The mass of helium in white dwarf stars and the formation and evolution of hydrogen-deficient post-AGB stars

We report results from an XMM-Newton observation of the broad-line radio galaxy 3C109 (z=0.3056). Confirming previous results, an Fe emission line is detected, possibly comprising a broad component. However, the data cannot distinguish between an untruncated accretion disc and a case in which the innermost 20-30 gravitational radii are missing. In fact, a partial-covering plus narrow Fe line scenario is not ruled out statistically. 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 3C109 is of the order of 2-3x10^45 erg/s which, combined with a recent black hole mass estimate of ~2x10^8 M_sun, implies an Eddington ratio greater than unity. 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. We also confirm previous findings on the detection of low energy absorption in excess of the Galactic value. 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 3C109. [abriged]