Radiative feedbacks as drivers for quasi-periodic-oscillation activity in black-hole X-ray binaries

TL;DR

This study models the disk-corona feedback in black-hole X-ray binaries to demonstrate how radiative interactions can produce limit-cycle oscillations resembling observed type-C QPOs.

Contribution

It introduces a radiation-based model showing that disk-corona feedback can generate QPO-like oscillations with properties matching observations.

Findings

Limit-cycle oscillations occur when soft photon reprocessing dominates electron cooling.

Oscillation frequencies depend on coronal radius and energization timescale.

X-ray spectra from oscillations are power-law-like up to 100 keV.

Abstract

Black-hole X-ray binaries (BHXRBs) in the hard and hard-intermediate spectral states commonly exhibit prominent type-C quasi-periodic oscillations (QPOs) in their X-ray power spectra. Despite extensive observational and theoretical efforts, the physical mechanism responsible for these oscillations has not yet been firmly established. The disk-corona system in BHXRBs is radiatively coupled, as hard X-ray emission from the corona can be reprocessed by the accretion disk and re-emitted as soft photons that contribute to cooling the coronal electrons. Aim of the present study is to examine whether this feedback can give rise to limit cycles having the spectro-temporal properties of QPOs. We model the coronal emission using a one-zone radiation framework and solve the time-dependent kinetic equations for electrons and photons. Electrons are energized by some unspecified process and cool via inverse Compton scattering of soft photons originating from (i) the accretion disk and (ii) disk reprocessing of the hard radiation produced in the corona. When electron cooling is dominated by soft photons reprocessed in the accretion disk, the disk-corona system undergoes limit-cycle oscillations. For a subset of the model parameters, these oscillations reproduce key properties of type-C QPOs observed in BHXRBs. The oscillation frequency depends on the coronal radius and on the energization timescale, while the resulting X-ray spectra are well described by power laws extending up to energies of ~ 10-100 keV. These calculations confirm and extend earlier semi-analytical results obtained with simplified treatments. Owing to the scale-invariant nature of the model, the results can be readily extrapolated to other accreting systems, such as Active Galactic Nuclei.