Fe K Line Profile in Low-redshift Quasars: Average Shape and Eddington Ratio Dependence

We analyze X-ray spectra of 43 Palomar-Green quasars observed with XMM-Newton in order to investigate their mean Fe K line profile and its dependence on physical properties. The continuum spectra of 39 objects are well reproduced by a model consisting of a power law and a blackbody modified by Galactic absorption. The spectra of the remaining four objects require an additional power-law component absorbed with a column density of ∼ 10cm. A feature resembling an emission line at 6.4 keV, identified with an Fe K line, is detected in 33 objects. Approximately half of the sample show an absorption feature around 0.65–0.95 keV, which is due to absorption lines and edges of O VII and O VIII. We fit the entire sample simultaneously to derive average Fe line parameters by assuming a common Fe line shape. The Fe line is relatively narrow (σ = 0.36 keV), with a center energy of 6.48 keV and a mean equivalent width (EW) of 248 eV. By combining black hole masses estimated from the virial method and bolometric luminosities derived from full spectral energy distributions, we examine the dependence of the Fe K line profile on Eddington ratio. As the Eddington ratio increases, the line becomes systematically stronger (EW = 130 to 280 eV), broader (σ = 0.1 to 0.7 keV), and peaks at higher energies (6.4 to 6.8 keV). This result suggests that the accretion rate onto the black hole directly influences the geometrical structure and ionization state of the accretion disk. We also examine a two-component model consisting of a Gaussian and a diskline to constrain the intensity of the broad line. The mean equivalent widths are ≈ 70−180eV for the four Eddington ratio groups, although the standard deviations in each group are very large. This suggests that the broad line is not ubiquitous. Subject headings: accretion, accretion disks — galaxies: active — galaxies: nuclei — galaxies: Seyfert — quasars: general — X-rays: general