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

TL;DR

This study investigates how the angular distribution of X-ray emission from accretion disks affects black hole spin measurements, finding that assumptions about emission directionality can introduce significant uncertainties.

Contribution

We introduce a new extension to the KY spectral fitting software incorporating emission angular distribution, and quantify its impact on spin determination accuracy.

Findings

Uncertainty in emission angular distribution causes about 20% error in spin measurement.

Isotropic emission assumption provides the best fit to our model data.

Incorrect limb darkening assumptions can mimic steeper radial emissivity profiles.

Abstract

Spin of an accreting black hole can be determined by spectroscopy of the emission and absorption features produced in the inner regions of an accretion disc. We discuss the method employing the relativistic line profiles of iron in the X-ray domain, where the emergent spectrum is blurred by general relativistic effects. Precision of spectra fitting procedure could be compromised by inappropriate account of the angular distribution of the disc emission. Often a unique profile is assumed, invariable over the entire range of radii in the disc and energy in the spectral band. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emitted photons. We find that the uncertainty of the directional emission distribution translates to 20% uncertainty in determination of the marginally stable orbit. By assuming a rotating black hole in the centre of an accretion disc, we perform radiation transfer computations of an X-ray irradiated disc atmosphere to determine the directionality of outgoing X-rays in the 2-10 keV energy band. We implemented the simulation results as a new extension to the KY software package for X-ray spectra fitting of relativistic accretion disc models. Although the parameter space is rather complex, leading to a rich variety of possible outcomes, we find that on average the isotropic directionality reproduces our model data to the best precision. Our results also suggest that an improper usage of limb darkening can partly mimic a steeper profile of radial emissivity. We demonstrate these results on the case of XMM-Newton observation of the Seyfert galaxy MCG-6-30-15, for which we construct confidence levels of chi squared statistics, and on the simulated data for the future X-ray IXO mission.