Revealing a hard X-ray spectral component that reverberates within one light hour of the central supermassive black hole in Ark 564

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: m.giustini@sron.nl
² XMM-Newton Science Operations Centre, ESA/ESAC, Villafranca del Castillo, Apartado 78, 28692 Villanueva de la Cañada, Spain
³ Center for Space Science and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21250, USA
⁵ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
⁶ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
⁷ Institute for Astrophysics and Computational Sciences, Department of Physics, The Catholic University of America, Washington, DC 20064, USA

Received: 5 November 2014
Accepted: 26 January 2015

Abstract

Context. Arakelian 564 (Ark 564, z = 0.0247) is an X-ray-bright narrow-line Seyfert 1 galaxy. By using advanced X-ray timing techniques, an excess of “delayed” emission in the hard X-ray band (4−7.5 keV) following about 1000 s after “flaring” light in the soft X-ray band (0.4 − 1 keV) was recently detected.

Aims. We report on the X-ray spectral analysis of eight XMM-Newton and one Suzaku observation of Ark 564. Our aim is to characterise the X-ray spectral properties of the source in the light of these recently reported results.

Methods. High-resolution spectroscopy was performed with the RGS in the soft X-ray band, while broad-band spectroscopy was performed with the EPIC-pn and XIS/PIN instruments. We analysed time-averaged, flux-selected, and time-resolved spectra.

Results. Despite the strong variability in flux during our observational campaign, the broad-band spectral shape of Ark 564 does not vary dramatically and can be reproduced either by a superposition of a power law and a blackbody emission or by a Comptonized power-law emission model. High-resolution spectroscopy revealed ionised gas along the line of sight at the systemic redshift of the source, with a low column density (N_H ~ 10²¹ cm^-2) and a range of ionisation states (−0.8 < log (ξ/erg cm s^-1) < 2.4). Broad-band spectroscopy revealed a very steep intrinsic continuum (photon index Γ ~ 2.6) and a rather weak emission feature in the iron K band (EW ~ 150 eV); modelling this feature with a reflection component requires highly ionised gas, log (ξ/erg cm s^-1) > 3.5. A reflection-dominated or an absorption-dominated model are similarly able to well reproduce the time-averaged data from a statistical point of view, in both cases requiring contrived geometries and/or unlikely physical parameters. Finally, through time-resolved analysis we spectroscopically identified the “delayed” emission as a spectral hardening above ~4 keV; the most likely interpretation for this component is a reprocessing of the “flaring” light by gas located at 10−100 r_g from the central supermassive black hole that is so hot that it can Compton-upscatter the flaring intrinsic continuum emission.