Chandra Letgs Spectroscopy Of The Quasar Mr 2251-178 And Its Warm Absorber

摘要

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 N_H ≈ 2 × 10~(21) 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 2.5 8 × 10~(-11) ergscm~(-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 × 10~(20) 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 N_(OI) ≈ 2 × 10~(19) cm~(-2) and N_(OI) ≈ 9 × 10~(19) cm~(-2), respectively.