How to estimate the distance to the warm absorber in AGN from photoionized models

¹ Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland e-mail: agata@camk.edu.pl
² Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
³ Observatoire Astronomique de Strabsourg, Université Louis Pasteur, CNRS, 11 rue de l'Université, 67000 Strasbourg, France
⁴ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁵ CAAUL, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal

Received: 8 February 2008
Accepted: 22 May 2008

Abstract

Aims. We present a method that allows us to estimate the distance from the continuum source located in the center of AGN to the highly ionized gas called warm absorber.

Methods. We computed a set of constant total pressure photoionization models compatible with the warm absorber conditions, where a metal-rich gas is irradiated by a continuum in the form of a double power law. The first power law is hard up to 100 keV and represents radiation from an X-ray source, while the second power law extends up to several eVs and illustrates radiation from an accretion disk.

Results. When the ionized continuum is dominated by the soft component, the warm absorber is heated by free-free absorption, instead of Comptonization, and the transmitted spectra show different absorption line characteristics for different values of the hydrogen number density at the cloud illuminated surface.

Conclusions. This fact results in the possibility of deriving the number density on the cloud illuminated side from observations and hence the distance to the warm absorber.