Astron. Astrophys. 342, 57-68 (1999)

1. Introduction

The properties of the X-ray emission of the powerful radio galaxies have been investigated in the literature (Fabbiano et al. 1984, Crawford & Fabian 1995, 1996).

In the framework of the AGNs unification models (Barthel 1989, Urry & Padovani 1995) the nuclear X-ray emission of the FRII radio galaxies is expected to be heavily absorbed along the line of sight by a circumnuclear molecular torus. Strong absorption has indeed been detected in the X-ray observations of the narrow line FRII radio galaxies Cyg A (Ueno et al. 1994) and 3C 194 (Crawford & Fabian 1996); moreover, intrinsic absorption has been discovered in the X-ray spectra of the broad line radio galaxies (BLRGs) 3C 109 (Allen & Fabian 1992) and 3C 287.2 (Crawford & Fabian 1995).

Worrall et al. (1994) have suggested that part of the soft X-ray emission of some strong FRII radio galaxies is synchrotron self-Compton (SSC) in the AGN jets. SSC may also explain the X-ray emission observed with ROSAT in two emission regions coincident with the radio hot-spots of Cygnus A (Harris et al. 1994), but it is generally too weak to be detected even in the nearby radio galaxies (Hardcastle et al. 1998a).

It is well known that non-thermal mechanisms can produce extended X-ray emission in the lobes of the radio galaxies where the relativistic electrons can interact with the microwave background photons (CMB) and radiate via the inverse Compton (IC) process. The relativistic particle densities and magnetic field strengths in the extended lobes of radio galaxies are usually estimated on the basis of the minimum energy assumption (equipartition). The detection of IC scattered X-rays from the radio lobes could provide an invaluable tool to determine the value of these important physical parameters. Unfortunately, this process is not particularly efficient at low redshifts (the IC emissivity increases as (1 + z)⁴) so that ROSAT and ASCA have failed to detect this effect in nearby radio galaxies with a few exceptions (notably, Fornax A: Feigelson et al. 1995, Kaneda et al. 1995; Cen B: Tashiro et al. 1998).

In a recent paper Brunetti et al. (1997) have shown that in the framework of the unification scheme linking strong FRII radio galaxies and radio loud quasars it is possible to predict large X-ray fluxes by the IC scattering of the far/near IR photons from a typical "hidden" quasar with the relativistic electrons in the radio lobes. Even if this effect is expected to dominate the IC contribution also at high redshifts, and predicts X-ray fluxes and spectral shapes consistent with those observed for a number of strong distant FRII radio galaxies (redshift z 1), it is difficult to be tested in detail because of the low spatial resolution and sensitivity of the available X-ray telescopes. The far/near IR photons from the "hidden" quasar are scattered in the X-ray band mainly by mildly relativistic electrons () which are not those responsible for the synchrotron radio emission (typically ). Therefore, the detection of the IC emission predicted by Brunetti et al. (1997) model would not only provide additional evidence in favour of the radio galaxy/quasar unification scheme, but it would also provide an unique handle to probe the spectral and spatial distributions of the relativistic particles over a wider energy range.

Nevertheless, optical and radio observations have suggested that relatively distant radio galaxies (z 0.3) lie at the center of moderately rich clusters of galaxies (Yates et al. 1989, Hill & Lilly 1991) with a dense hot intracluster medium (Garrington & Conway 1991). In addition, X-ray data are consistent with a scenario in which more than two third of the 50, or so, nearby brightest clusters have cooling flows (Fabian 1994). As a consequence in a number of powerful radio galaxies the thermal emission from the intracluster medium is expected to dominate the observed soft X-ray flux, as in Cyg A (Reynolds & Fabian 1995), producing extended X-ray structures and shadowing any other contribution. This scenario is further complicated by the fact that deep ROSAT HRI observations of nearby radio galaxies have revealed X-ray deficit associated with the radio lobes possibly originating from the hydrodynamical interaction of the jets and radio hot-spots with the intracluster medium (Cyg.A: Carilli et al. 1994; NGC 1275: Böhringer et al. 1993; 3C 449: Hardcastle et al. 1998b).

We present here a deep ROSAT HRI observation of 3C 219, a nearby (z=0.1744) powerful radio source identified with a cD galaxy of magnitude M_V=-21.4 (Taylor et al.1996), belonging to a non Abell cluster (Burbidge & Crowne 1979). Despite of its cluster membership, the ROSAT PSPC and ASCA archive data suggest that thermal emission, if present, is negligible so that this source could be a good candidate to detect possible IC contribution to the X-ray flux.

The radio structure is well studied (Perley et al. 1980, Bridle et al. 1986, Clarke et al. 1992): it is a classical double-lobed FRII radio galaxy that spreads over on the sky plane corresponding to a projected size 460 Kpc.  ¹ A strong jet extends over ( 50 Kpc) through the south-western lobe, while a weak counterjet is visible in the north-eastern lobe. The total radio spectrum, largely dominated by the extended radio structure, between 178 and 750 MHz is (, Laing et al. 1983).

Fabbiano et al. (1986) have reported the detection of a broad Paschen- line in excess of the predictions of case B recombination implying the presence of an absorbed broad line region with mag. More recent optical studies have shown that the 3C 219 spectrum is dominated by a starlight continuum, but a non-stellar component and emission line features are also present (Lawrence et al. 1996); from the broad line ratio , assuming typical broad line region parameters, we infer . In a H+[N II] image taken with the 4m Kitt Peak telescope the galaxy appears to be point-like (Baum et al. 1988) with a nearby, possibly interacting companion to the south-east. An image of 3C 219 has been taken with the HST telescope's WFPC2 through a broadband red (F702W) filter (de Koff et al. 1996); the galaxy is well resolved and does not present relevant features (dusty lane, distortions) on a scale pc (i.e. HST resolution).

The data analysis is presented in Sects. 2 and 3 while the proposed interpretation is discussed in Sect. 4.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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