A broad-band X-ray view of the warm absorber in radio-quiet quasar MR 2251-178

Abstract

We present the analysis of a new broad-band X-ray spectrum (0.6-180.0 keV) of the radio-quiet quasar MR 2251-178 which uses both Suzaku and Swift/Burst Alert Telescope data. In accordance with previous observations, we find that the general continuum can be well described by a power law with Γ= 1.6 and an apparent soft excess below 1 keV. Warm absorption is clearly present, and absorption lines due to the Fe unresolved transition array, Fe L (Fe XXIII-XXIV), S XV and S XVI are detected below 3 keV. At higher energies, Fe K absorption from Fe XXV-XXVI is detected and a relatively weak (EW = 25⁺¹²_-8 eV) narrow Fe Kα emission line is observed (E= 6.44 ± 0.04 keV) which is well modelled by the presence of a mildly ionized (ξ≲ 30) reflection component with a low reflection fraction (R < 0.2). At least five ionized absorption components with 10²⁰≲N_H≲ 10²³ cm^-2 and 0 ≲ log ξ/erg cm s^-1≲ 4 are required to achieve an adequate spectral fit. Alternatively, we show that the continuum can also be fit if a Γ∼ 2.0 power law is absorbed by a column of N_H∼ 10²³ cm^-2 which covers ∼30 per cent of the source flux. Independent of which continuum model is adopted, the Fe L and Fe XXV Heα lines are well fit by a single absorber outflowing with v_out∼ 0.14c. Such an outflow/disc-wind is likely to be substantially clumped (b∼ 10^-3) in order to not vastly exceed the likely accretion rate of the source.