Heavy absorption and soft X-ray emission lines in the XMM-Newton spectrum of the type 2 radio-loud quasar 3C 234

Abstract

Aims. We report results on a 40 ks XMM-Newton observation of the type 2 quasar 3C 234. Optical spectropolarimetric data have demonstrated the presence of a hidden broad-line region in this powerful (M V ≤ −24.2 after reddening and starlight correction) narrow-line FRII radio galaxy. Our analysis is aimed at investigating the X-ray spectral properties of this peculiar source that have remained poorly known so far. Methods. We analyze the 0.5-10 keV spectroscopic data collected by the EPIC cameras in 2006.Results. The X-ray spectrum of this radio-loud quasar is typical of a local Compton-thin Seyfert 2 galaxy. It exhibits strong absorption (N H ∼ 3.5 × 10 23 cm −2 ) and a narrow, neutral Fe Kα emission line with an equivalent width of ≈140 ± 40 eV. Our observation also reveals that the soft portion of the spectrum is characterized by strong emission lines with a very low level of scattered primary continuum. A possible explanation of these features in terms of thermal emission from a two-temperature, collisionally ionized plasma emission seems to be unlikely due to the high luminosity estimated for this component (L 0.5−2 ∼ 6 × 10 42 erg s −1 ). It is likely that most of the soft X-ray emission originates from a photoionized plasma as commonly observed in obscured, radio-quiet Seyfertlike AGNs.Conclusions. This X-ray observation has definitively confirmed the presence of a hidden quasar in 3C 234. The line-rich spectrum and the steepness of the hard X-ray continuum (Γ ≈ 1.7) found in this source weaken the hypothesis that the bulk of the X-ray emission in radio-loud AGNs with high-excitation optical lines arises from jet non-thermal emission.

The high-energy view of the broad-line radio galaxy 3C 111

“…Similar features, produced by photoionized gas, have been detected in two other FR II radio galaxies, the BLRG 3C 445 (e.g. Grandi et al 2007; Sambruna et al 2007) and the narrow‐line radio galaxy 3C 234 (Piconcelli et al 2008).…”

On the nature of the absorber in IRAS 09104+4109: the X-ray and mid-infrared view

“…It is also possible that there might be a contribution to soft X‐rays from photoionized gas at the inner nucleus, as suggested by I01; this hypothesis would be consistent with the hard‐to‐soft X‐ray flux ratio (≈4 per cent) observed in I09104, which is comparable to that typically measured in Seyfert 2 galaxies (e.g. Matt et al 2004; Piconcelli et al 2008, and references therein). However, a detailed analysis of the soft X‐ray emission, limited by the low number of counts below 2 keV (≈200) and the lack of deep X‐ray grating data, is beyond the purposes of this paper, since the main goal of our work consists of a proper characterization of the hard (>2 keV) emission.…”

3C 33: another case of photoionized soft X-ray emission in radio galaxies

“…The lines in Tables 2 and 4 are likely produced by H-and He-like ions of light elements such as oxygen and neon, often detected in obscured AGNs which is indicative of photoionized gas (Bianchi & Guainazzi 2007;Grandi et al 2007;Sambruna et al 2007;Piconcelli et al 2008). Although the moderate ACIS energy resolution does not allow us to draw strong conclusions, we emphasize that the centroids of the lines occurs where two strong features produced by photoionized gas are expected, i.e., the OVII recombination feature and the NeIX triplet lines.…”

“…Recently the discovery of emission lines produced by the photoionized gas in the soft X-ray spectrum of 3C 445 (Grandi et al 2007;Sambruna et al 2007) and 3C 234 (Piconcelli et al 2008) provided an alternative way of interpreting the soft excess.…”