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

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


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 (NH ~ 1021 cm-2) and a range of ionisation states (-0.8 < log?(?/erg cm?s-1) < 2.4). Broad-band spectroscopy revealed a very steep intrinsic continuum (photon index G ~ 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?(?/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 rg from the central supermassive black hole that is so hot that it can Compton-upscatter the flaring intrinsic continuum emission.

Acceptance Date Jan 26, 2015
Publication Date Apr 23, 2015
Print ISSN 0004-6361
Publisher EDP Sciences
Keywords galaxies: active, galaxies: individual: Ark 564, galaxies: Seyfert, X-rays: individuals: Ark 564, black hole physics
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