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

V. Spin determination from disc-Comptonisation efficiency method

¹ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
e-mail: delphine.porquet@lam.fr
² Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
³ Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, Roma 00146, Italy
⁵ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, Firenze 50125, Italy
⁷ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, Merate 23807, LC, Italy
⁸ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, Strasbourg 67000, France
⁹ Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama-shi, Saitam 337-8570, Japan
¹⁰ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹¹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹² Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹³ Eureka Scientific Inc., 2452 Delmer Street, Suite 100, Oakland, CA 94602, USA
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 169-221, Pasadena, CA 91109, USA
¹⁵ Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332-0430, USA
¹⁶ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA

Received: 17 October 2018
Accepted: 18 December 2018

Abstract

Context. The spin of supermassive black holes (SMBH) in active galactic nuclei (AGN) can be determined from spectral signature(s) of relativistic reflection such as the X-ray iron Kα line profile, but this can be rather uncertain when the line of sight intersects the so-called warm absorber and/or other wind components as these distort the continuum shape. Therefore, AGN showing no (or very weak) intrinsic absorption along the line-of-sight such as Ark 120, a so-called bare AGN, are the ideal targets for SMBH spin measurements. However, in our previous work on Ark 120, we found that its 2014 X-ray spectrum is dominated by Comptonisation, while the relativistic reflection emission only originates at tens of gravitational radii from the SMBH. As a result, we could not constrain the SMBH spin from disc reflection alone.

Aims. Our aim is to determine the SMBH spin in Ark 120 from an alternative technique based on the global energetics of the disc-corona system. Indeed, the mass accretion rate (Ṁ) through the outer disc can be measured from the optical-UV emission, while the bolometric luminosity (L_bol) can be fairly well constrained from the optical to hard X-rays spectral energy distribution, giving access to the accretion efficiency η = L_bol/(Ṁc²) which depends on the SMBH spin.

Methods. The spectral analysis uses simultaneous XMM-Newton (OM and pn) and NuSTAR observations on 2014 March 22 and 2013 February 18. We applied the OPTXCONV model (based on OPTXAGNF) to self consistently reproduce the emission from the inner corona (warm and hot thermal Comptonisation) and the outer disc (colour temperature corrected black body), taking into account both the disc inclination angle and relativistic effects. For self-consistency, we modelled the mild relativistic reflection of the incident Comptonisation components using the XILCONV convolution model.

Results. We infer a SMBH spin of 0.83^+0.05_−0.03, adopting the SMBH reverberation mass of 1.50 × 10⁸ M_⊙. In addition, we find that the coronal radius decreases with increasing flux (by about a factor of two), from 85⁺¹³₋₁₀R_g in 2013 to 14 ± 3 R_g in 2014.

Conclusions. This is the first time that such a constraint is obtained for a SMBH spin from this technique, thanks to the bare properties of Ark 120, its well determined SMBH reverberation mass, and the presence of a mild relativistic reflection component in 2014 which allows us to constrain the disc inclination angle. We caution that these results depend on the detailed disc-corona structure, which is not yet fully established. However, the realistic parameter values (e.g. L_bol/L_Edd, disc inclination angle) found suggest that this is a promising method to determine spin in moderate-Ṁ AGN.