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Chandra LETGS spectroscopy of the quasar MR 2251-178 and its warm absorber

Abstract:

We present an analysis of our Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation of the quasar MR 2251-178. The warm absorber of MR 2251-178 is well described by a hydrogen column density NH ≈2 × 1021 cm-2 and an ionization parameter log(ξ) ≈ 0.6. We find in the spectrum weak evidence for narrow absorption lines from carbon and nitrogen which indicate that the ionized material is in outflow. We note changes (in time) of the absorption structure in the band 0.6-1 keV (around the unresolved transition arrays [UTAs] plus the O VII and O VIII K edges) at different periods of the observation. We measure a 0.1 - 2 keV flux of 2.58 × 10-11 ergs cm-2 s-1. This flux implies that the nuclear source of MR 2251-178 is in a relatively low state. No significant variability is seen in the light curve. We do not find evidence for extra cold material in the line of sight, and set an upper limit of N H ≈ 1.2 × 1020 cm-2. The X-ray spectrum does not appear to show evidence for dusty material, although an upper limit in the neutral carbon and oxygen column densities can only be set to NC 1 ≈ 2 × 1019 cm -2 and N O 1 ≈ 9 × 1019 cm-2, respectively. © 2008. The American Astronomical Society. All rights reserved.