Transient obscuration event captured in NGC 3227

III. Photoionization modeling of the X-ray obscuration event in 2019

¹ Department of physics, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
e-mail: mao@astro.hiroshima-u.ac.jp
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513 2300 Leiden, The Netherlands
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁶ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁷ Department of Astronomy, Nanjing University, Nanjing 210093, PR China
⁸ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
⁹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁰ INAF-IASF Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
¹¹ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Italian Space Agency (ASI), Via del Politecnico snc, 00133 Roma, Italy
¹⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁶ Departament de Física, EEBE, Universitat Politècnica de Catalunya, Av. Eduard Maristany 16, 08019 Barcelona, Spain
¹⁷ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁸ Max Planck Institute fur Extraterrestriche Physik, 85748 Garching, Germany
¹⁹ 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
²⁰ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel

Received: 11 November 2021
Accepted: 14 April 2022

Abstract

Context. A growing number of transient X-ray obscuration events in type I active galactic nuclei suggest that our line of sight to the central engine is not always free. Multiple X-ray obscuration events were reported in the nearby Seyfert 1.5 galaxy NGC 3227 from 2000 to 2016. In late 2019, another X-ray obscuration event was identified with Swift. Two coordinated target-of-opportunity observations with XMM-Newton, NuSTAR, and the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) were triggered in November and December 2019 to study this obscuration event.

Aims. We aim to constrain the physical properties of the absorbing material (i.e., the obscurer) that caused the X-ray obscuration event in late 2019. We also aim to compare the handful of obscuration events in NGC 3227 and other Seyfert galaxies.

Methods. For each observation, we analyzed the time-averaged X-ray spectra collected with XMM-Newton and NuSTAR. We performed photoionization modeling with the SPEX code, which allows us to constrain the intrinsic continuum simultaneously with various photoionized absorption and emission components.

Results. Similar to previous transient X-ray obscuration events in NGC 3227, the one caught in late 2019 is short-lived (less than five months). If the obscurer has only one photoionized component, the two X-ray observations in late 2019 cannot be explained by the same obscurer that responds to the varying ionizing continuum. Due to the unknown geometry of the obscurer, its number density and distance to the black hole cannot be well constrained. The inferred distance covers at least two orders of magnitude, from the broad-line region to the dusty torus. Unlike some other X-ray obscuration events in Seyfert galaxies, such as NGC 5548 and NGC 3783, no prominent blueshifted, broad absorption troughs were found in the 2019 HST/COS spectra of NGC 3227 when compared with archival UV spectra. This might be explained if the X-ray obscurer does not intercept our line of sight to (a significant portion of) the UV-emitting region. Understanding the variety of observational differences in the X-ray obscuration events observed so far is not straightforward. Future observations with high-quality data are needed to unveil the nature of the X-ray obscuration events.