Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC 5548

II. Physical conditions in the X-ray absorber

¹ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: K.C.Steenbrugge@sron.nl
² Department of Physics and Astronomy, Georgia State University, Astronomy Offices, One Park Place South SE, Suite 700, Atlanta, GA 30303, USA
³ Catholic University of America, USA
⁴ Laboratory for Astronomy and Solar Physics, NASA's Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
⁵ CASA, University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA
⁶ Joint Center for Astrophysics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ Laboratory for High Energy Astrophysics, Code 660, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Physics Department, Lawrence Livermore National Laboratory, PO Box 808, L-41, Livermore, CA 94550, USA
⁹ Columbia Astrophysics Laboratory, Columbia University, 538W. 120th Street, New York, NY 10027, USA
¹⁰ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 26 January 2004
Accepted: 4 January 2005

Abstract

We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened (full width half maximum = 8000 km s^-1) emission lines of  and  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 ~300 ks. No variability in the warm absorber was detected between the spectra from the first 170 ks 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 -1040 km s^-1, 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 (, ) 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^-4 sr. 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 0.3  yr^-1 for the mass loss from the galaxy due to the observed outflows.