Interference as an origin of the peaked noise in accreting X-ray binaries

TL;DR

This paper introduces a physical model explaining peaked noise in X-ray binaries' power spectra as interference between two Comptonization processes, linking spectral features to accretion flow properties.

Contribution

It presents a novel interpretation of peaked noise as interference of two Comptonization continua and demonstrates how to extract accretion flow parameters from observed spectral features.

Findings

Model explains harmonic relations in power spectra

Delay times relate to accretion flow parameters

Applied to black hole binaries to constrain flow properties

Abstract

We propose a physical model for the peaked noise in the X-ray power density spectra of accreting X-ray binaries. We interpret its appearance as an interference of two Comptonization continua: one coming from the up-scattering of seed photons from the cold thin disk and the other fed by the synchrotron emission of the hot flow. Variations of both X-ray components are caused by fluctuations in mass accretion rate, but there is a delay between them corresponding to the propagation timescale from the disk Comptonization radius to the region of synchrotron Comptonization. If the disk and synchrotron Comptonization are correlated, the humps in the power spectra are harmonically related and the dips between them appear at frequencies related as odd numbers 1:3:5. If they are anti-correlated, the humps are related as 1:3:5, but the dips are harmonically related. Similar structures are expected to be observed in accreting neutron star binaries and supermassive black holes. The delay can be easily recovered from the frequency of peaked noise and further used to constrain the combination of the viscosity parameter and disk height-to-radius ratio $\alpha (H/R)^2$ of the accretion flow. We model multi-peak power spectra of black hole X-ray binaries GX 339$-$4 and XTE J1748$-$288 to constrain these parameters.