Anatomy of the AGN in NGC 5548: VI. Long-term variability of the warm absorber

Context. We observed the archetypal Seyfert 1 galaxy NGC 5548 in 2013-2014 in the context of an extensive multiwavelength campaign involving several satellites, which revealed the source to be in an extraordinary state of persistent heavy obscuration. Aims. We re-analyzed the archival grating spectra obtained by XMM-Newton and Chandra between 1999 and 2007 in order to characterize the classic warm absorber (WA) using consistent models and up-to-date photoionization codes and atomic physics databases and to construct a baseline model that can be used as a template for the physical state of the WA in the 2013 observations. Methods. We used the latest version of the photoionization code CLOUDY and the SPEX fitting package to model the X-ray grating spectra of the different archival observations of NGC 5548. Results. We find that the WA in NGC 5548 is composed of six distinct ionization phases outflowing in four kinematic regimes. The components seem to be in the form of a stratified wind with several layers intersected by our line of sight. Assuming that the changes in the WA are solely due to ionization or recombination processes in response to variations in the ionizing flux among the different observations, we are able to estimate lower limits on the density of the absorbing gas, finding that the farthest components are less dense and have a lower ionization. These limits are used to put stringent upper limits on the distance of the WA components from the central ionizing source, with the lowest ionization phases at several pc distances (<50, <20, and <5 pc, respectively), while the intermediately ionized components lie at pc-scale distances from the center (<3.6 and