Equivalent width EW of the absorption line system between 7 and 9 keV as a function of the estimated N_H for the UFOs in Table A.1 (black points). The gray points are from a large number of XSTAR models with 10²² < N_H < 10²⁴ cm⁻², 10³ < ξ < 10⁵ ergs/s cm, and 1000 < b < 30000 km/s. The black line is the upper envelope of all XSTAR models and all observed points. It represents the minimum N_H corresponding to a given EW.