Abstract

The properties of the iron Kα line emitted by a photoionized accretion disc have been calculated for different source geometries.

The properties of the iron Kα fluorescence line emitted by an α-viscosity accretion disc illuminated by an external X-ray source have been calculated for different values of the disc accretion rate ṁ. The vertical ionization structure of the matter has been computed by using the numerical code described in. Two different source geometries have been studied: a point source located at 20 rg (=GM/c2) above the disc on its symmetry axis, and an extended source above the innermost part (r = 6 – 50rg) of the disc. Assuming α=0.1, a Schwarzschild black hole and a hard luminosity equal to the disc luminosity, we find that for large values of ṁ (≲ 0.2, in units of the critical value) the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source (while for neutral matter it is ∼150eV for a face-on disc). The line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of ṁ (≲ 0.5) leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc.