The puzzling X-ray continuum of the quasar MR 2251-178
  We report on a comprehensive X-ray spectral analysis of the nearby
radio-quiet quasar MR 2251-178, based on the long-look (~ 400 ks) XMM-Newton
observation carried out in November 2011. As the properties of the multiphase
warm absorber (thoroughly discussed in a recent, complementary work) hint at a
steep photoionizing continuum, here we investigate into the nature of the
intrinsic X-ray emission of MR 2251-178 by testing several physical models. The
apparent 2-10 keV flatness as well as the subtle broadband curvature can be
ascribed to partial covering of the X-ray source by a cold, clumpy absorption
system with column densities ranging from a fraction to several x10^23 cm^-2.
As opposed to more complex configurations, only one cloud is required along the
line of sight in the presence of a soft X-ray excess, possibly arising as
Comptonized disc emission in the accretion disc atmosphere. On statistical
grounds, even reflection with standard efficiency off the surface of the inner
disc cannot be ruled out, although this tentatively overpredicts the observed ~
14-150 keV emission. It is thus possible that each of the examined physical
processes is relevant to a certain degree, and hence only a combination of
high-quality, simultaneous broadband spectral coverage and multi-epoch
monitoring of X-ray spectral variability could help disentangling the different
contributions. Yet, regardless of the model adopted, we infer for MR 2251-178 a
bolometric luminosity of ~ 5-7 x 10^45 erg/s, implying that the central black
hole is accreting at ~ 15-25 per cent of the Eddington limit.
