X-ray narrow line region variability as a geometry probe: The case of
  NGC 5548
  We study the long time scale variability of the gas responsible for the X-ray
narrow emission lines in the Seyfert 1 galaxy NGC 5548, in order to constrain
the location and geometry of the emitting gas. Using X-ray spectra taken with
the Chandra-LETGS and HETGS instruments and with XMM-Newton RGS and combining
them with long-term monitoring observations of the Rossi X-ray Timing Explorer
(RXTE), we perform a correlation analysis in order to try constrain the time
scale on which the narrow line emitting gas responds to variations of the
continuum flux. With the inclusion of the 2007 Chandra-LETGS observation we
have an additional observation at an historically low flux level. We conclude
that the NLR in NGC 5548 is in the form of an ionization cone, compact in size,
and located between 1 and 15 pc from the central source, depending on the exact
geometry of the NLR.
