Arakelian 564: An unusual component in the X-ray spectra of narrow-line Seyfert 1 galaxies

We present an ASCA observation of the narrow-line Seyfert 1 (NLSy1) galaxy Arakelian 564. The X-ray light curve shows rapid variability, but no eviden ce for energy dependence to these variations, within the 0.6-10 keV bandpas s. A strong (EW similar to 70 eV) spectral feature is observed close to 1 k eV. A similar feature has been observed in Ton S180, another member of the NLSy1 class of objects, but it has not been observed in broad-line Seyfert galaxies. The feature energy suggests a large contribution from Fe L-shell lines, but its intensity is difficult to explain in terms of emission and/o r absorption from photoionized gas. The models that predict most emission a t 1 keV are characterized by extreme values of column density, Fe abundance , and ionization parameter. Models based on gas in thermal equilibrium with kT similar to 1 keV provide an alternative parameterization of the soft sp ectrum. The latter may be interpreted as the hot intercloud medium, undergo ing rapid cooling and producing strong Fe L-shell recombination lines. In a ll cases the physical conditions are rather different from those observed i n broad-line Seyfert galaxies. The hard X-ray spectrum shows a broad and as ymmetric Fe Ka line of large equivalent width (similar to 550 eV), suggesti ve of significant emission from the inner accretion disk. The profile can b e explained by a neutral disk viewed at similar to 60 degrees to the line o f sight, contrary to the hypothesis that NLSy1's are viewed pole-on. The la rge EW of this line, the strong 1 keV emission, and the strong optical Fe e mission lines all suggest an extreme Fe abundance in this and perhaps other NLSy1's.