The Spectral Energy Distribution and Emission-Line Properties of the Narrow-Line Seyfert 1 Galaxy Arakelian 564

We present the intrinsic spectral energy distribution (SED) of the narrow-line Seyfert 1 galaxy (NLS1) Ark 564, constructed with contemporaneous data obtained during a multiwavelength, multisatellite observing campaign in 2000 and 2001. We compare this SED with that of the NLS1 Ton S180 and with those obtained for broad-line Seyfert 1 galaxies to infer how the relative accretion rates vary among the Seyfert 1 population. Although the peak of the SED is not well constrained, in our parameterization most of the energy of this object is emitted in the 10-100 eV regime, constituting roughly half of the emitted energy in the optical/X-ray ranges. This is consistent with a primary spectral component peaking in the extreme-UV/soft X-ray band and with disk-corona models, hence high accretion rates. Indeed, we estimate that ≈ 1. We also address the issue of the energy budget in this source by examining the emission lines observed in its spectrum, and we constrain the physical properties of the line-emitting gas through photoionization modeling. The available data suggest that the line-emitting gas is characterized by log n ≈ 11 and log U ≈ 0 and is stratified around log U ≈ 0. Our estimate of the radius of the Hβ-emitting region R ≈ 10 ± 2 lt-days is consistent with the R-luminosity relationships found for Seyfert 1 galaxies, which indicates that the narrowness of the emission lines in this NLS1 is not due to the broad-line region being relatively farther away from the central mass than in broad-line Seyfert 1 galaxies of comparable luminosity. We also find evidence for supersolar metallicity in this NLS1. We show that the emission lines are not good diagnostics for the underlying SEDs and that the absorption line studies offer a far more powerful tool to determine the ionizing continuum of active galactic nuclei, especially if one is comparing the lower and higher ionization lines.