does particle acceleration occur? What is jet plasma made of? What does X-ray emission tell us about the dynamics and energetics of radio plasma/gas interactions? Is a jet's fate determined by the central engine?

Table of Contents

THE STAGE IS SET

Historical perspective

Radiation processes

Generic classes of jets

Lifetimes and duty cycles

ARE THE RADIO STRUCTURES IN A STATE OF MINIMUM ENERGY?

Calculation of the minimum-energy field

Using X-rays to test minimum energy

DO POWERFUL LARGE-SCALE JETS HAVE FAST SPINAL SPEEDS?

The impetus from PKS 0637-752

The dependence of beamed iC-CMB on beaming factors and redshift

How is the beamed iC-CMB model faring under scrutiny?

Synchrotron emission as an alternative

WHAT KEEPS JETS COLLIMATED?

WHERE AND HOW DOES PARTICLE ACCELERATION OCCUR?

The link with synchrotron X-ray emission

Particle acceleration in knotty structures

Incorporating polarization data

WHAT IS JET PLASMA MADE OF?

WHAT DOES X-RAY EMISSION TELL US ABOUT THE DYNAMICS AND ENERGETICS OF RADIO PLASMA/GAS INTERACTIONS?

Expectations for FRIIs

Dynamics of FRIs in clusters

Centaurus A

The effect of galaxy mergers

IS A JET'S FATE DETERMINED BY THE CENTRAL ENGINE?

An evolutionary cycle?

The rôle played by accretion processes

The rôle of the environment

Information from beamed sources

SUMMARY AND CONCLUDING REMARKS

REFERENCES

1. THE STAGE IS SET

1.1. Historical perspective

In the 1970s and 1980s the powerful capabilities of radio interferometry gave birth to the study of extragalactic radio jets. It became clear that radio jets are plasma outflows originating in the centres of active galaxies, seen through their synchrotron emission. After much debate, properties such as the relative one-sidedness of the jets, and the measurement of apparent superluminal expansion, by Very Long Baseline Interferometry (VLBI), were accepted as due to the outflows having relativistic bulk speeds. Early attempts at unifying source populations based on special relativity and apparent source properties (e.g. 175) have developed over the years into comprehensive unified schemes (e.g. 8) whereby quasars are explained as radio galaxies whose jets are at small angles to the line of sight and so are boosted by relativistic effects.

By the mid 1990s, the study of radio jets had reached something of a hiatus, and major groups around the world turned their attention to other pursuits such as gravitational lensing and the study of the Cosmic Microwave Background (CMB) radiation. A turning point was the sensitivity and high-fidelity mirrors of the Chandra X-ray Observatory [209], which resulted in the detection of resolved X-ray emission from many tens of well-known extragalactic radio sources (see [103] for a source compilation as of 2006: the number continues to increase). When combined with X-ray measurements of the ambient gas made with Chandra and XMM-Newton, and multiwavelength data, many important questions related to the physics of jets can be addressed. Progress towards answering those questions is the substance of this review.

The enhanced capabilities for the X-ray study of jets have coincided with strong interest from the wider astronomical community in the growth of supermassive black holes (SMBHs), following the links that have been made between SMBH and galaxy growth (e.g. 165, 79). SMBHs (and indeed compact objects of stellar mass) commonly produce jets, as an outcome of accretion processes responsible also for black-hole growth. It is also clear that extragalactic jets are capable of transferring large amounts of energy to baryonic matter in the host galaxies and surrounding clusters at large distances from the SMBH. The way in which heating during the jet mode of AGN activity might overcome the problem of fast radiative cooling in the centre of clusters is now intensely studied in nearby objects (see Section 7), and heating from `radio mode' activity is included in simulations of hierarchical structure formation (e.g. 57). We need therefore to understand what regulates the production of jets and how much energy they carry. X-ray measurements of nuclear emission probe the fueling and accretion processes, and those of resolved jet emission and the surrounding gaseous medium probe jet composition, speed, dynamical processes, energy deposition, and feedback.a

1.2. Radiation processes

The two main jet radiation processes are synchrotron radiation and inverse-Compton scattering. Their relative importance depends on observing frequency, location within the jet, and the speed of the jet. The thermally X-ray-emitting medium into which the jets propagate plays a major rôle in the properties of the flow and the appearance of the jets. The physics of the relevant radiation processes are well described in published work (e.g. 86, 26, 153, 189, 168, 174, 137), and most key equations for the topics in this review, in a form that is independent of the system of units, can be found in [220].

It is particularly in the X-ray band that synchrotron radiation and inverse-Compton emission are both important. X-ray synchrotron emission depends on the number of high-energy electrons and the strength and filling factor of the magnetic field in the rest frame of the jet. Inverse Compton X-ray emission depends on the number of low-energy electrons, the strength of an appropriate population of seed photons (such as the CMB, low-energy jet synchrotron radiation, or emission from the central engine), and the geometry of scattering in the rest frame of the jet. In an ideal world, observations would be sufficient to determine the emission process, and this in turn would lead to measurements of physical parameters. In reality, X-ray imaging spectroscopy, even accompanied by good measurements of the multiwavelength spectral energy distribution (SED), often leaves ambiguities in the dominant emission process. Knowledge is furthered through intensive study of individual sources or source populations.

1.3. Generic classes of jets

In discussing jets, it is useful to refer to the Fanaroff and Riley [74] classification that divides radio sources broadly into two morphological types, FRI and FRII. A relatively sharp division between FRIs and FRIIs has been seen when sources are mapped onto a plane of radio luminosity and galaxy optical luminosity [136] - the so called Ledlow-Owen relation. FRIIs are of higher radio luminosity, with the separation between the classes moving to larger radio luminosity in galaxies that are optically more massive and luminous. The distinct morphologies (e.g. 150) are believed to be a reflection of different flow dynamics (e.g. 134).

FRI sources (of lower isotropic radio power, with BL Lac objects as the beamed counterpart in unified schemes) have broadening jets feeding diffuse lobes or plumes that can show significant gradual bending, usually thought to be due to ram-pressure as the source moves relative to the external medium. The jet emission is of high contrast against diffuse radio structures, implying that the jet plasma is an efficient radiator. kpc-scale jets are usually brightest at a flaring point some distance from the active galactic nucleus, and then fade gradually in brightness at larger distances from the core, although this pattern is often interrupted by bright knots seen when the jet is viewed in the radio or the X-ray. Such an example is shown in Figure 1 ¹. The jets are believed to slow from highly-relativistic to sub-relativistic flow on kpc-scales from entrainment of the external interstellar medium (ISM), perhaps enhanced by stellar mass loss within the jet. The strong velocity shear between the jet flow and the almost stationary external medium must generate instabilities at the interface [20], and drive the flow into a turbulent state. The physics of the resulting flow is far from clear, although it can be investigated with simplifying assumptions (e.g., 14, 15 and see Section 4).

Figure 1. Roughly 6.6 kpc (projected) of the inner jet of the z = 0.0165 FRI radio galaxy NGC 315. Left: 5 GHz VLA radio map showing a knotty filamentary structure in diffuse emission. Right: Smoothed Chandra X-ray image of ~ 52.3 ks livetime also showing knotty structure embedded in diffuse emission. The ridge-line defined by the radio structure is shown in white, and indicates a level of correspondence between the radio and X-ray knots. Figure adapted from [221].

FRII sources (of higher isotropic radio power, with quasars as the beamed counterpart in unified schemes) have narrower jets that are sometimes faint with respect to surrounding lobe plasma and that terminate at bright hotspots (Fig. 2). The jets are often knotty when observed with high resolution, and the jets can bend abruptly without losing significant collimation (see Section 3.1 and Section 5.3 for examples). The bending is often large in quasar jets, supporting the conjecture that quasars are viewed at small angle to the line of sight and that bends are amplified through projection. In contrast to FRI jets which are in contact with the external medium, the standard model for FRII jets is that they are light, embedded in lobe plasma, and remain supersonic with respect to the external gas out to the hotspots. The energy and momentum fluxes in the flow are normally expected to be sufficient to drive a bow shock into the ambient medium. The ambient gas, heated as it crosses the shock, forces old jet material that has passed through the hotspots into edge-brightened cocoons. FRII jets are thus low-efficiency radiators but efficient conveyors of energy to large distances. They are often hundreds of kpc in length (particularly when deprojected for their angles to the line of sight), crossing many scale heights of the external medium from relatively dense gas in a galaxy core to outer group or cluster regions where the external density and pressure are orders of magnitude lower. State-of-the-art three-dimensional magneto-hydrodynamical simulations that incorporate particle transport and shock acceleration do well at reproducing the essential characteristics of synchrotron emission from such a source, and suggest that the shock and magnetic-field structures of the hotspots and lobes are extraordinarily complex and unsteady [201, 202].

Figure 2. The z = 0.458 FRII radio galaxy 3C 200. A smoothed 0.3-5 keV Chandra X-ray image of ~ 14.7 ks livetime is shown with radio contours from a 4.86 GHz VLA radio map [135] (beam size 0.33" × 0.33"). Both nuclear [11] and extended X-ray emission are detected. A rough correspondence of some of the extended X-ray emission with the radio lobes has resulted in the claim for inverse-Compton scattering of the CMB by electrons in the lobes [55], but most of the extended emission over larger scales is now attributed to cluster gas [12].

1.4. Lifetimes and duty cycles

Individual FRI and FRII radio galaxies are thought to live for at most some tens of millions of years (e.g. 140, 118). Age estimates are based on measuring curvature in the radio spectra caused by radiative energy losses of the higher-energy electrons over the lifetime of the sources (e.g. 3). In contrast to the relative youth of observed radio structures, present-day clusters were already forming in the young Universe. Ideas that radio sources have an important rôle in heating cluster gas (see Section 7) then require a correct balance between the duty-cycle of repeated radio activity and heating efficiency as a function of jet luminosity. The duty cycle can be probed by searching for evidence of repeated activity from individual sources. Radio sources classified as GHz-Peaked Spectrum (GPS) or Compact Steep Spectrum (CSS) are small and believed to be either young or have their growth stunted by the external medium [151], and source statistics suggest that if they evolve to kpc-scale sizes they must dim while so doing [162]. VLBI kinematic studies provide convincing evidence that sources in the Compact Symmetric Object (CSO) subset, at least, are young, with current ages less than 10⁴ years [52]. The fact that it is relatively uncommon to see GPS sources with extended radio emission that may be a relic of previous activity has been used to argue that periods between sustained activity are generally at least ten times longer than the radiative lifetime of the radio emission from the earlier activity [190]. This is consistent with a time between episodes of activity in FRIIs of between about 5 × 10⁸ and 10⁹ years that is estimated using optical- and radio-catalog cross correlations coupled with an average source lifetime of about 1.5 × 10⁷ years from modelling projected source lengths [18]. Of course, within the lifetime of an individual radio source there might be shorter-term interruptions or variations of activity (see Section 8.1).

¹ Values for the cosmological parameters of H₀ = 70 km s^-1 Mpc^-1, _m0 = 0.3, and ₀ = 0.7 are adopted throughout this review. Back.

2. ARE THE RADIO STRUCTURES IN A STATE OF MINIMUM ENERGY?

2.1. Calculation of the minimum-energy field

The magnetic field strength and particle spectrum are important for jet physics as they define the internal pressure. The level of synchrotron radiation depends on the magnetic-field strength and the number of relativistic electrons and positrons, but these quantities are inseparable based on the observed synchrotron radiation alone. To progress further it is usual to assume that the source is radiating such that its combined energy in relativistic particles and magnetic field is a minimum [35]. In this situation the energy in the magnetic field is ~ 3/4 of the energy in the relativistic particles, and so this is similar to the condition in which the two are equal and the source is in `equipartition'. A change in any direction of the ratio of energy density in particles to magnetic field increases the total energy and pressure in the emitting plasma.

The minimum-energy magnetic field for a power-law spectrum of electrons producing radiation of a measured flux density at a particular frequency can be calculated analytically (e.g. 220), and for more complicated spectra the results can be obtained via numerical integration. Physical insight can be gained by considering a power-law spectrum where electrons give rise to a synchrotron luminosity, L, at a given frequency of the form

(1)

It is now normally thought preferable to define the spectral limits via a minimum and maximum Lorentz factor for the electrons in the source frame, _min and _max, (e.g. 220), rather than as synchrotron frequencies in the observer's frame (e.g. 147), since the former is related to acceleration processes and has the potential for being chosen on a physical basis. Except in the special case of = 0.5, the minimum-energy magnetic field strength, B_me, is given by

(2)

where V is the source volume, and C₁ and C₂ are combinations of fundamental physical constants and functions of given by synchrotron theory (for details see [220]). Following the notation of [147], K is the ratio of energy in other relativistic particles to that in the electron and positron component, and is the fraction of the volume filled by particles and fields (the so-called filling factor). The true minimum energy is when the only relativistic particles are radiating leptons, and the volume is completely and uniformly filled with radiating particles and fields. Some authors consistently use these assumptions when calculating B_me. If K > 0 or < 1 then B_me is increased. Results for B_me are more strongly dependent on _min than _max, since > 0.5 for most observed radio spectra.

Relativistic beaming of a source affects B_me (as considered later in Section 3.2). Since there is inevitably uncertainty in the value of beaming parameters, B_me is best measured in components for which bulk relativistic motion is believed to be small or negligible. Of course, even in the absence of relativistic beaming, the angle to the line of sight, , enters into the calculation via a correction from projected linear size into true source volume, V. Typical values found for B_me in radio lobes and hotspots are 2-200 µGauss (0.2-20 nT) (e.g. 119), although a hotspot field as large as 3000 µGauss has been measured [91].

Figure 3 shows the dependence of B_me on _min, K, , and , separately for electrons giving rise to synchrotron spectra with = 0.6 and = 1.1. The former slope is as expected from electrons undergoing highly relativistic shock acceleration [1], and the latter where energy losses have steepened the spectrum. The curves show that B_me changes rather little (within factors of at most a few) for rather large changes in the input assumptions.

Figure 3. Effect on the calculated minimum-energy magnetic field if a parameter value is varied from its nominal value (left-hand-side of plot). Results are for a power-law electron spectrum, extending from Lorentz factor min to max = 105, that gives rise to a synchrotron spectrum S - with = 0.6 (solid lines) and = 1.1 (dashed lines). (a): increasing min from a value of 10. (b): increasing the ratio of energy in other particles to that in electrons, K, from a value of zero. (c): decreasing the filling factor, , from a value of 1. (d) decreasing the angle to the line of sight, and thus increasing the source volume from the projected size at = 90°.

2.2. Using X-rays to test minimum energy

The minimum-energy assumption can be tested by combining measurements of synchrotron and inverse-Compton emission from the same electron population. If the inverse Compton process is responsible for most of the X-ray radiation that is measured, and the properties of the photon field are known, the X-ray flux density is proportional merely to the normalization of the electron spectrum, , if the usual power-law form

(3)

is assumed, where N_e^(rel) is the number of relativistic electrons per unit . The upscattered photons might be the CMB, whose properties are well known. Alternatively they could be the radio synchrotron radiation itself, in the process known as synchrotron self-Compton (SSC), or photons from the active nucleus, particularly at infrared through ultraviolet frequencies. Since the available photons range in frequency, so too do the energies of electrons responsible for scattering them into the X-ray, and these are rarely the same electrons for which the magnetic field is probed through synchrotron radiation. Nevertheless, it is usual to assume that the magnetic field, photons, and relativistic electrons are co-located, with the synchrotron photon density proportional to B¹⁺. Here is defined as in Equation 1, and theory gives = (p - 1) / 2. The combination of synchrotron (radio) flux density and inverse Compton (X-ray) flux density then allows a value for the magnetic field strength, B_SiC, to be inferred and compared with B_me.

Since the modelling requires that the volume and any bulk motion of the emitting plasma be known, the best locations for testing minimum energy are the radio hotspots, which are relatively bright and compact, and are thought to arise from sub-relativistic flows at jet termination (but see [80]), and old radio lobes where the plasma may be relatively relaxed. There is no reason to expect dynamical structures to be at minimum energy.

It was anticipated that Chandra and XMM-Newton would make important advances in tests of minimum energy, since already with ROSAT and ASCA there were convincing detections of inverse Compton X-ray emission from the hotspots and lobes of a handful of sources (e.g. 101, 76, 195), and pioneering work on the hotspots of Cygnus A had found good agreement with minimum energy [101]. Chandra and XMM-Newton have allowed such tests to be made on a significant number of lobes and hotspots, with results generally finding magnetic field strengths within a factor of a few of their minimum energy (equipartition) values for K = 0 and = 1 (e.g. 95, 33, 110, 51, 30, 10, 54, 148). A study of ~ 40 hotspot X-ray detections concludes that the most luminous hotspots tend to be in good agreement with minimum-energy magnetic fields, whereas in less-luminous sources the interpretation is complicated by an additional synchrotron component of X-ray emission [98]. Considerable complexity of structure is seen where hotspots are close enough for X-ray images to have kpc-scale or better resolution (e.g. 126).

Figure 4. The amount by which the fraction of the total X-ray flux density attributable to inverse Compton radiation, X-rayiC / X-raytotal, would have to be reduced for a result of BSiC / Bme = 0.5 to be increased. Solid and dashed curves are for = 0.6 and = 1.1, respectively.

For radio lobes, the largest systematic study where it is assumed that all the X-ray emission is inverse Compton radiation is of 33 FRII lobes, and finds 0.3 < B_SiC / B_me < 1.3 [55]. Since the asymmetry is on the side of B_SiC < B_me, it is important to recognize that the analysis may not have accurately taken into account contributions to the lobe X-ray emission from cluster gas, now commonly detected away from the lobe regions in FRII radio galaxies ([12], and see Fig. 2). However, as seen in Figure 4, the lobe X-ray emission from cluster gas would have to be far brighter than that from inverse Compton scattering to cause B_SiC / B_me to increase significantly (e.g., from 0.5 to 1.0), and this is incompatible with the observation that lobes stand out in X-rays as compared with adjacent regions.

Better agreement between B_SiC and B_me would be achieved if B_me has been overestimated. Figure 3 shows that decreasing the filling factor or including relativistic protons that energetically dominate the electrons have the opposite effect. A decrease in B_me is found if the source has been assumed to be in the plane of the sky whereas it is really at a small angle, with the structures having more volume. However, the small angles required to make an appreciable difference would be inconsistent with random sampling. More promising would be if _min were higher than typically assumed, as stressed by [27] who claim evidence for a value of _min as high as ~ 10⁴ in the hotspot of one FRII radio galaxy, with a lower value of _min ~ 10³ in the lobes as a result of adiabatic expansion. This is in line with earlier measurements of spectral flattening at low radio frequencies in hotspot spectra, suggestive of values of _min no lower than a few hundred (e.g. 133, 41). Why there might be such a _min in a hotspot is discussed by [91].

It is important to stress that finding B_SiC / B_me within a factor of a few of unity does not allow strong constraints to be placed on physical parameters. As shown in Figure 3, large changes in input parameters do not change B_me, and thus B_SiC / B_me, by a large amount. It is often pointed out that if the magnetic-field strength is a factor of a few below B_me, the energy in relativistic electrons must dominate the magnetic-field energy by orders of magnitude. While this is relevant for understanding the state of the plasma, does this really matter from the point of view of source energetics? The increase in combined electron and magnetic-field energy over the minimum energy is relatively modest as long as the electron spectrum is not very steep and the field strength is no less than about a third of B_me (Fig. 5).

Figure 5. The ratio of total energy in electrons and magnetic field, computed from combined X-ray inverse Compton and radio synchrotron measurements, to that calculated for minimum energy, for the range of BSiC / Bme typically observed. Solid and dashed curves are for = 0.6 and = 1.1, respectively.

In any case, it is clear that application of minimum energy over large regions is an oversimplification. Three-dimensional magneto-hydrodynamical simulations that incorporate particle transport and shock acceleration [201, 202] find much substructure of particle distributions and fields within the volumes typically integrated over observationally. Complexity on a coarser scale is seen in some observations (e.g. 110, 148).

3. DO POWERFUL LARGE-SCALE JETS HAVE FAST SPINAL SPEEDS?

3.1. The impetus from PKS 0637-752

Chandra is central to the current debate concerning jet speed in the powerful radio jets of quasars. The work was kick-started unexpectedly. Observing quasars was not initialy a high scientific priority for Chandra, as it was recognized that the cores were bright, and the likelihood of multiple photons arriving between CCD readouts was high, leading to distorted spectral measurements (so called `pileup'). It was thus fortuitous that a radio-loud quasar was the chosen target for in-flight focus calibration, since this led to the detection of resolved jet emission from the z = 0.651 quasar PKS 0637-752 ([177, 45], and see Fig. 6).

Figure 6. The z = 0.651 quasar PKS 0637-752, using data from [177]. The plot shows a smoothed Chandra X-ray image of ~ 35 ks exposure with radio contours from an 8.64 GHz ATCA radio map (beam size 0.96" × 0.81"). X-ray emission is detected from the nucleus and from the western radio jet before it bends north. The bright jet region 7.8" west of the nucleus is known as Knot WK7.8.

Several possible origins for PKS 0637-752's jet X-rays were considered. The level of optical emission was too low to explain the X-rays as the synchrotron radiation from a single population of electrons, and SSC was disfavoured as it would require strong dominance of the energy in relativistic electrons over that in magnetic field, giving a total energy in particles and field that is ~ 1000 times that given by minimum energy [177]. A more promising explanation allowed the jet to be at minimum energy but required it to have fast bulk motion (a Lorentz factor of ~ 20 at ~ 5° to the line of sight), in which case it would see boosted CMB in its rest frame and emit beamed X-rays in the observer's frame [197, 44]. Although such a speed and angle are consistent with VLBI measurements on pc scales [139], the fast speed must persist up to hundreds of kpc from the core (after projection is taken into account) for the X-rays to be produced by this mechanism, which I will call "beamed iC-CMB". This explanation ran counter to the common wisdom of the time, based on radio data, that the bulk relativistic speed of quasar jets on the large scale is ~ 2 (e.g. 31, 208). To overcome the contradiction, it was suggested that quasar jets have a fast-moving central spine responsible for the observed X-rays, and a slower-moving outer region that emits the bulk of the observed radio emission [44]. This follows the same pattern as the transverse velocity structures, conjectured for FRI jets, that are thought to result from the entrainment of external material (see Section 4).

3.2. The dependence of beamed iC-CMB on beaming factors and redshift

In modelling beamed iC-CMB emission, most authors use the approximation that CMB photons, isotropic in the observer's frame, are scattered into directions in the jet frame that are parallel to the instantaneous velocity vectors of the scattering electrons (e.g. 58, 102). This has been shown to be an excellent approximation for calculating the X-ray emissivity as long as the jet's bulk motion has Lorentz factor 2 [59], which is, in any case, required for the mechanism to be effective at producing strong X-ray fluxes. The basic physics of the formalism is particularly clearly presented in [58], and here those formulae are presented in a slightly different form which is independent of the system of units.

We consider a source travelling at speed c and bulk Lorentz factor towards the observer at an angle to the line of sight, so that the bulk relativistic Doppler factor, , is given by

(4)

An electron of Lorentz factor will scatter a CMB photon that has a characteristic frequency today of _CMB to an observed frequency, , given by

(5)

where the spectral redistribution function is approximated as a delta function (equation (7) of [58], written in the notation of this paper). A delta-function approximation is also used for the synchrotron spectral distribution function such that an electron of Lorentz factor radiates at frequency

(6)

where _g is the non-relativistic electron gyrofrequency, which is proportional to the magnetic field strength, B. Written in SI units, _g = eB / 2 m_e 30 B GHz, where B is in units of Tesla. For a CMB that is monochromatic at a frequency of _CMB at redshift equal to zero, then the ratio of inverse Compton to synchrotron flux density at a fixed frequency in the observer's frame is simply given by

(7)

where u_CMB is the energy-density of the CMB at a redshift of zero and u_B is the energy density in the magnetic field in the rest-frame of the jet. Noting that u_B B_int² and _g B_int, where B_int is the intrinsic magnetic-field strength in the rest-frame of the jet,

(8)

If the modelling assumes minimum energy in relativistic particles and fields, then Equation 2 can be used. The luminosity density can be written in terms of the observable synchrotron flux density using

(9)

where D_L is the luminosity distance. The volume of a radio source can be specified in terms of its angular component sizes, _x, _y and path length through the source, d, as

(10)

Substituting for L and V (Equations 9 and 10) in Equation 2 then gives

(11)

i.e.,

(12)

Substituting for B_int = B_me in Equation 8 gives

(13)

Equation 9 (and thus Equations 11, 12 and 13) applies to a spherical blob in which S_syn ³⁺: for a continuous jet where S_syn ²⁺, B_me 1 / ^{(2+) / (3+)}, and Equation 13 has a slightly more complicated dependence on . Also, Equation 10 adopts the assumption that the pathlength through the jet is independent of redshift. Alternative assumptions could be adopted, modifying the redshift dependencies in Equations 11, 12 and 13.

3.3. How is the beamed iC-CMB model faring under scrutiny?

It was obvious that there were important consequences if the beamed iC-CMB interpretation of the X-ray emission from the resolved jet of PKS 0637-752 is correct, and holds for other quasar jets. In particular, increasing from the previously accepted value of ~ 2 to ~ 20 means increasing the jet power by a factor of ~ 100, or more if cold ions are an important contributor to the jet composition (see appendix B of [178]).

Programs targeting the resolved radio jets of core-dominated quasars with Chandra followed the work on PKS 0637-752 [170, 171, 141]. The detection success rate of roughly 50 per cent in relatively short exposures made it clear that PKS 0637-752 is not an outlier. Longer Chandra observations were made of some of the X-ray brightest and morphologically most interesting sources (e.g. 142, 169, 181, 182, 183, 115, 116, 178, 180, 179, 200). The combination of surveys and long pointed observations have made it possible to look critically at the application of the beamed iC-CMB model to these sources.

The high X-ray detection rate of quasar jets in short exposures is notable. In most Chandra observations of FRII radio galaxies at similar redshifts to the quasars, the jets (as opposed to the terminal hotspots) are not detected (e.g. 218, 12). This can be understood in the framework of quasar/radio-galaxy unification with reference to Figure 7 (based on Equation 8) which shows that for jets that are intrinsically the same, the ratio of beamed-iC to synchrotron radiation strongly decreases with increasing jet angle to the line of sight. The observed quasar X-ray jet emission is normally one-sided and on the same side as the brighter radio jet, in support of relativistic beaming. Where two-sided X-ray emission has been seen, explanations can be found which are not in violation of fast jet speeds (e.g. 71, 120).

Figure 7. Normalized ratio of inverse Compton to synchrotron flux density for a fixed intrinsic magnetic field strength for different jet angles to the line of sight. The normalization is to the value for a jet with a Lorentz factor of 15 and = 1.1 at 0° to the line of sight. Solid and dashed curves are for = 0.6 and = 1.1, respectively. Each set has curves for = 15, = 10, and = 5, in descending order at = 0°. Based on Equation 8.

In general the jets contain multiple knots that can be fitted independently to the beamed iC-CMB model with minimum-energy magnetic field strengths of order 10-20 µG (1-2 nT) (e.g. 178). Note, however, that there are insufficient observational constraints to fit the two free parameters of angle to the line of sight and bulk Lorentz factor separately, and an assumption must be made on one of these parameters. It has been common to assume sin = 1 / (i.e., = ), although this is not particularly sensible for sources where multiple knots in the same source give different values for , since it can lead to a jet that bends more erratically than makes physical sense. In some cases the results can be shown to agree with the estimates of speed and power from simple models for the pc-scale emission (e.g., 116, 200, and see Section 8.4), although with rather large uncertainties.

There is, however, a major difficulty with the beamed iC-CMB interpretation that arises from a detailed comparison between radio and X-ray emission. Figure 8 (based on Equation 5) shows the mean Lorentz factor of electrons that scatter photons from the peak of the CMB spectrum into the X-ray at 1 keV, for various jet bulk Lorentz factors and angles to the line of sight. The synchrotron emission from these electrons will be at a peak frequency of ² _g 30² B GHz, where _g is the gyrofrequency and B is magnetic field strength in Tesla. For a typical field of 2 nT, the radio synchrotron emission from these electrons is at 0.3 MHz if = 100, or 20 MHz if = 10³, both below the observable radio band. Under the beamed iC-CMB model, which requires small angle to the line of sight, , to be effective, the X-ray emission thus arises from lower-energy electrons than the radio emission. These electrons have long synchrotron energy-loss lifetimes. However, observations sometimes show X-ray emission that weakens relative to the radio towards the downstream regions of the jets and in some cases in individual knots, indicating that the population of low-energy electrons is being depleted more rapidly than the population of high-energy electrons, contrary to expectations based on radiation losses. This was seen in PKS 0637-752 ([177, 45], and see Fig. 9), and such behaviour is also seen strikingly in several other sources including 3C 273 [142, 169], quasar 0827+243 [115], PKS 1127-145 [181] and PKS 1136-135 [172]. Various suggestions have been made to overcome the problem within the framework of the beamed iC-CMB model, but none is uniformly regarded as satisfactory.

Figure 8. Mean Lorentz factor, , of electrons which scatter CMB photons near the black-body peak to X-ray photons of 1 keV. Results are shown for an emission region at selected angles to the line of sight over a range of bulk Lorentz factor, . Based on Equation 5.

Figure 9. The X-ray and radio profiles down the jet of PKS 0637-752 (see Fig 6). The X-ray intensity drops before the radio at large jet angles. 1" corresponds to a projected linear distance of 6.93 kpc.

It has been suggested that strong clumping in the jets may resolve the problem through adiabatic energy losses [198]. However, it is not clear that the beamed iC-CMB mechanism is then required, since such clumping would increase the SSC yield for a slow jet at minimum energy [177]. Alternatively, it has been suggested that jet deceleration is important, perhaps through entrainment of external gas (e.g. 81, 172, 199). A problem with this as a general solution is that, as shown in Figure 10 (based on Equations 8 and 13), the ratio of inverse Compton to synchrotron emission only falls for a decelerating jet over particular ranges of bulk Lorentz factor for jets at an angle of less than about 5° to the line of sight. This means that any source for which the X-ray drops off faster than the radio with downstream distance would need to be at particularly small angle to the line of sight or rather slow (but see [81] for a more detailed treatment that includes compression of the magnetic field and thus relative amplification of the radio synchrotron emission downstream). Jet deceleration is potentially testable through looking at the X-ray and radio profiles of source samples.

Figure 10. Normalized ratio of inverse Compton to synchrotron flux density for a fixed jet angle to the line of sight with decreasing Lorentz factor, . Left: with fixed intrinsic magnetic field. Right: with minimum-energy magnetic field. In each panel separately the curves are normalized to the value of SiC / Ssyn for a jet with a Lorentz factor of 15, = 1.1, at 5° to the line of sight. Solid and dashed curves are for = 0.6 and = 1.1, respectively. Each set has curves for = 5°, = 10°, and = 15°, in descending value on the y-axis at = 15.

A point in favour of the beamed iC-CMB explanation is that the particularly straight knotty jet in the quasar 4C 19.44 shows one of the most uniform X-ray to radio ratios over almost a dozen discrete knots in its straightest section ([179], and see Fig. 11). In contrast to PKS 0637-752, the radio drops more rapidly than the X-ray at the end of the straight, well-collimated jet beyond about 15" from the nucleus (Fig. 12). This might suggest that drops in the level of X-ray to radio emission along other jets are the result of the jets bending out of the line of sight. Since quasar jets are selected for observation based partly on their core radio emission, any bending downstream is more likely in a direction away from the line of sight than towards it. A large change in jet angle could easily produce the typical decreases in X-ray to radio ratio (a factor of a few to about 10; compare with Fig. 7). However, it is difficult to understand how a real change in angle of a ~ 20 flow by more than about a degree could occur without severe jet decollimation ². As apparent from Figure 7, more than a factor of about two decrease in X-ray to radio ratio is then not expected from bending alone.

Figure 11. The z = 0.72 quasar 4C 19.44, using data from [179]. The plot show an unsmoothed Chandra X-ray image of ~ 189.4 ks exposure with radio contours from a 4.86 GHz VLA radio map (beam size 0.47" × 0.43"). X-ray emission is detected from the nucleus, the southern radio jet, the northern hotspot and southern radio lobe. The excess X-ray counts in a line running NE-SW centred on the nucleus are a frame-readout artifact.

Figure 12. The X-ray and radio profiles down the jet of 4C 19.44 (see Fig. 11). In contrast to PKS 0637-752 (Fig. 9), the radio intensity drops before the X-ray at large jet angles. 1" corresponds to a projected linear distance of 7.23 kpc.

A test that the beamed iC-CMB explanation must pass concerns the redshift dependency. The increase in CMB energy density with redshift means that the X-ray to radio ratio should increase with redshift by a factor of something like (1 + z)² (Equation 13: the precise dependence on redshift depends on assumptions concerning minimum energy and whether or not the path length through the source is redshift dependent). Such a redshift effect is not ruled out [141] although a larger sample is needed for a more definitive test.

3.4. Synchrotron emission as an alternative

The fast jet speed required for the beamed iC-CMB explanation of quasar X-ray emission disappears if an alternative explanation can be found for the X-rays. It is then natural to invoke synchrotron radiation, the mechanism producing the X-rays in low-power FRI jets (see Section 5.1). However, whereas for FRI jets the SED can normally be modelled with a broken power-law spectrum from the radio, through the optical to the X-ray (e.g. 29, 96, 21), PKS 0637-752 has too little optical emission to allow this, and a separate population of electrons with an anomalously high low-energy cutoff would be required [177]. Figure 13 compares the spectral distribution of the FRI radio galaxy M 87, where a broken-power-law synchrotron components fits well, with that of the FRII quasar PKS 0637-752.

Figure 13. Spectral distributions from the radio to X-ray. Left: The integrated emission from Knots A, B and C of M 87, using data from [28] and [13], fits a broken power law synchrotron spectrum, although the change of 1.5 in electron spectral slope is greater than expected from a simple model for synchrotron energy losses. Right: A broken power-law spectrum does not fit through the emission from Knot WK7.8 of PKS 0637-752 (taken from [45]) although a synchrotron component with an exponential cutoff and either a beamed iC-CMB component or a separate synchrotron component with an anomalously high low-energy cutoff can be made to fit the data.

Most of the several tens of current quasar X-ray jet detections were found through targeted Chandra programs to observe bright, prominent, one-sided radio jets. In most cases there was no pre-existing reported optical jet detection, but there has been reasonable success from follow-up work. The level of such optical detections often lies below an interpolation between the radio and X-ray spectra, supporting the idea that synchrotron emission from a single power-law distribution of electrons is not responsible for all the emission (e.g. 171).

However, the conclusion regarding synchrotron emission is not quite as clear cut, since a single-component electron spectrum will harden at high energies if inverse-Compton losses are also important (since this loss process is less efficient in the Klein-Nishina regime), and the consequent spectral hardening in the synchrotron spectrum might then better match observations [60].

As long as electrons can be accelerated to high energy (and they can be in FRIs) they will produce synchrotron radiation at some level. Radio galaxies are at large angle to the line of sight and any iC-CMB emission will be beamed out of the line of sight of the observer (an extension of to large angle shows that, even for the most optimistic case, the ratio of iC-CMB to synchrotron emission drops three orders of magnitude between = 0 and = 80°). Indeed, synchrotron X-ray emission from knots in the radio jets of nearby FRII radio galaxies is reported (e.g. 212, 219, 125, 120). When optical detections are also available, the energy distributions [125, 219] are of similar simple form to those in FRIs (see Fig. 14), not requiring the complex electron spectral forms generally needed to explain quasar X-ray emission as synchrotron radiation. Spatial offsets reminiscent of those seen in FRIs and which are presumably a feature of the particle acceleration processes (see Section 5.2) are also seen [219, 120].

Figure 14. The spectrum from the radio to X-ray for the knot in the FRII radio galaxy 3C 346 (see Fig. 16) fits a broken-power-law synchrotron model.

It remains uncertain as to whether or not in quasars it is necessary to explain the jet X-ray emission as the synchrotron output of a distorted electron spectrum [60] or from separate populations of electrons (e.g. 7), as an alternative to the beamed iC-CMB model. In the case of 3C 273, the run of X-ray spectral slope down the jet rules out a simple beamed iC-CMB interpretation, but a two-zone iC-CMB model with a faster spine, although disfavoured, cannot be ruled out ([112]). If a synchrotron interpretation is sought, similar, simple electron spectra in all jet regions do not fit observations (e.g. 166, 169, 142). A two-zone model with faster spine has been proposed, where, unlike for beamed iC-CMB in which the X-rays are from the spine, the X-rays would arise from the shear layer through electron acceleration to very high energy [112].

It is important to understand the primary X-ray emission in quasar jets, and this remains an observational problem - more work on samples and further detailed, deep, multiwavelength observations of individual sources are needed. Predictions for yields at higher energies also differ according to the X-ray emission mechanism, and so there is a prospect that the new Fermi Gamma-ray Space Telescope will help in finding solutions (e.g. 60, 83). Optical polarimetry is potentially a strong discriminant since, unlike for optical synchrotron emission, the optical emission should be essentially unpolarized if it is a lower-energy extension of X-ray emission that is produced via the beamed iC-CMB mechanism (e.g. 113, 203).

² Large changes in jet angle in projection are observed in many sources, but Figure 7 relates to the true jet angle to the line of sight. Back.

4. WHAT KEEPS JETS COLLIMATED?

X-ray measurements of the external medium support arguments that low-power FRI jets slow through entrainment of this gas.

For the few low-power radio galaxies with heavily studied, straight, radio jets and counterjets (and so lying relatively close to the plane of the sky and presumably in relatively relaxed environments), kinematic models have been constructed to fit the jet-counterjet asymmetry [131, 38, 39, 132]. Typically, the jets start fast (relativistic) and relatively faint with a small opening angle. Then they go through a flaring region where they steadily broaden and are typically bright both in radio and X-ray (Fig. 1), and finally the opening angle changes and the jet becomes fainter, particularly at X-ray energies (e.g. 221). It is in this final region, beyond that shown for NGC 315 in Figure 1, that the jets are modelled as decelerating steadily as they collect mass from the external medium or stellar winds [122]. Buoyancy forces are then important for much of the flow further downstream, as the jets adjust to changes in the density of the external medium, causing deflections from straight-line motion.

In ongoing work, these kinematic models are being extended into dynamical models, based on conservation laws for mass, momentum, and energy [16], and are being tested for self consistency with the density and pressure of the external medium. For one source so far, 3C 31, excellent self consistency has been found [130]. This lends confidence to an understanding of the basic flow behaviour of these sources.

Deceleration via mass entrainment is consistent with a range of observational evidence at radio frequencies [129], and naturally leads to the outer parts of the jet (sheath) being decelerated before the inner (spine). Applied to more central regions, the consequence that emission from a slower sheath becomes relatively more important in jets at larger angle to the line of sight then resolves difficulties in models that unify BL Lac objects with FRI radio galaxies (e.g. 48).

It has been known since the Einstein and ROSAT X-ray observatories that the minimum pressure in low-power FRI jets (calculated without relativistic protons) is normally below that of the external X-ray-emitting medium (e.g. 149, 121, 77, 216). The model for 3C 31 [130] demonstrates that entrainment of the external medium explains the jet dynamics in the deceleration region, and pressure balance can be achieved by adding relativistic protons (with neutrality preserved by balancing proton and electron number densities) or extending the electron spectrum to lower energies (if electron-positron charge balance is enforced). Recent work [56] has claimed a greater pressure imbalance in FRI jets that are more in contact with external gas (less in contact with the plumes or lobes of older jet plasma), and speculates that the pressure is balanced by heated entrained material, with an entrainment rate or a heating efficiency that is higher where jets are in greater direct contact with the X-ray-emitting atmosphere. This seems in conflict with the entrainment model for the quasar PKS 1136-135 in the context of the beamed iC-CMB model, where a standard model would have the jets heavily embedded in old lobe plasma and yet where the estimated entrainment rate is an order of magnitude higher than for 3C 31 [199].

While the X-ray-emitting interstellar or intergalactic medium can thus be controlling the flow where FRI jets are decelerating, and indeed where buoyancy forces or an excess of gas pressure dominate (e.g. 215, 222), FRI radio jets are highly overpressured in their inner regions close to the nucleus (e.g. 130). Here the X-ray emission has yet to contribute in a significant way to the collimation debate.

The jets of FRII radio galaxies are not significantly in contact with the external medium for most of their length, so the external medium is unlikely to control jet collimation, although entrainment of external gas might be significant over their long propagation paths [199]. Current uncertainties in the jet X-ray emission mechanism, and thus the particle content and energy, make direct comparison of the internal and external pressures difficult, except in the large-scale lobes if dynamical effects are ignored.

5. WHERE AND HOW DOES PARTICLE ACCELERATION OCCUR?

5.1. The link with synchrotron X-ray emission

Chandra found X-ray synchrotron emission to be common in the resolved kpc-scale jets of FRI radio sources [217]. The X-ray jets are readily detected in sources covering the whole range of orientation in unified schemes. The several tens of detected sources range from beamed jets in BL Lac objects [21, 157, 173] to two-sided jets in radio galaxies [46, 97], with most X-ray jets corresponding to the brighter radio jet, (e.g. 217, 96, 104, 143, 68, 221). Several of the observations have been targeted at sources already known to have optical jets, from ground-based work or HST. However, it's proved easier to detect X-ray jets in modest Chandra exposures than to detect optical jets in HST snapshot surveys, because of better contrast with galaxy emission in the X-ray band than in the optical [217].

Inverse Compton models for any reasonable photon field suggest an uncomfortably large departure from a minimum-energy magnetic field in most low-power X-ray jets (e.g., 96), although the beamed iC-CMB model is a contender for the emission from some BL Lac objects (e.g., 173). Otherwise synchrotron mission from a single electron population, usually with a broken power law, is the model of choice to fit the radio, optical, and X-ray flux densities and the relatively steep X-ray spectra (e.g., 29, 96). Given Equation 6, X-ray synchrotron radiation at 1 keV requires electrons of energy ~ 10¹³ eV (Lorentz factor 2 × 10⁷) if the magnetic field strength is of order 20 nT (200 µG; the electron energy scales as B^-1/2). Averaging over pitch-angle distribution, the lifetime of synchrotron-emitting electrons is given by

(14)

where m_e is the electron mass, _T is the Thomson cross section, and u_B is the energy density in the magnetic field. We thus see that electrons emitting 1 keV synchrotron radiation in a 20 nT magnetic field have an energy-loss lifetime of about 30 years (lifetime scales as B^-3/2). The electrons must therefore be accelerated in situ, since their lifetimes against synchrotron losses are less than the minimum transport times from the active nuclei, or even from side to side across the jet. (This should not be the case if proton synchrotron radiation is important [2], since lifetime scales as (m_p / m_e)^5/2.) Particle acceleration is generally discussed for the cases of a particle interacting with a distributed population of plasma waves or magnetohydrodynamic turbulence, or shock acceleration (see e.g., 25, 66, 108, 5).

For electrons, particle acceleration and energy losses are in competition (e.g., 106), no more so than in hotspots of FRIIs (e.g., 34), which mark the termination points of the beam. Hotspots display considerable complexity in the X-ray, with synchrotron components seen in the less powerful sources indicating that TeV electrons are present (e.g., 98, 126). It has been suggested that the low-energy radio spectral-slope change seen in hotspots may mark a transition between electrons that are accelerated through electron-proton cyclotron resonance and those (at higher energy) that are simply undergoing shock acceleration (e.g., 192, 91). If in FRIs the far-IR spectral break consistently maps electrons of a particular energy, it is possible that the break here is also more related to acceleration than loss processes [22].

Whether or not particle acceleration is required along the jets of quasars depends on the emission process at high energies. If the beamed iC-CMB model holds, then the electrons participating in radiation at wavelengths currently mapped are generally of low enough energy to reach the end of the jet without significant energy loss, except if a relatively high level of optical emission must be explained as synchrotron radiation. The knotty nature could then be understood as variable output in the jet (e.g., 191). However, in nearby FRII radio-galaxy jets, where synchrotron X-ray emission is seen (Section 3.4), the need for particle acceleration is secure, and similar underlying processes are expected in quasars even where the synchrotron X-rays might be outshone by beamed iC-CMB emission.

Details of the regions of particle acceleration are best studied in the closest sources. Cen A (Fig. 15) and NGC 315 (Fig. 1) are particularly good examples of FRI jets where the X-ray jet emission is resolved across as well as along the jet, and X-ray knots are embedded in more diffuse structure [97, 221, 100, 223]. The fact that the X-ray emission is not just confined to regions within energy-loss light travel distances of the knots shows that particle acceleration can occur also in diffuse regions. The relatively soft X-ray spectrum seen in the diffuse emission in Cen A has been used to argue that something other than shock acceleration (proposed for the knots) might be taking place in the diffuse regions [100], although no specific explanation is suggested, and the competition between energy losses and acceleration may be more important here.

Figure 15. A rotated image of a roughly 4.5 kpc (projected) length of the 0.8-3 keV X-ray jet of Cen A from combining six deep (~ 100 ks) Chandra exposures. Image taken from [223].

5.2. Particle acceleration in knotty structures

The model of jet deceleration through entrainment (Section 4) leaves unanswered important questions about the origins of the bright knots that appear in many jets, particularly FRIs, and that are usually interpreted as the sites of strong shocks. Radio studies have searched for high-speed knot motions, with apparent speeds greater than the speed of light having been noted in M 87 [19]. A proper-motion study of the knots in Cen A over a 10-year baseline found that some knots, and even some more diffuse emission, travel at about 0.5c, indicative of bulk motion rather than pattern speed [97]. This motion, coupled with the jet-to-counter jet asymmetry, suggests considerable intrinsic differences in the two jets, to avoid the jets being at an implausibly small angle to the line of sight.

Other knots in Cen A appear to be stationary, which might suggest that they result from intruders in the flow, such as gas clouds or high-mass stars (e.g., 75, 97). Some of these have emission profiles in the X-ray and radio that are unexpected from a simple toy model where the electrons are accelerated and then advect down the jet, losing energy from synchrotron radiation. Instead the bulk of the radio emission peaks downstream from the X-ray within these knots, leading to suggestions that both radio and X-ray-emitting electrons are accelerated in the standing shock of a stationary obstacle, and a wake downstream causes further acceleration of the low-energy, radio-emitting, electrons [97]. The resulting radio-X-ray offsets, averaged over several knots, could give the radio-X-ray offsets commonly seen in more distant jets (e.g., 96, 219, 63).

The knots of Cen A are not highly variable in observations to date [100], but dramatic variability on a timescale of months is seen in a knot in the jet of M 87, and the X-ray, optical and radio light curves are broadly consistent with shock acceleration, expansion, and energy losses, although the timeline is currently too short for strong conclusions to be drawn [105].

It is important to study the location of jet knots within the flow, to see if that can provide a clue as to their nature. A particularly interesting example is NGC 315 [221]. Here the diffuse emission contains a knotty structure in the radio and X-ray that appears to describe an oscillatory filament (Fig. 1). Although the structure could be the result of a chance superposition of non-axisymmetric knots, the level of coherence led to suggestions that the knots might be predominantly a surface feature residing in the shear layer between the fast spine and slower, outer, sheath plasma. If this interpretation is correct, we might expect the X-ray spectra of the knots to be similar across the transverse width of the jet. However, the distinct knotty emission is only about 10% of the total in X-rays and radio along the ~ 2.5 kpc of projected jet length over which it is detected, and with a source distance of ~ 70 Mpc the observations did not allow the spectra of the knot and diffuse emission to be separated.

At 3.7 Mpc, Centaurus A is a much closer example of an FRI radio galaxy whose knots and diffuse emission are seen over a similar projected linear distance to that of NGC 315. An X-ray spectral study of Cen A's knots found a spectral steepening with increasing lateral distance from the jet axis, disfavouring these knots all residing in a shear layer [223]. A flatter X-ray spectrum is seen more central to the flow, and an alternative explanation to acceleration in stationary shocks is that the knots here might be formed by stronger turbulent cascades with more efficient particle acceleration. Knot migration under the influence of the shear flow might then be expected, and proper-motion studies might then distinguish between this interpretation and stationary shocks from stellar or gaseous intruders entering the flow [223].

5.3. Incorporating polarization data

There are no current X-ray missions with polarization capabilities. However, the radio and optical bands probe electron populations responsible for the X-ray emission, albeit at different electron energies. If the emission is synchrotron, polarization data provide our best handle on the direction and relative degree of alignment of the magnetic field. Radio observations show that the fields are relatively well ordered, although there is much complexity. Broadly, the magnetic fields in FRII jets tend to be parallel to the jet axis, whereas in FRI jets they are either predominantly perpendicular, or perpendicular at the jet centre and parallel near the edges, with the mixed configurations pointing to perpendicular fields associated with shocks and parallel fields from shear or oblique shocks [32].

Optical polarization measurements of resolved jet structures have been made with HST. So far these have mostly concentrated on nearby FRI radio galaxies, where the optical features are brighter and the emission mechanism is synchrotron radiation (for an atlas of polarization images see [156]). Work is under way to explore optical polarization in the jets of FRII radio galaxies and quasars. As mentioned in Section 3.4, the optical emission should be essentially unpolarized if it is an extension of a beamed iC-CMB X-ray component, in contrast to being of synchrotron origin.

The first jet to be studied in detail in both its radio and optical polarized emission was M 87, where there is evidence for strong shock acceleration in compressed transverse magnetic fields at the base of bright emitting regions, although the polarization fraction becomes low at the flux maxima [155]. Significant differences between the polarization structures seen in the optical and radio suggest that the sites of acceleration are different for different electron energies, with the strongest shocks, that provide acceleration to the highest energies, appearing in the most central parts of the jet [155]. Detailed work on 3C 15 shows a jet that narrows from the radio to the optical to the X-ray, showing that acceleration to the highest energies occurs more centrally to the flow, and a mixture of strong shocks and stratified flows can account for the broad features seen in the optical and radio polarization [63].

A third source for which optical and radio polarization data have been important is 3C 346 (Fig. 16). Here X-ray emission is associated with a bright radio and optical knot where the jet bends by 70° in projection (the X-ray emission peaks somewhat upstream of the radio, as seen in other sources), leading to a suggestion that the bending and X-ray brightening are the result of a strong oblique shock located in the wake of a companion galaxy [219]. Polarization data has supported the model by revealing a compressed and amplified magnetic field in a direction consistent with that of the proposed shock, in both the radio and optical ([64] and see Fig. 16).

Figure 16. 3C 346. Upper: Schematic showing an oblique shock formed in the wake of the passage of a companion galaxy to 3C 346, and how it affects the radio jet, from [219]. Circles are the galaxies and red marks the path of the radio jet. Lower: Radio intensity contours and polarization vectors (rotated through 90° roughly to represent the magnetic-field direction) on a smoothed Chandra X-ray image, indicating compressed field lines aligned with the proposed shock, from [64].

Jets are presumed to obtain much of their energy from the infall of matter into a supermassive black hole. It is then natural to suppose that electromagnetic radiation would carry much of the energy from the system on the smallest scales, since a plausible mechanism for the extraction of energy is the twisting of magnetic field linked to the accretion disk (e.g., 138). Fast interactions with the plasma environment and efficient particle acceleration should load the field with matter. In the resulting magnetohydrodynamic flow much of the momentum would be carried by particles, although Poynting flux may carry a significant fraction of the total energy [163, 6].

Polarized radiation is the signpost to significant energy in relativistic particles and magnetic fields. Jet plasma must be neutral, on average, to remain collimated, but this can be achieved by various combinations of relativistic and cold electrons, positrons, and protons. Alternatively, it has been suggested that some of the energy is transported in a decaying neutral beam of ultra-high-energy neutrons and -rays [7].

Several quantities are available to help sort out the jet composition.

1. The synchrotron emission. Since the electron rest mass is only 1/1836 that of a proton, and since synchrotron energy loss rates are proportional to the inverse square of mass, the observation of synchrotron radiation is usually used to infer the presence of relativistic electrons (and perhaps positrons), although an alternative model produces the synchrotron radiation from protons accelerated to energies greater than ~ 10¹⁸ eV [2].
2. The jet power. All the particles, relativistic and thermal, combine with the magnetic field strength and bulk Lorentz factor to produce this quantity (see appendix B of [178]). It should be no smaller than the radiative power of the old lobe material (the energy sink), averaged over the lifetime of the source. In cases where jets have excavated cavities in the external gas, the enthalpy can be estimated as that required to displace the gas (e.g., 23, 65, 4).
3. Faraday rotation. The contribution from thermal particles must not be so high as to exceed constraints placed by Faraday rotation, or by Faraday depolarization for extended regions.
4. The jet pressure. Relativistic particles and magnetic field are thought to dominate this quantity, which is 1/3 of their energy density, and which can be compared to the external gas pressure. If X-ray inverse Compton emission is observed the internal energy density can be estimated using the radio synchrotron and X-ray flux densities (Section 2.2). Otherwise it is usual to assume minimum energy (Section 2.1). A difficulty is that relativistic electron-proton and electron-positron jets give similar pressures with different assumptions about the least energetic particles, for which observational constraints are poor at best. The contribution of thermal particles to the pressure is usually taken to be small.

Radiation drag and observational constraints on Comptonized radiation by cold electrons and positrons seriously hamper electron-positron jets formed close to the central black hole [185, 186]. In the cores of some quasars the radiated power is too large to be met by that contained in a jet of magnetic field and relativistic leptons close to minimum energy, and observational constraints on Comptonized radiation limit the density of cold leptons, so that a significant proton component is required if the energy carrier is indeed particles [196]. Thus, an electron-proton plasma is usually favoured when jets are discussed.

The presence of relativistic protons is supported for some FRI radio galaxies: the lobes, if assumed to be lepton-dominated and radiating at minimum energy (Section 2.2) would collapse under the pressure of the X-ray-emitting medium unless there is an additional pressure source and, although there are several ways of boosting the internal pressure in such a situation, magnetic dominance would make the sources unusual, electron dominance is unlikely from constraints on inverse-Compton scattering of the CMB, and non-relativistic protons are disfavoured on grounds of Faraday rotation, leaving a relativistic proton component most likely (e.g., 53). However, decreased filling factors cannot be ruled out (e.g., 65), except perhaps where the radio structure has excavated a clear cavity in the X-ray-emitting atmosphere (e.g., 24). If indeed the extra pressure is from relativistic protons, it is uncertain as to how much arises from entrained material accelerated in the shear layer of the decelerating jet as compared to particles transported from the core (see Section 4).

FRII jets transport more energy to larger distances, and thus have more need than FRI jets for a non-radiating energy carrier with high momentum transport. Relativistic hydrodynamic simulations find that the key parameter in preventing jets from strongly decelerating in an external boundary layer is density contrast with the external medium, in the sense that denser jets can propagate further [167]. A more dominant relativistic proton content could provide this. Protons are also required if the low-frequency spectral turn-over in hotspots is the result of cyclotron resonant absorption (e.g., 91). On the other hand, pressure balance has been used to argue against relativistic protons in some FRII lobes. It is argued that the presence of relativistic protons is improbable since (a) the lobe magnetic field based on synchrotron and inverse Compton emission agrees with that from minimum energy calculated using relativistic leptons alone, and (b) that, even in the absence of such protons, the source is in pressure balance with the external medium (e.g., 10, 54). However, these calculations ignore possible dynamical effects in FRII lobes, and there are considerable additional sources of uncertainty (see Section 2.2 and item 4 above).

In the context of the beamed iC-CMB model for quasar jets (Section 3.2), it is possible to extend an argument limiting the density of cold electron-positron pairs [186] to kpc-scale regions [82]. In the case of PKS 0637-752, upper limits on Comptonized CMB radiation from Spitzer are sufficiently low to place stringent limits on the mass flux carried by cold lepton pairs, with the implication that this jet is indeed made electrically neutral through a strong presence of protons [203]. However, this argument relies on the beamed iC-CMB model being correct, with a large kinematic power being sustained throughout the jet (see Section 3.3).

Jet composition remains uncertain, and various degeneracies between physical quantities and observable parameters render it difficult to make watertight arguments. However, X-ray measurements continue to provide important clues to the puzzle.

7. WHAT DOES X-RAY EMISSION TELL US ABOUT THE DYNAMICS AND ENERGETICS OF RADIO PLASMA/GAS INTERACTIONS?

7.1. Expectations for FRIIs

The energy and momentum fluxes in FRII jets are expected to be sufficient to drive a bow shock at supersonic speed into the ambient medium (e.g., 134). Ambient gas crossing the bow shock will be heated. For a shock advance speed relative to the speed of light of v_adv / c, the Mach number, , in monatomic gas of normal cosmic abundances with thermal energy kT in units of keV, is given by

(15)

For a non-relativistic equation of state ( = 5/3), the jump conditions for a non-radiating shock (e.g., 189) find that pressure, density, and temperature ratios between gas that has crossed the shock and the ambient medium are

(16)

(17)

(18)

where subscripts 2 and 1 refer to post-shock and pre-shock conditions, respectively. For high advance speed and large Mach number the density contrast reaches a factor of four, resulting in enhanced X-ray emissivity from shocked gas. The visibility in observations will depend on the relative volumes of shocked and unshocked gas along given lines of sight.

Complications apply in reality. Firstly, there is observational evidence that in supernova remnants the post-shock electrons are cooler than the ions (e.g., 109, 160). Secondly, a bow shock around a lobe is oblique away from its head, with a consequent change in the jump conditions and the emissivity contrast [210]. The closer a structure is to a spherical expansion, the more normal the shock will be everywhere and the better the applicability of the above equations.

ROSAT data revealed the presence of X-ray cavities coincident with the inner parts of the radio lobes of Cygnus A, and these were interpreted as due to the contrast between undisturbed ambient gas and gas around the lobes that had been heated in the past but has now expanded and cooled to a low emissivity [42], although the parameters of the shock are not effectively constrained by the data. More recent Chandra observations of Cygnus A find gas at the sides of the lobes to have kT ~ 6 keV, slightly hotter than the value of 5 keV from ambient medium at the same cluster radius, but the gas may have cooled after bow-shock heating, and again the data do not usefully constrain model parameters [188]. Evidence of strong shock heating around more distant FRII radio galaxies has yet to be seen.

CSS and GPS sources have been examined for evidence of shock heating. These are good places to look as the radio sources are generally considered to be in an early stage of expansion and they are overpressured with respect to even a cluster ambient medium (e.g., 184). The disadvantage is that source sizes are small so that even Chandra will have difficulty in separating emission from the nuclei, radio structures, and ambient medium from that of any shocked gas. The best evidence for detection of shocked gas thus arises from deep XMM-Newton spectroscopy, and in particular that of the CSS source 3C 303.1 [152]. The X-ray spectrum contains soft emission (associated with the ambient galaxy atmosphere) and a hard component. Since nuclear emission is undetected in the radio, it is reasonable to associate the hard emission with shocked gas, and a model can be constructed [152] that has an expansion velocity consistent with cooling-time arguments for optical emission-line gas [61].

7.2. Dynamics of FRIs in clusters

Low-power sources are closer and more amenable to detailed study, since the various components of X-ray emission are more easily separated. The medium plays an important rôle in the deceleration of the jets, which share momentum and energy with entrained material (Section 4).

The Einstein and ROSAT missions found evidence that the radio lobes of NGC 1275 have pushed Perseus-cluster gas aside (e.g., 28), and now many clusters and groups are found to harbour gas cavities containing radio plasma that originates from active galaxies (e.g., 23). Rather than expansion at high Mach number, the displacement of the gas appears normally to create low-density, rising bubbles in rough pressure balance with the surrounding medium (e.g., 50). NGC 1275, M 87, and Hydra A are showcase examples with deep Chandra exposures and complex bubble and cavity systems [72, 78, 213]. Radio bubbles in clusters are sufficiently common that they are an important heat source today, with enough power to balance the radiative cooling of dense gas in clusters (e.g., 65, 159), although the total energies and lifetimes of individual bubbles are considerably uncertain. An issue of particular interest that follows from this is the potential for the associated heating and cooling to forge the link between black-hole and galaxy growth. A recent review is available [144], and so the topic is not dealt in depth here.

It is noteworthy that the luminosity function of radio sources places the energetically dominant population to be roughly at the FRI/FRII boundary (e.g., 136), rather than within the more numerous but lower power population of FRIs studied in nearby clusters (although there are claims that total jet power scales slightly less than linearly with radio power (e.g., 211, 24). It thus remains possible that the rather gentle heating around currently studied sources does not provide us with the complete picture, and violent shock heating around more powerful sources is energetically important but currently eluding detection.

7.3. Centaurus A

The best example of supersonic expansion is not in an FRII radio source but associated with the inner southwest radio lobe of Cen A ([124] and see Fig. 17 for a more recent, deeper, Chandra image). Cen A is our nearest radio galaxy, where 1 arcmin corresponds to ~ 1.1 kpc. The full extent of Cen A's radio emission covers several degrees on the sky [117]. Within this lies a sub-galaxy-sized double-lobed inner structure [36] with a predominantly one-sided jet to the NE and weak counter-jet knots to the SW [97] that are embedded in a radio lobe with pressure at least ten times larger than that of the ambient ISM [124]. The lobe should be expanding and be surrounded by a shock. The associated structure is exquisitely seen in Figure 17. Although the capped SW lobe is around the weak counterjet, so it is not evident that the lobe is being thrust forward supersonically with respect to the external interstellar medium (ISM) by the momentum flux of an active jet, the high internal pressure in the radio lobe ensures its strong expansion.

Figure 17. Radio contours on a deep Chandra image of Cen A, showing the core and NE jet crossed by absorption stripes corresponding to NGC 5128's dust lanes, the SW lobe, structures associated with the NE lobe, the position of a merger-related gas discontinuity that shows up better at lower energies, and many XRBs in NGC 5128 (see [100, 114, 223, 127, 187]).

The density contrast between post-shock and pre-shock gas in Cen A inferred by [124] was larger than four, which is not allowed by Equation 17, and so straightforward modelling was not possible. New modelling is underway using results from the new deep observation. However, the conclusion that the lobe's kinetic energy exceeds its thermal energy, and the thermal energy of the ISM within 15 kpc of the centre of the galaxy, is unlikely to change. As the shell dissipates, most of the kinetic energy should ultimately be converted into heat and this will have a major effect on Cen A's ISM, providing distributed heating.

There is much still to be learned about how gas is displaced by radio structures, and the processes of heat transfer. A new view will be possible with the high-resolution spectroscopic capabilities of the International X-ray Observatory currently under study by ESA and NASA. This will provide the vital ingredient of useful velocity data, giving a handle also on such issues as turbulence and non-perpendicular velocities at shocks.

7.4. The effect of galaxy mergers

It is important to understand what triggers radio activity and what causes it to cease, particularly since radio sources are now recognized as an important heat source for large-scale structure (Section 7.2). It has long been recognized that mergers may be important in triggering radio activity, and this is consistent with the preference for low-power radio galaxies to reside in clusters and rich groups. For example, NGC 1275 and M 87 (7.2) are the dominant galaxies of the Perseus and Virgo clusters, respectively. Cen A (7.3) is hosted by NGC 5128 which in turn hosts an inner warped disk suspected to be the merger remnant of a small gas-rich spiral galaxy (e.g. 158).

Mergers leave an imprint on the temperature, density, and metallicity structures of the gas. Due to good linear resolution it is again Cen A that shows such effects particularly well, with clear indications that even the hot X-ray-emitting gas is poorly mixed. The merger appears to be having an important influence on the evolution of the northeast radio jet and inner lobe [127].

In the more extreme case of 3C 442A (Figure 18) there is evidence that a merger may have smothered a previously active jet, leaving a large volume of decaying radio plasma, while at the same time re-starting jet activity in the nucleus of one of the galaxies [222]. Here the merger gas has sufficiently high pressure for the radio lobes to be riding on the pressure front of the merger gas that is sweeping them apart. The energy in the merger gas will eventually be dissipated in the outer regions of the group atmosphere - an additional source of heating to that arising from both the old and new merger-induced radio activity. The radio spectrum from the old decaying radio lobes is flatter where they are being compressed by the expanding merger gas, suggesting that energy from the gas has a second effect, in re-exciting relativistic electrons through compression and adiabatic heating [222]. While it is undoubtedly true that mergers produce messy substructures, the example of 3C 442A suggests that there is some prospect that the switching on and off of radio activity by mergers can be timed (albeit roughly) using the morphology of the stellar component of the galaxies and spectral changes in the radio plasma, and that this can be combined with the measured energy content of the gas and radio plasma to trace the history of radio outbursts and their effectiveness in heating gas.

Figure 18. Chandra contours (logarithmic spacing) on a color radio image of 3C 442A, taken from [222]. Bright X-ray emission from the merger atmospheres of NGC 7236 and NGC 7237 fills the gap between the radio lobes which are no longer fuelled by an active jet.

8.1. An evolutionary cycle?

The Ledlow-Owen relation (Section 1.3) showed that a galaxy of a given optical luminosity can host either an FRI or FRII radio source. This resulted in renewed speculation in the 1990s that there may be an evolution between FRII and FRI activity controlled by external influences. Such speculation was supported by evidence of FRIIs associated with galaxy mergers (distorted isophotes and higher amounts of high-excitation ionized gas) and FRIs associated with galaxies in more relaxed dynamical states [17]. Evolutionary ideas have also arisen from the so called `fundamental plane' that places AGN on an extension of the relationship between inner jet radio power, X-ray luminosity and black-hole mass found for X-ray binaries (XRBs) [146, 73]. It has been suggested that the changes in X-ray spectrum and jet luminosity that accompany changes in accretion characteristics in an XRB could apply to AGN, such that an individual object may go though transitions between an FRI and FRII, and indeed to becoming radio quiet (e.g., 123).

Observationally, kpc-scale jets accompany AGN with accretion flows that in the extreme are either geometrically-thick and radiatively inefficient or geometrically-thin and radiatively efficient, with the latter accompanied by high-excitation optical emission lines. It is possible that an AGN changes over the lifetime of a radio source, such that the observed kpc-scale radio structures are the result of ejection from an AGN evolving through different states. Some sort of intermittency of the central engine over timescales of ~ 10⁴ - 10⁶ years (shorter than the lifetime of radio sources, Section 1.4) gains support from observational and theoretical considerations (e.g., 164, 176, 111, 191). Multiple changes to the central structure over the lifetime of the radio source would be required to reconcile the claim that a geometrically-thick flow is needed to sustain a significant jet (with the most powerful requiring a spinning black hole) [145] with the observation that many AGN with powerful jets currently show geometrically-thin disks and high-excitation emission lines (see below).

Closer examination is needed of the extent to which the observed powers and structures of jets relate either to the accretion processes or to large-scale environmental effects. Both appear to play a rôle.

8.2. The rôle played by accretion processes

Broadly, powerful jets of FRII structure are associated with AGN showing high-excitation optical emission lines, while lower-power jets, normally but not always of FRI structure, are associated with AGN showing low-excitation lines. This suggests that the central engine has at least some influence on the power and large-scale structure of the jets (e.g., 9).

A correlation between the core radio emission and low-energy (~ 1 to 2 keV) nuclear X-ray output of radio galaxies has been known since the Einstein and ROSAT missions, and has been used to argue that the soft X-rays arise from pc-scale jets [70, 214, 37, 93]. An optical core is often seen with HST, and is interpreted as synchrotron emission from a similar small-scale emitting region [47, 94, 40, 49, 207]. Such pc-scales jets protrude from any gas and dust torus invoked by AGN unified models, and so this component should not be greatly affected by absorption, although relativistic effects will cause jet orientation to affect the level of X-ray flux observed.

Since jet emission dominates at low X-ray energies, it has been important to obtain sensitive spectral measurements that extend to the higher X-ray energies accessible to Chandra and XMM-Newton in order to probe the region closer to the SMBH and representative of the bolometric power of the central engine. At these energies any strong emission from the AGN should dominate jet emission even if it is largely absorbed at lower energies by a gas torus. Results find a number of radio galaxies showing clear evidence of a hard continuum, sometimes accompanied by Fe-line emission, and presumed to be emission associated with an accretion-disk corona (e.g., 204, 224, 90). Both the jet and central-engine X-ray components can sometimes be distinguished in the same spectrum (e.g., 54, 67, 225).

The hard component is more often detected in FRIIs than in FRIs. Of course, greater absorption from a torus could potentially combine with lower X-ray luminosity in causing the non-detection of the second component in most FRIs, and so particular reliability can be placed on the results of a study of nearby (z < 0.1) radio galaxies that has allowed for absorption in placing upper limits on the luminosity of undetected nuclear components [69]. The radiative efficiency of the central engine was then found by correcting the X-ray luminosity to a bolometric luminosity and combining it with the inferred SMBH mass. In powerful FRIIs, radiatively-efficient accretion associated with a thin disk surrounded by an obscuring torus is normally inferred. FRII radio galaxies at z ~ 0.5 also show an absorbed X-ray component [11]. In contrast, in z < 0.1 FRIs, all the nuclear X-ray emission can normally be interpreted as jet related, and usually only upper limits are found for accretion-related emission [69]. Any X-ray luminosity associated with a non-jet central-engine component in low-power sources is normally sufficiently low to support earlier speculations based on the Ledlow-Owen relation that the physical difference between the two types of radio source arises from the different nature of their accretion disks and efficiency of accretion [85]. Further support for these ideas comes from Spitzer results for the z < 0.1 sample [22] that show an additional component of hot dust only in FRIIs.

While results at first-look appear quite convincing of a connection between large-scale radio power and the structure of the central engine, there are sources which defy the trend. Both Cen A and NGC 4261 have large-scale FRI structures, and yet contain absorbed, hard, luminous X-ray components characteristic of the coronae of thin accretion disks seen through an obscuring torus [67, 225]. This might suggest that something relatively recent (perhaps the galaxy merger in the case of Cen A [69]) has provided additional material for accretion and affected the central engine in a way that has yet to be reflected in the power and structure of the large-scale radio emission. The difficulty is that merger and source-development time scales are expected to be comparable. A further complication is the tendency for any X-ray accretion-related components in FRII low-excitation radio galaxies to be less luminous than those seen in a typical FRII high-excitation radio galaxy [99], as was known for the optical continuum [49, 206]. This means that not all FRIIs have equivalent central engines. However, it is is hard to treat as a coincidence the tendency for the most powerful FRIIs with the least evidence for external disruption to arise from AGN showing high-excitation optical emission lines and evidence for thin accretion disks.

In the normally inferred absence of thin radiatively-efficient accretion disks in FRIs, it has been argued in several cases that sufficient X-ray-emitting hot gas is present in their galaxies and clusters to produce the required jet power through a geometrically-thick Bondi accretion flow (e.g., 62, 4). Here the jet power is inferred from the energy required to excavate the cavities observed in the X-ray-emitting gas, i.e., a more direct method than scaling from radio power (e.g., 211) as is normal in the absence of other information. Recent work confirms that the most luminous FRIIs also tend to lie in luminous X-ray clusters [12], and it is reasonable to assume that they experience similar or greater supplies of galaxy and cluster hot gas. However jet powers are also higher (how much so rests on uncertainties in speed and composition), consistent with requiring an extra energy source in the form of stars and gas clouds fuelling a thin accretion disk. A major outstanding problem is a full understanding of the mechanisms which convert gas infall into two different accretion structures. Jets are expected to be more strongly coupled to the structure of the host stellar system, and hence to play a more major rôle in feedback, if the accreting gas originates predominantly from the reservoir contained in the potential well of the system as a whole, whether it be hot (e.g., 4) or cold (e.g., 161) in origin.

8.3. The rôle of the environment

Assuming that jets are genuinely symmetric at production, the environment appears to be, at a minimum, a strong secondary factor (with jet power being the likely primary influence) in shaping large-scale jet structure. For example, some radio sources show what appears to be FRI morphology on one side and FRII on the other, and this has been used to argue for different environmental effects on the two sides [92].

VLBI proper-motion studies find few, if any, differences in the speed or morphology of FRI and FRII radio jets in their initial stages of development from the central engine [154, 88]. However, the radiative powers are higher in FRIIs, but not in linear proportion to their total radio powers (e.g., 87, 89), suggesting that on the small scale a radio source has knowledge of how it will evolve. Particularly compelling evidence that the environment does have some influence is the recent discovery that quasars, traditionally the hosts only of FRII structures, can host FRI radio structures, with evidence that denser, more clumpy, environments at higher redshift are allowing this to occur [107]. The rôle of the X-ray-emitting environment in decelerating FRI jets was discussed in Section 4.

8.4. Information from beamed sources

The beamed counterparts of radio galaxies (quasars and BL Lac objects) do not allow the accretion structures to be probed in the X-ray, since the beamed jet emission swamps all other nuclear components; indeed it is sometimes dominant up to the TeV band. Multi-wavelength spectral energy distributions and variability time scales are used to probe the beaming parameters and the physical properties of the emitting regions (e.g., 84, 128, 193). Correlated flares are sometimes measured across wavebands, giving support to the presence of a dominant spatial region of emission (e.g., 205, 194), but otherwise uncertainties of size scales, geometries, and parameters for the competing processes of energy loss and acceleration often force the adoption of oversimplified or poorly-constrained models for individual jets. Much is published on the topic, and a review is beyond the scope of this work. Substantial progress in understanding is anticipated from multiwavelength programmes associated with the Fermi Gamma-ray Space Telescope.

VLBI radio-polarization studies have found systematic differences between powerful quasars (beamed FRIIs) and BL Lac objects (beamed FRIs) in core polarizations, the orientations of the magnetic fields in the inner jets, and in jet length, although it is difficult to separate intrinsic differences from the possible influence of the parsec-scale environment, such as the density and magnetic field contained in line-emitting gas [43].

9. SUMMARY AND CONCLUDING REMARKS

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, S_{1 keV} / S_{5 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.

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