2.1.1. Elemental abundance

Elemental abundances effect the equivalent width of the iron line both through the amount of iron that is present to fluoresce, and the absorption of the line photons by L-shell photoelectric absorption of iron and K-shell photoelectric absorption of lower-Z elements. These competing effects, together with the fact that the edge is saturated (i.e. most incident photons just above the photoelectric edge are absorbed by iron ions), leads to a roughly logarithmical dependence on abundance. For example, using the cosmic abundance values from Anders & Grevesse (1982), the equivalent width as a function of the iron abundance A_Fe is given by

(1)

(2)

where

(3)

(4)

(5)

where A_Fe = 1 refers to cosmic abundances (Matt, Fabian & Reynolds 1997).