BeppoSAX spectroscopy of MR 2251-178: a test for ionized reflection in radio quiet QSOs

Abstract

We present the 0.1-200 keV BeppoSAX spectrum of MR 2251-178 observed at two epochs in 1998 separated by 5 months. Both epochs show identical spectral shape and X-ray flux. Analysis of the combined spectra allow us to confirm the presence of the ionized Fe K-alpha line detected by ASCA and to test the presence of reflection from ionized material. The best self-consistent spectral fit is obtained when including a contribution from a mildly ionized reflector (Xi(0.01-100) ~ 1625 erg cm s-1, i.e. U(0.1-10) ~ 0.14) with a reflection fraction R_ion ~ 0.11. An exponential cut-off to the direct power-law continuum is then required at E ~ 100 keV. The low energy spectrum is absorbed by ionized matter with a column density N_W = (8.1 (-1.2/+2.1))*10^21 cm^-2 and an X-ray ionization parameter U(0.1-10)= 0.06 +/- 0.01. The warm absorber column is slightly lower than that measured with ASCA. This change could be caused by bulk motion. The BeppoSAX absorber ionization parameter globally agrees with the ``U versus Flux'' correlation found for MR 2251-178 with EXOSAT, Ginga and ASCA. This suggests that the absorber is in a state of instantaneous ionization equilibrium.