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The analysis of the broad iron line profile in the X-ray spectra of activegalactic nuclei and black hole X-ray binaries allows us to constrain the spinparameter of the black hole. We compare the constraints on the spin value fortwo X-ray sources, MCG-6-30-15 and GX 339-4, with a broad iron line usingpresent relativistic line models in XSPEC - LAOR and KYRLINE. The LAOR modelhas the spin value set to the extremal value a=0.9982, while the KYRLINE modelenables direct fitting of the spin parameter. The spin value is constrainedmainly by the lower boundary of the broad line, which depends on the innerboundary of the disc emission where the gravitational redshift is maximal. Theposition of the inner disc boundary is usually identified with the marginallystable orbit which is related to the spin value. In this way the LAOR model canbe used to estimate the spin value. We investigate the consistency of the LAORand KYRLINE models. We find that the spin values evaluated by both models agreewithin the general uncertainties when applied on the current data. However, theresults are apparently distinguishable for higher quality data, such as thosesimulated for the International X-ray Observatory (IXO) mission. We find thatthe LAOR model tends to overestimate the spin value and furthermore, it hasinsufficient resolution which affects the correct determination of thehigh-energy edge of the broad line.

Comparison of Relativistic Iron Line Models