A deep X-ray view of the bare AGN Ark 120

VI. Geometry of the hot corona from spectroscopic and polarization signatures

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
e-mail: marinucci@fis.uniroma3.it
² Aix-Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ Astronomical Institute of the Czech Academy of Sciences, 14100 Prague, Czech Republic
⁴ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
⁵ Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁶ INAF-Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
⁷ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK
⁸ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹⁰ INAF-Istituto di Astrofisica e Planetologie Spaziali, Via Fosso del Cavaliere, 00133 Roma, Italy

Received: 18 October 2018
Accepted: 16 November 2018

Abstract

Context. The spectral shape of the hard X-ray continuum of Active Galactic Nuclei (AGN) can be ascribed to inverse Compton scattering of optical/UV seed photons from the accretion disc by a hot corona of electrons. This physical process produces a polarization signal which is strongly sensitive to the geometry of the scattering medium (i.e. the hot corona) and of the radiation field.

Aims. MoCA (Monte Carlo code for Comptonisation in Astrophysics) is a versatile code which allows for different geometries and configurations to be tested for Compton scattering in compact objects. A single photon approach is considered as well as polarisation and Klein–Nishina effects. In this work, we selected four different geometries for the scattering electrons cloud above the accretion disc, namely an extended slab, an extended spheroid and two compact spheroids.

Methods. We discuss the first application of the MoCA model to reproduce the hard X-ray primary continuum of the bare Seyfert 1 galaxy Ark 120, using different geometries for the hot corona above the accretion disc. The lack of extra-Galactic absorption along the line of sight makes it an excellent target for studying the accretion disc-corona system. We report on the spectral analysis of the simultaneous 2013 and 2014 XMM-Newton and NuSTAR observations of the source.

Results. A general agreement is found between the best fit values of the hot coronal parameters obtained with MoCA and the ones inferred using other Comptonisation codes from the literature. The expected polarization signal from the best fits with MoCA is then presented and discussed, in view of the launch in 2021 of the Imaging X-ray Polarimetry Explorer (IXPE).

Conclusions. We find that none of the tested geometries for the hot corona (extended slab and extended/compact spheroids) can be statistically preferred, based on spectroscopy solely. In the future, an IXPE observation less than 1 Ms long will clearly distinguish between an extended slab or a spherical hot corona.