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Spin of an accreting black hole can be determined by spectroscopy of theemission and absorption features produced in the inner regions of an accretiondisc. We discuss the method employing the relativistic line profiles of iron inthe X-ray domain, where the emergent spectrum is blurred by generalrelativistic effects. Precision of spectra fitting procedure could becompromised by inappropriate account of the angular distribution of the discemission. Often a unique profile is assumed, invariable over the entire rangeof radii in the disc and energy in the spectral band. We study how sensitivethe spin determination is to the assumptions about the intrinsic angulardistribution of the emitted photons. We find that the uncertainty of thedirectional emission distribution translates to 20% uncertainty indetermination of the marginally stable orbit. By assuming a rotating black holein the centre of an accretion disc, we perform radiation transfer computationsof an X-ray irradiated disc atmosphere to determine the directionality ofoutgoing X-rays in the 2-10 keV energy band. We implemented the simulationresults as a new extension to the KY software package for X-ray spectra fittingof relativistic accretion disc models. Although the parameter space is rathercomplex, leading to a rich variety of possible outcomes, we find that onaverage the isotropic directionality reproduces our model data to the bestprecision. Our results also suggest that an improper usage of limb darkeningcan partly mimic a steeper profile of radial emissivity. We demonstrate theseresults on the case of XMM-Newton observation of the Seyfert galaxyMCG-6-30-15, for which we construct confidence levels of chi squaredstatistics, and on the simulated data for the future X-ray IXO mission.

Role of emission angular directionality in spin determination of accreting black holes with broad iron line