Anatomy of the AGN in NGC 5548

IV. The short-term variability of the outflows

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: L.di.Gesu@sron.nl
² XMM-Newton Science Operations Centre, ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
³ Leiden Observatory, Leiden University, Post Office Box 9513, 2300 RA Leiden, The Netherlands
⁴ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁷ INAF-IASF, Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Department of Physics and Astronomy, The John Hopkins University, Baltimore, MD 21218, USA
¹⁰ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
¹¹ Max-Planck-Institute fur extraterrestrische Physik, Giessebachstrasse, 85748 Garching, Germany
¹² INAF/IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹³ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
¹⁴ Department of Astronomy, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
¹⁵ Institute for Astronomy, University of Cambridge, Madingley Rd, Cambridge, CB3 0HA, UK
¹⁶ Instituto de Astronomia, Universidad Catolica del Norte, Avenida Angamos 0610, 1280 Casilla, Antofagasta, Chile
¹⁷ Department of Physics,University of Oxford, Keble Road, Oxford, OX1 3RH, UK

Received: 20 February 2015
Accepted: 5 May 2015

Abstract

During an extensive multiwavelength campaign that we performed in 2013−2014, we found the prototypical Seyfert 1 galaxy NGC 5548 in an unusual condition of heavy and persistent obscuration. The newly discovered “obscurer” absorbs most of the soft X-ray continuum along our line of sight and lowers the ionizing luminosity received by the classical warm absorber. We present the analysis of the high resolution X-ray spectra collected with XMM-Newton and Chandra throughout the campaign, which are suitable to investigate the variability of both the obscurer and classical warm absorber. The time separation between these X-ray observations range from two days to eight months. On these timescales the obscurer is variable both in column density and in covering fraction. This is consistent with the picture of a patchy wind. The most significant variation occurred in September 2013 when the source brightened for two weeks. A higher and steeper intrinsic continuum and a lower obscurer covering fraction are both required to explain the spectral shape during the flare. We suggest that a geometrical change of the soft X-ray source behind the obscurer causes the observed drop in the covering fraction. Because of the higher soft X-ray continuum level, the September 2013 Chandra spectrum is the only X ray spectrum of the campaign in which individual features of the warm absorber could be detected. The spectrum shows absorption from Fe-UTA, O iv, and O v, consistent with belonging to the lower-ionization counterpart of the historical NGC 5548 warm absorber. Hence, we confirm that the warm absorber has responded to the drop in the ionizing luminosity caused by the obscurer.