The XMM-Newton X-ray Spectra of the Most X-ray Luminous Radio-quiet
  ROSAT Bright Survey-QSOs: A Reference Sample for the Interpretation of
  High-redshift QSO Spectra
  We present the broadband X-ray properties of four of the most X-ray luminous
(L_X >= 10^{45} erg/s in the 0.5-2 keV band) radio-quiet QSOs found in the
ROSAT Bright Survey. This uniform sample class, which explores the extreme end
of the QSO luminosity function, exhibits surprisingly homogenous X-ray spectral
properties: a soft excess with an extremely smooth shape containing no obvious
discrete features, a hard power law above 2 keV, and a weak narrow/barely
resolved Fe K-alpha fluorescence line for the three high signal-to-noise ratio
(S/N) spectra. The soft excess can be well fitted with only a soft power law.
No signatures of warm or cold intrinsic absorbers are found. The Fe K-alpha
centroids and the line widths indicate emission from neutral Fe (E=6.4 keV)
originating from cold material from distances of only a few light days or
further out. The well-constrained equivalent widths (EW) of the neutral Fe
lines are higher than expected from the X-ray Baldwin effect which has been
only poorly constrained at very high luminosities. Taking into account our
individual EW measurements, we show that the X-ray Baldwin effect flattens
above L_X ~ 10^{44} erg/s (2-10 keV band) where an almost constant <EW> of ~100
eV is found.
  We confirm the assumption of having very similar X-ray AGN properties when
interpreting stacked X-ray spectra. Our stacked spectrum serves as a superb
reference for the interpretation of low S/N spectra of radio-quiet QSOs with
similar luminosities at higher redshifts routinely detected by XMM-Newton and
Chandra surveys.
