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

IV. XMM-Newton and NuSTAR spectra dominated by two temperature (warm, hot) Comptonization processes

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
e-mail: delphine.porquet@astro.unistra.fr
² Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Keele ST5 5BG, UK
³ CSST, 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, 00146 Roma, Italy
⁵ INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁶ INAF–Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
⁷ Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332-0430, USA
⁸ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
⁹ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
¹⁰ Dr Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, 96049 Bamberg, Germany
¹¹ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
¹² Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁶ INAF – IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁷ NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA

Received: 1 June 2017
Accepted: 24 July 2017

Abstract

Context. The physical characteristics of the material closest to supermassive black holes (SMBHs) are primarily studied through X-ray observations. However, the origins of the main X-ray components such as the soft X-ray excess, the Fe Kα line complex, and the hard X-ray excess are still hotly debated. This is particularly problematic for active galactic nuclei (AGN) showing a significant intrinsic absorption, either warm or neutral, which can severely distort the observed continuum. Therefore, AGN with no (or very weak) intrinsic absorption along the line of sight, so-called “bare AGN”, are the best targets to directly probe matter very close to the SMBH.

Aims. We perform an X-ray spectral analysis of the brightest and cleanest bare AGN known so far, Ark 120, in order to determine the process(es) at work in the vicinity of the SMBH.

Methods. We present spectral analyses of data from an extensive campaign observing Ark 120 in X-rays with XMM-Newton (4 × 120 ks, 2014 March 18–24), and NuSTAR (65.5 ks, 2014 March 22).

Results. During this very deep X-ray campaign, the source was caught in a high-flux state similar to the earlier 2003 XMM-Newton observation, and about twice as bright as the lower-flux observation in 2013. The spectral analysis confirms the “softer when brighter” behavior of Ark 120. The four XMM-Newton/pn spectra are characterized by the presence of a prominent soft X-ray excess and a significant Fe Kα complex. The continuum is very similar above about 3 keV, while significant variability is present for the soft X-ray excess. We find that relativistic reflection from a constant-density, flat accretion disk cannot simultaneously produce the soft excess, broad Fe Kα complex, and hard X-ray excess. Instead, Comptonization reproduces the broadband (0.3–79 keV) continuum well, together with a contribution from a mildly relativistic disk reflection spectrum.

Conclusions. During this 2014 observational campaign, the soft X-ray spectrum of Ark 120 below ~0.5 keV was found to be dominated by Comptonization of seed photons from the disk by a warm (kT_e ~ 0.5 keV), optically-thick corona (τ ~ 9). Above this energy, the X-ray spectrum becomes dominated by Comptonization from electrons in a hot optically thin corona, while the broad Fe Kα line and the mild Compton hump result from reflection off the disk at several tens of gravitational radii.