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 2011 November. 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 broad-band 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 ×10²³ 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 broad-band 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 × 10⁴⁵ erg s^-1, implying that the central black hole is accreting at ∼15-25 per cent of the Eddington limit.