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2.1. Cygnus A

Cygnus A is sufficiently powerful and close to show a full array of X-ray emission components. Its surrounding cluster emission is strong, but when modelled and subtracted from the ~ 0.2 - 2 keV ROSAT High Resolution Imager (HRI) image, Carilli et al. (1994) found pronounced soft-excess emission associated with the radio hotspots, core, and two (possibly three) regions around the limb of the lobe plasma, together with an X-ray deficit in the inner lobes. With reference to a standard model for a powerful radio source with supersonic jet (Fig. 1), it is interesting to speculate that the excesses around the lobes are parts of ring structures pinching the contact discontinuity. There was no evidence for increased X-ray emission due to higher gas density ahead of the beam, in the cocoon between the contact discontinuity (containing the radio lobes) and bow shock, as might confirm the standard model, but Carilli et al. (1994) argue that the increased luminosity due to higher density may be offset by the effect of heating, which would tend to remove X-ray emission to an energy band above that to which ROSAT is sensitive.

Figure 1

Figure 1. Sketch of the termination region of a powerful radio jet viewed in the rest frame of the bow shock. Radio lobe emission fills the region inside the contact discontinuity. Between the contact discontinuity and the bow shock we expect the ambient X-ray-emitting medium to be both compressed and heated with respect to the medium in front of the bow shock.

X-ray spectroscopy in the 2-10 keV energy band finds a poor fit to cluster gas alone, and argues for the presence of non-thermal emission seen through a large absorbing column, NH ~ 4 × 1023 cm-2, and interpreted as emission from a heavily obscured central AGN (Arnaud et al. 1987, Ueno et al. 1994). Interestingly, this absorbed core emission cannot be the soft-X-ray core excess in the ROSAT HRI image (Harris et al. 1994b), because such a high column density has a disastrous effect on soft X-rays (Fig. 2). Instead, the soft X-rays may arise from a central region in the radio source where the only line-of-sight absorption is the Galactic column, NH ~ 3 × 1021 cm-2. Indirect support for this suggestion comes from the fact that the ratio of the unabsorbed X-ray to core-radio luminosity is then very similar to that of core-dominated quasars and those high-redshift counterparts of Cygnus A for which the core soft X-ray emission is separated from cluster emission (Fig. 3), although the cluster-scale cooling flow in Cygnus A (Reynolds & Fabian 1996) should contribute at some level to the HRI soft X-ray core excess.

Figure 2

Figure 2. As the excess (intrinsic) column density rises to more than a few 1022 cm-2, the counts measured with ROSAT quickly fall. The example is for the PSPC detector, assuming a source with a power-law spectrum of alpha = 1.0 (fnu propto nu-alpha) and galactic column density of 1.1 × 1020 cm-2, but a similar situation applies to the HRI and its measurement of the X-ray core of Cygnus A.

Figure 3

Figure 3. The two high-redshift (z > 0.6) radio galaxies for which core soft X-ray emission is separated from cluster emission, 3C 280 (Worrall et al. 1994) and 3C 220.1 (Hardcastle et al. 1998b), are roughly consistent with an extrapolation of the core radio/X-ray correlation for core-dominated quasars of comparable redshift (Worrall et al. 1994). Since these quasars are believed through Unification models to be powerful radio galaxies oriented with their jets in the line of sight (e.g. Barthel 1989), the correlation supports the interpretation of the core soft X-ray emission from these radio galaxies as being beamed and associated with the radio jet. Cygnus A, although local, has comparable radio-core luminosity to 3C 280, and fits remarkably well on the correlation when the HRI core X-ray emission is interpreted as radio-related.

2.2. Observational Biases

Because radio galaxies are multi-component X-ray emitters, the energy-band, sensitivity, and spatial and spectral resolution of the observing instrument influence what is measured. Focussing X-ray optics have the major advantage of decreasing the background, and so Einstein was the first mission to detect some tens of radio galaxies in soft X-rays (e.g. Fabbiano et al. 1984) and to separate components in nearby objects such as Cen A and M 87 (Feigelson et al. 1981, Schreier et al. 1982). The mission contributing most to the subject over the last decade has been ROSAT. The combination of improved sensitivity (roughly twice Einstein's collecting area at ~ 1 keV) and a longer mission have permitted relatively large samples of radio galaxies to be studied, and the point response function (PRF) of ~ 15" Half Energy Width (HEW) for the Position Sensitive Proportional Counter (PSPC) and ~ 4" HEW for the HRI (although with poorer sensitivity and no spectral resolution) has led to component separation in many sources for which pointed observations were made. An object can look rather different when viewed with the PSPC and HRI, with the former emphasizing extended emission and the latter the compact components (Fig 4). ASCA has increased the number of radio galaxies with spectroscopic component separation (e.g. Sambruna et al. 1999) although, with its relatively poor spatial resolution (HEW approx 3'), weak sources often give ambiguous results, with different combinations of spectral models giving similarly good fits to the data. The payload of BeppoSax covers the broad energy range from soft X-rays to 300 keV, although mostly with non-focussing optics (Boella et al. 1997); its strengths are therefore in broad-band studies of bright beamed counterparts of radio galaxies, although there are tentative claims for the detection of heavily-obscured AGN nuclei (as for Cygnus A) in some FRIs (Trussoni et al. 1998).

Figure 4

Figure 4. The ROSAT PSPC and HRI are sensitive to different structures in a complex source, such as J2310-437 (Worrall et al. 1999), a BL Lac object in an X-ray cluster. The PSPC is more sensitive to low surface brightness extended structures (left panel shows X-ray contours from a 4 ks PSPC exposure), whilst the superior spatial resolution of the HRI pin-points features which are unresolved or of small spatial scale (right panel shows 31 ks HRI exposure). Lowest contours are at 3.8sigma, and grey-scale is R-band CCD image.

Observational biases occur from redshift effects, not only in the sense that in flux-limited samples the more distant sources are the more powerful. Extended X-ray emission tends to be seen around high-redshift radio sources only if it is of cluster size and strength; around low-redshift sources it is easier to detect the more compact gaseous components than larger-scale emission which fills the detector field of view. Spectral measurements attempt to measure excess absorption over that in the line of sight in our Galaxy, and small fitted excesses become large intrinsic excesses when transformed to the rest frame of a high-redshift source, where this would not happen for a more local source.

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