Anatomy of the AGN in NGC 5548

V. A clear view of the X-ray narrow emission lines

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT UK
e-mail: m.whewell@ucl.ac.uk
² SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Department of Physics and Astronomy, Universiteit Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280, Antofagasta, Chile
⁶ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁷ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
⁸ European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ INAF− Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁰ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
¹³ Department of Astronomy, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
¹⁴ Department of Astronomy, The Ohio State University, 140 W 18th Avenue, Columbus, OH 43210, USA
¹⁵ Center for Cosmology & AstroParticle Physics, The Ohio State University, 191 West Woodruff Ave., Columbus, OH 43210, USA
¹⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
¹⁷ CNRS, IPAG, 38000 Grenoble, France
¹⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK

Received: 15 June 2015
Accepted: 15 July 2015

Abstract

Context. Our consortium performed an extensive multi-wavelength campaign of the nearby Seyfert 1 galaxy NGC 5548 in 2013−14. The source appeared unusually heavily absorbed in the soft X-rays, and signatures of outflowing absorption were also present in the UV. He-like triplets of neon, oxygen and nitrogen, and radiative recombination continuum (RRC) features were found to dominate the soft X-ray spectrum due to the low continuum flux.

Aims. Here we focus on characterising these narrow emission features using data obtained from the XMM-Newton RGS (770 ks stacked spectrum).

Methods. We use spex for our initial analysis of these features. Self-consistent photoionisation models from Cloudy are then compared with the data to characterise the physical conditions of the emitting region.

Results. Outflow velocity discrepancies within the O VII triplet lines can be explained if the X-ray narrow-line region (NLR) in NGC 5548 is absorbed by at least one of the six warm absorber components found by previous analyses. The RRCs allow us to directly calculate a temperature of the emitting gas of a few eV (~10⁴ K), favouring photoionised conditions. We fit the data with a Cloudy model of log ξ = 1.45 ± 0.05 erg cm s^-1, log N_H = 22.9 ± 0.4 cm^-2 and log  v_turb = 2.25 ± 0.5 km s^-1 for the emitting gas; this is the first time the X-ray NLR gas in this source has been modelled so comprehensively. This allows us to estimate the distance from the central source to the illuminated face of the emitting clouds as 13.9 ± 0.6 pc, consistent with previous work.