Revealing a hard X-ray spectral component reverberating within one light hour of the central Supermassive Black Hole in Ark 564

M. Giustini, T. J. Turner, J. N. Reeves, L. Miller, E. Legg, S. B. Kraemer, I. M. George

Published 2015-02-04Version 1

Ark 564 (z=0.0247) is an X-ray bright NLS1. By using advanced X-ray timing techniques, Legg et al. (2012) discovered an excess of "delayed" emission in the hard X-ray band (4-7.5 keV) following about 1000 seconds after "flaring" light in the soft X-ray band (0.4-1 keV). We report on the X-ray spectral analysis of eight XMM-Newton and one Suzaku observation of Ark 564. 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. Despite the large variability in flux, the broad band spectral shape of Ark 564 is not dramatically varying 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 the presence of ionised gas along the line of sight at the systemic redshift of the source, with a low column density and a range of ionisation states. Broad band spectroscopy revealed a very steep intrinsic continuum and a rather weak emission feature in the iron K band; modelling this feature with a reflection component requires the presence of highly ionised gas. Either a reflection-dominated or an absorption-dominated model are 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 discovered by Legg et al. (2012) as a spectral hardening above ~4 keV; the most likely interpretation for this component is reprocessing of the "flaring" light by gas located at 10-100 r_g from the central supermassive black hole and so hot to be able to Compton upscatter the flaring intrinsic continuum emission.