GRMHD prediction of coronal variability in accreting black holes

TL;DR

This study uses 3D general relativistic MHD simulations to predict the variability of coronal luminosity in accreting black holes, revealing a power-law behavior driven by turbulent accretion rate fluctuations.

Contribution

It provides a novel prediction of coronal variability characteristics based on energy-conserving GRMHD simulations, linking physical turbulence to observed power spectra.

Findings

Power spectrum follows a -2 power-law over relevant frequencies.

Variability driven by chaotic MHD turbulence in accretion flows.

Radial epicyclic motions modulate accretion rate without dissipation.

Abstract

On the basis of data from an energy-conserving 3D general relativistic MHD simulation, we predict the statistical character of variability in the coronal luminosity from accreting black holes. When the inner boundary of the corona is defined to be the electron scattering photosphere, its location depends only on the mass accretion rate in Eddington units (\dot{M}). Nearly independent of viewing angle and \dot{M}, the power spectrum over the range of frequencies from approximately the orbital frequency at the innermost stable circular orbit (ISCO) to ~100 times lower is well approximated by a power-law with index -2, crudely consistent with the observed power spectra of hard X-ray fluctuations in AGN and the hard states of Galactic binary black holes. The underlying physical driver for variability in the light curve is variations in the accretion rate caused by the chaotic character of MHD turbulence, but the power spectrum of the coronal light output is significantly steeper. Part of this contrast is due to the fact that the mass accretion rate can be significantly modulated by radial epicyclic motions that do not result in dissipation, and therefore do not drive luminosity fluctuations. The other part of this contrast is due to the inward decrease of the characteristic inflow time, which leads to decreasing radial coherence length with increasing fluctuation frequency.