2005A&A...434..569S

We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened (full width half maximum = 8000km/s) emission lines of ion OVII and ion CVI in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30% increase in luminosity in the second half of the observation. The gradual increase in luminosity occurred over a timescale of ∼300ks. No variability in the warm absorber was detected between the spectra from the first 170ks and the second part of the observation. The longer wavelength range of the LETGS resulted in the detection of absorption lines from a broad range of ions, in particular of C, N, O, Ne, Mg, Si, S and Fe. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at -1040km/s, becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (ion MgXII, ion SiXIV) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10^–4sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. We determine an upper limit of 0.3M_☉/yr for the mass loss from the galaxy due to the observed outflows.