Transient obscuration event captured in NGC 3227

IV. Origin of the obscuring cloud variability

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
e-mail: sam.waters.17@ucl.ac.uk
² Department of Physics, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³ Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK
⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁷ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁸ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁹ INAF-IASF Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ Max Planck Institute fur Extraterrestriche Physik, 85748 Garching, Germany
¹⁴ Departament de Física, EEBE, Universitat Politècnica de Catalunya, Av. Eduard Maristany, 16, Barcelona 08019, Spain
¹⁵ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹⁶ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
¹⁷ Telespazio UK for the European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo, s/n, 28692 Villanueva de la Cañada, Madrid, Spain

Received: 30 March 2022
Accepted: 6 March 2023

Abstract

Context. Obscuration events in type I active galactic nuclei (AGN) have been detected more frequently in recent years. The strong flux decrease in the soft X-ray band between observations has been caused by clouds with large column densities transiting our line of sight (LOS) and covering the central AGN. Another event has been captured in NGC 3227 at the end of 2019, which was observed with XMM-Newton, NuSTAR, and the Hubble Space Telescope.

Aims. We aim to determine the nature and origin of the observed spectral variability in the 2019 obscuration event.

Methods. We study the evolution of the obscurer by splitting the two XMM-Newton observations from 2019 into timing bins of length ∼10 ks. We used the SPEX code to analyse the 0.35–10 keV EPIC-PN spectra of each timing bin.

Results. In the first observation (Obs 1), there is a strong anti-correlation between the column density (N_H) of the obscurer and the continuum normalisations of the X-ray power law and soft Comptonisation components (N_pow and N_comt, respectively). The power-law continuum models the hard X-rays produced by the corona, and the Comptonisation component models the soft X-ray excess and emission from the accretion disk. Through further testing, we conclude that the continuum is likely to drive the observed variability, but we cannot rule out a possible contribution from N_H of the obscurer if it fully transverses across the ionising source within our LOS during the observation. The ionisation parameter (ξ) of the obscurer is not easily constrained, and therefore it is not clear whether it varies in response to changes in the ionising continuum. The second observation (Obs 2) displays a significantly lower count rate due to the combination of a high N_H and covering fraction of the obscurer, and a lower continuum flux.

Conclusions. The observed variability seen during the obscuration event of NGC 3227 in 2019 is likely driven by the continuum, but the obscurer varies at the same time, making it difficult to distinguish between the two possibilities with full certainty.