2015A&A...577A...8G

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 (4-7.5keV) following about 1000s after ''flaring'' light in the soft X-ray band (0.4-1keV) was recently detected. 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. 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 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∼10²¹cm^–2) and a range of ionisation states (-0.8<log(ξ/erg cm/s)<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∼150eV); modelling this feature with a reflection component requires highly ionised gas, log(ξ/erg cm/s)>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 ∼4keV; the most likely interpretation for this component is a reprocessing of the ''flaring'' light by gas located at 10-100r_g from the central supermassive black hole that is so hot that it can Compton-upscatter the flaring intrinsic continuum emission.