X-ray/UV campaign on the Mrk 279 outflow: Density diagnostics in Active Galactic Nuclei using O v K-shell absorption lines

¹ SRON National Institute for Space Research Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: J.Kaastra@sron.nl
² Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ CASA, University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA
⁴ Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ KIPAC/SLAC, 2575 Sand Hill Road M/S 29, Menlo Park, Ca 94025, USA

Received: 8 June 2004
Accepted: 12 August 2004

Abstract

One of the main problems in modeling the ionised outflows in Active Galactic Nuclei is the unknown distance of the outflowing wind to the central source. Only if the density is known this distance can be determined through the ionisation parameter. Here we study density diagnostics based upon transitions. is known to have metastable levels that are density dependent. We study the population of those levels under photoionisation equilibrium conditions and determine for which parameter range they can have a significant population. We find that resonance line trapping plays an important role in reducing the critical densities above which the metastable population becomes important. We investigate the K-shell absorption lines from these metastable levels. Provided that there is a sufficient population of the metastable levels, the corresponding K-shell absorption lines are detectable and are well separated from the main absorption line originating from the ground state. We then present the Chandra LETGS spectrum of the Seyfert 1 galaxy Mrk 279 that may show for the first time the presence of these metastable level absorption lines. A firm identification is not yet possible due to both uncertainties in the observed wavelength of the strongest line as well as uncertainties in the predicted wavelength. If the line is indeed due to absorption from , then we deduce a distance to the central source of one light week to a few light months, depending upon the importance of additional heating processes.