Comparison of Relativistic Iron Line Models

TL;DR

This paper compares two relativistic iron line models, LAOR and KYRLINE, for constraining black hole spin from X-ray spectra, highlighting their agreement on current data and differences with higher quality future data.

Contribution

It evaluates the consistency and limitations of LAOR and KYRLINE models in estimating black hole spin from X-ray spectral data.

Findings

Both models agree within uncertainties on current data.

LAOR tends to overestimate spin and has resolution issues.

Differences become significant with higher quality simulated data.

Abstract

The analysis of the broad iron line profile in the X-ray spectra of active galactic nuclei and black hole X-ray binaries allows us to constrain the spin parameter of the black hole. We compare the constraints on the spin value for two X-ray sources, MCG-6-30-15 and GX 339-4, with a broad iron line using present relativistic line models in XSPEC - LAOR and KYRLINE. The LAOR model has the spin value set to the extremal value a=0.9982, while the KYRLINE model enables direct fitting of the spin parameter. The spin value is constrained mainly by the lower boundary of the broad line, which depends on the inner boundary of the disc emission where the gravitational redshift is maximal. The position of the inner disc boundary is usually identified with the marginally stable orbit which is related to the spin value. In this way the LAOR model can be used to estimate the spin value. We investigate the consistency of the LAOR and KYRLINE models. We find that the spin values evaluated by both models agree within the general uncertainties when applied on the current data. However, the results are apparently distinguishable for higher quality data, such as those simulated for the International X-ray Observatory (IXO) mission. We find that the LAOR model tends to overestimate the spin value and furthermore, it has insufficient resolution which affects the correct determination of the high-energy edge of the broad line.