Density constraint of the warm absorber in NGC 5548

Context. Ionized outflows in active galactic nuclei are thought to influence the evolution of their host galaxies and supermassive black holes. Taking distance into account is important when deriving the kinetic power of the outflows as a cosmic feedback channel. However, the distance between the outflows and the central engine is poorly constrained. The density of the outflows is an essential parameter for estimating this distance. NGC 5548 exhibits a variety of spectroscopic features in its archival spectra, which can be used for density analysis. Aims. We used the variability in the absorption lines from the archival spectra to obtain a density constraint and then estimate the distance to the outflows. Methods. We used the archival observations of NGC 5548 taken with Chandra in January 2002 to search for variations in the absorption lines. Results. We find that the Mg XII Ly α and the O VIII Ly β absorption lines vary significantly on the ∼144 ks and ∼162 ks timescales during the different observation periods. Based on the variability timescales and the physical properties of the variable components that dominated these two absorption lines, we derived a lower limit on the density of the variable warm absorber components in the range ∼7.2–9.0 × 10 11 m −3 , and an upper limit on their distance from the central source in the range ∼0.2–0.5 pc.