Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

Abstract

ABSTRA C T We present simultaneous ASCA and RXTE observations of Ark 564, the brightest known ‘narrow-line’ Seyfert 1 in the 2‐10 keV band. The measured X-ray spectrum is dominated by a steepOG < 2:7U power-law continuum extending to at least 20 keV, with imprinted Fe K-line and edge features and an additional ‘soft excess’ below ,1:5 keV. The energy of the iron K-edge indicates the presence of highly ionized material, which we identify in terms of reflection from a strongly irradiated accretion disc. The high reflectivity of this putative disc, together with its strong intrinsic O viii Lya and O viii recombination emission, can also explain much of the observed soft excess flux. Furthermore, the same spectral model also provides a reasonable match to the very steep 0.1‐2 keV spectrum deduced from ROSAT data. The source is much more rapidly variable than ‘normal’ Seyfert 1s of comparable luminosity, increasing by a factor of ,50 per cent in 1.6 h, with no measurable lag between the 0.5‐2 keV and 3‐12 keV bands, consistent with much of the soft excess flux arising from reprocessing of the primary power-law component in the inner region of the accretion disc. We note, finally, that if the unusually steep power-law component is a result of Compton cooling of a disc corona by an intense soft photon flux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.