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

We present simultaneous ASCA and RXTE observations of Ark 564, the brightes t known 'narrow-line' Seyfert 1 in the 2-10 keV band. The measured X-ray sp ectrum is dominated by a steep (Gamma approximate to 2.7) power-law continu um extending to at least 20 keV, with imprinted Fe K-line and edge features and an additional 'soft excess' below similar to 1.5 keV. The energy of th e iron K-edge indicates the presence of highly ionized material, which we i dentify in terms of reflection from a strongly irradiated accretion disc. T he high reflectivity of this putative disc, together with its strong intrin sic O VIII Ly alpha and O VIII recombination emission, can also explain muc h of the observed soft excess flux. Furthermore, the same spectral model al so 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' Se yfert Is of comparable luminosity, increasing by a factor of similar to 50 per cent in 1.6 h, with no measurable lag between the 0.5-2 keV and 3-12 ke V bands, consistent with much of the soft excess flux arising from reproces sing of the primary power-law component in the inner region of the accretio n disc. We note, finally, that if the unusually steep power-law component i s a result of Compton cooling of a disc corona by an intense soft photon fl ux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.