2006MNRAS.371..283M

We report results from an XMM-Newton observation of the broad-line radio galaxy 3C 109 (z = 0.3056). Previous ASCA data revealed the presence of a broad iron line from the accretion disc with which the XMM-Newton spectrum is fully consistent. However, although improving the ASCA constraints on the line parameters, the quality of the data is not high enough to distinguish between an untruncated accretion disc extending down to small radii close to the black hole and a scenario in which the innermost 20-30 gravitational radii are missing. For this reason, our results are model-dependent and the hard data can be modelled equally well by considering an absorption scenario in which a large column of neutral gas partially covers the X-ray continuum source. However, the absorber would have to comprise hundreds/thousands very compact clouds close to the X-ray source, which seems rather extreme a requirement. The 2-10 keV intrinsic luminosity of 3C 109 is of the order of 2-3x10⁴⁵ erg/s regardless of the adopted model. A recent black hole mass estimate of ∼2x10⁸ M_☉implies that L_bol/L_Edd> 1. If partial covering is excluded, the observed reflection fraction (of the order of unity), steep photon index (1.86), and Fe line equivalent width (about 100 eV) all suggest to exclude that the X-ray continuum is strongly beamed indicating that the large Eddington ratio is associated with a radiatively efficient accretion process and making it unlikely that the innermost accretion disc is replaced by a thick radiatively inefficient medium such as in advection-dominated accretion models. We also confirm previous findings on the detection of low energy absorption in excess of the Galactic value, where we find excellent agreement with previous results obtained in X-rays and at other wavelengths (optical and infrared). The better quality of the XMM-Newton data enables us to attribute the excess absorption to slightly ionized gas in the line of sight, located at the redshift of 3C 109. The most likely interpretation for the excess absorption is that the line of sight is grazing the obscuring torus of unified models, which is consistent with the inclination inferred from the Fe line profile (about 40°) and with the hybrid radio-galaxy/quasar nature of 3C 109.