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 data obtained with both Suzaku and the Swift/BAT. In accordance with previous observations, we find that the general continuum can be well described by a power-law with {\Gamma}=1.6 and an apparent soft-excess below 1 keV. Warm absorption is clearly present and absorption lines due to the Fe UTA, 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 K{\alpha} emission line is observed at E=6.44\pm0.04 keV. The Fe K{\alpha} emission is well modelled by the presence of a mildly ionised ({\xi}\leq30) reflection component with a low reflection fraction (R<0.2). At least 5 ionised absorption components with 10^{20} \leq N_H \leq 10^{23} cm^{-2} and 0 \leq log({\xi})/erg cm s^{-1} \leq 4 are required to achieve an adequate spectral fit. Alternatively, we show that the continuum can also be fit if a {\Gamma}~2.0 power-law is absorbed by a column of N_H~10^{23} cm^{-2} which covers ~30% of the source flux. Independent of which continuum model is adopted, the Fe L and Fe XXV He{\alpha} lines are described by a single absorber outflowing with v_out~0.14 c. Such an outflow/disk wind is likely to be substantially clumped (b~10^{-3}) in order to not vastly exceed the likely accretion rate of the source.
Highlights
It is well established that the soft X-ray spectrum of at least half of all Seyfert 1 galaxies is characterized by regions of photoionized ‘warm’ absorption along the line of sight (Blustin et al 2005; McKernan, Yaqoob & Reynolds 2007)
The normalization on the Swift/Burst Alert Telescope (BAT) was left as a free parameter throughout, and was found to be 4–5 per cent greater than that of the X-ray imaging spectrometer (XIS) instruments
The outflow velocity found for this absorber is consistent with an identification of the Fe K absorption with Fe XXV Heα, the velocity is substantially larger than that found if the 1.29 keV line is identified as solely Fe XXIV
Summary
It is well established that the soft X-ray spectrum of at least half of all Seyfert 1 galaxies is characterized by regions of photoionized ‘warm’ absorption along the line of sight (Blustin et al 2005; McKernan, Yaqoob & Reynolds 2007). There is a large body of observational evidence for blueshifted absorption lines at rest-frame energies greater than 7 keV in many AGN (e.g. PG 1211+143, Pounds et al 2003; PDS 456, Reeves, O’Brien & Ward 2003; NGC 1365, Risaliti et al 2005; MCG -5-23-16, Braito et al 2007; H 1413+117, Chartas et al 2007; Mrk 766, Miller et al 2007; NGC 3516, Turner et al 2008; Mrk 509, Cappi et al 2009; 3C 445, Reeves et al 2010; Braito et al 2011; Tombesi et al 2010a,b) This absorption is generally attributed to Fe XXV/XXVI in an outflowing accretion disc wind and requires both a high column density (i.e. NH ∼ 1023–1024 cm−2), high ionization parameter (i.e. log ξ /erg cm s−1 ∼ 3–6) and an outflow velocity ∼0.1 c.
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