Iron line variability and geometry in Seyfert galaxies

Abstract

Significant Iron features were recently found in several bright active galactic nuclei (AGN) with Tenma, Exosat and Ginga. When detected, the 6.4 keV fluorescent line equivalent width (EW) is equal to a few 0.1 keV. Such large equivalent widths are the signature of optically thick matter reprocessing X-ray photons. This matter largely covers the hard X-ray source independently of geometry (Makishima, 1987). We show here that variable Iron line equivalent widths are found in four Seyfert galaxies. For these objects, the Iron line equivalent widths are decreasing with increasing X-ray continuum flux. In NGC 5548 and NGC 3516, these variations are correlated with those of the ratio of hard and soft X-ray fluxes. Such behavior could be understood with geometrical variations where the soft X-ray luminosity is proportional to the square of the distance separating the X-ray source and the reprocessing matter. Such geometrical variations cannot be fitted with an extended reprocessing thin disk. Possible geometries for the soft X-ray source include the central part of a geometrically thick disk.