Dependence of the broad Fe Kα line on the physical parameters of AGN

In this paper, the dependence of the broad Fe Kα line on the physical parameters of AGN, such as the black hole mass M_BH, accretion rate (equivalently represented by Eddington ratio λ_Edd) and optical classification is investigated by applying the X-ray spectra stacking method to a large sample of AGN which have well measured optical parameters. A broad-line feature is detected (>3σ) in the stacked spectra of the high λ_Edd sub-sample (log λ_Edd > -0.9). The profile of the broad line can be well fitted with relativistic broad-line model, with the line energy consistent with highly ionized Fe Kα line (I.e. Fe XXVI). A model consisting of multiple narrow lines cannot be ruled out, however. We found hints that the Fe K line becomes broader as the λ_Edd increases. No broad-line feature is shown in the sub-sample of broad-line Seyfert 1 (BLS1) galaxies and in the full sample, while a broad line might be present, though at low significance, in the sub-sample of narrow-line Seyfert 1 (NLS1) galaxies. We find no strong dependence of the broad line on black hole masses. Our results indicate that the detection/properties of the broad Fe Kα line may strongly depend on λ_Edd, which can be explained if the ionization state and/or truncation radius of the accretion disc changes with λ_Edd. The non-detection of the broad line in the BLS1 sub-sample can be explained if the average equivalent width of the relativistic Fe Kα line is weak or/and the fraction of sources with relativistic Fe Kα line is small in BLS1 galaxies.