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 × 10²³ 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 × 10⁴² 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 Seyfert-like 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.