Fe K$\alpha$ Profiles from Simulations of Accreting Black Holes

TL;DR

This paper introduces a novel simulation-based method to predict Fe Kα emission profiles from accreting black holes, linking theoretical models directly to observable X-ray spectral features.

Contribution

It presents a self-consistent approach combining GRMHD simulations with radiation transport to produce realistic Fe Kα line profiles without phenomenological assumptions.

Findings

Fe Kα surface brightness peaks just outside the innermost stable orbit

Predicted line profiles and equivalent widths resemble observed spectra

Profiles depend on disk ionization and coronal geometry

Abstract

We present first results from a new technique for the prediction of Fe K$\alpha$ profiles directly from general relativistic magnetohydrodynamic (GRMHD) simulations. Data from a GRMHD simulation are processed by a Monte Carlo global radiation transport code, which determines the X-ray flux irradiating the disk surface and the coronal electron temperature self-consistently. With that irradiating flux and the disk's density structure drawn from the simulation, we determine the reprocessed Fe K$\alpha$ emission from photoionization equilibrium and solution of the radiation transfer equation. We produce maps of the surface brightness of Fe K$\alpha$ emission over the disk surface, which---for our example of a $10 M_\odot$, Schwarzschild black hole accreting at $1\%$ the Eddington value---rises steeply one gravitational radius outside the radius of the innermost stable circular orbit and then falls $\propto r^{-2}$ at larger radii. We explain these features of the Fe K$\alpha$ radial surface brightness profile as consequences of the disk's ionization structure and an extended coronal geometry, respectively. We also present the corresponding Fe K$\alpha$ line profiles as would be seen by distant observers at several inclinations. Both the shapes of the line profiles and the equivalent widths of our predicted K$\alpha$ lines are qualitatively similar to those typically observed from accreting black holes. Most importantly, this work represents a direct link between theory and observation: in a fully self-consistent way, we produce observable results---iron fluorescence line profiles---from the theory of black hole accretion with almost no phenomenological assumptions.