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

5. Conclusions

A deep ROSAT HRI X-ray image of the FRII radio galaxy 3C 219 shows a point-like source, coincident with the nucleus of the galaxy and accounting for 74% of the total net counts (60% of the total net counts by taking into account the spectral information), and an emission aligned with the radio structure that extends from the innermost region of the radio galaxy to the hundred-kpc scale.

The 0.1-10 keV spectrum is well represented by a partial covering model in which 74% of a power law spectrum () is absorbed by a column density cm^-2. This spectral slope coincides with that of the synchrotron emission of the radio lobes. The spectral analysis indicates that at most 10% of the X-ray flux could be due to a thermalized gas with a temperature of 1.5 keV. The point-like source, whose de-absorbed (isotropic) luminosity in the 0.1-2.4 keV band is erg s^-1, can be identified with the emission of a quasar hidden in the nucleus of 3C 219. This lends further support to the unification of FRII radio galaxies with radio loud quasars.

Subtraction of the point-like source leaves an extended, unabsorbed circumnuclear component (named C) whose luminosity in the 0.1-2.4 keV band is erg s^-1. It appears unlikely that a cooling flow may explain this component because the X-ray emission from the associated intracluster gas would exceed by far the observed counts. As a matter of fact, we have no evidence of a diffuse hot intracluster gas.

In agreement with the unification scheme, we have constructed a simplified model in which the main features of the C-component can be satisfactorily explained as IC scattering of the IR-optical radiation from the hidden quasar, and surrounding dusty/molecular torus, with the relativistic electrons uniformly distributed in the radio lobes. Since the C-component is significantly inclined with respect to the direction of the radio-jet, our model implies a large tilt () between the radio jet and the torus axis. Because of the limits one can place on the luminosity of the hidden quasar, we find that the X-ray flux of the C-component can be accounted for if the equipartition condition is violated, the energy in the relativistic particles being at least a factor 10 larger than that in equipartition conditions. The electrons mainly involved in the IC production of the soft X-rays have energies much lower () than those of the electrons producing the synchrotron radio emission. Therefore, if our model is correct, it can provide useful constraints on the particle acceleration mechanisms and aging of the radio source.

The X-ray structures (named S and N-components) observed in the outer regions cannot be explained by our model in which the relativistic particles are uniformly distributed within the radio lobes (it should be borne in mind that we have consistently included in our computations the contribution from the IC scattering of the CMB photons, which is of minor importance for the C-component, but dominant as one goes further out in the radio lobes). Accounting for these structures by the IC process requires positive fluctuations of a factor 2-2.5 in the column densities of the relativistic electrons. We tentatively associate this possibility with the evidence of back flows from the hot-spots. Alternatively, we cannot exclude the possibility of a thermal contribution from a hot (kT 1.5 keV) clumpy gas surrounding the radio lobes in localized regions. The presence of moderate depolarization and RM gradients from the radio maps supports this hypothesis in the case of the S-component. Observations with AXAF will provide an invaluable tool to verify our model and the nature of the X-ray emission of 3C 219.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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