X-ray narrow line region variability as a geometry probe: The case of NGC 5548

TL;DR

This study investigates the variability of X-ray narrow emission lines in NGC 5548 to determine the geometry and location of the emitting gas, revealing a compact ionization cone between 1 and 15 parsecs from the nucleus.

Contribution

It provides new constraints on the size and geometry of the narrow line region in NGC 5548 using long-term X-ray observations and variability analysis.

Findings

NLR in NGC 5548 is an ionization cone.

Located between 1 and 15 parsecs from the central source.

NLR is compact and responds to continuum flux changes.

Abstract

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.