Abstract
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 evidence for energy dependence to these variations, within the 0.6-10 keV bandpass. A strong (EW ~ 70 eV) spectral feature is observed close to 1 keV. 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/or absorption from photoionized gas. The models that predict most emission at 1 keV are characterized by extreme values of column density, Fe abundance, and ionization parameter. Models based on gas in thermal equilibrium with kT ~ 1 keV provide an alternative parameterization of the soft spectrum. The latter may be interpreted as the hot intercloud medium, undergoing rapid cooling and producing strong Fe L-shell recombination lines. In all cases the physical conditions are rather different from those observed in broad-line Seyfert galaxies. The hard X-ray spectrum shows a broad and asymmetric Fe Kα line of large equivalent width (~550 eV), suggestive of significant emission from the inner accretion disk. The profile can be explained by a neutral disk viewed at ~60° to the line of sight, contrary to the hypothesis that NLSy1's are viewed pole-on. The large EW of this line, the strong 1 keV emission, and the strong optical Fe emission lines all suggest an extreme Fe abundance in this and perhaps other NLSy1's.
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