Transient obscuration event captured in NGC 3227

I. Continuum model for the broadband spectral energy distribution

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: mmehdipour@stsci.edu
² SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁴ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁶ Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK
⁷ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁸ Department of Physics, Virginia Tech, Blacksburg VA 24061, USA
⁹ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
¹⁰ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹² Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹³ INAF-IASF Bologna, Via Gobetti 101, 40129 Bologna, Italy
¹⁴ Departament de Física, EEBE, Universitat Politécnica de Catalunya, Av. Eduard Maristany 16, 08019 Barcelona, Spain
¹⁵ Italian Space Agency (ASI), Via del Politecnico snc, 00133 Roma, Italy
¹⁶ Telespazio Vega 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
¹⁷ School of Physics and Astronomy and Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
¹⁸ Department of Astronomy, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
¹⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²⁰ INAF-IASF Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
²¹ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
²² Max Planck Institute fur Extraterrestriche Physik, 85748 Garching, Germany
²³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 17 May 2021
Accepted: 24 June 2021

Abstract

From Swift monitoring of a sample of active galactic nuclei (AGN) we found a transient X-ray obscuration event in Seyfert-1 galaxy NGC 3227, and thus triggered our joint XMM-Newton, NuSTAR, and Hubble Space Telescope (HST) observations to study this event. Here in the first paper of our series we present the broadband continuum modelling of the spectral energy distribution for NGC 3227, extending from near infrared (NIR) to hard X-rays. We use our new spectra taken with XMM-Newton, NuSTAR, and the HST Cosmic Origins Spectrograph in 2019, together with archival unobscured XMM-Newton, NuSTAR, and HST Space Telescope Imaging Spectrograph data, in order to disentangle various spectral components of NGC 3227 and recover the underlying continuum. We find the observed NIR-optical-UV continuum is explained well by an accretion disk blackbody component (T_max = 10 eV), which is internally reddened by E(B − V) = 0.45 with a Small Magellanic Cloud extinction law. We derive the inner radius (12 R_g) and the accretion rate (0.1 M_⊙ yr⁻¹) of the disk by modelling the thermal disk emission. The internal reddening in NGC 3227 is most likely associated with outflows from the dusty AGN torus. In addition, an unreddened continuum component is also evident, which likely arises from scattered radiation, associated with the extended narrow-line region of NGC 3227. The extreme ultraviolet continuum, and the ‘soft X-ray excess’, can be explained with a ‘warm Comptonisation’ component. The hard X-rays are consistent with a power-law and a neutral reflection component. The intrinsic bolometric luminosity of the AGN in NGC 3227 is about 2.2 × 10⁴³ erg s⁻¹ in 2019, corresponding to 3% Eddington luminosity. Our continuum modelling of the new triggered data of NGC 3227 requires the presence of a new obscuring gas with column density N_H = 5 × 10²² cm⁻², partially covering the X-ray source (C_f = 0.6).