A possible imprint of quasi-periodic oscillations in the X-ray spectra of black hole binaries

TL;DR

This study investigates how quasi-periodic oscillations (QPOs) in black hole binaries influence the time-averaged X-ray energy spectrum, revealing that QPOs can significantly affect spectral parameter estimations.

Contribution

The paper introduces a simple model demonstrating the impact of QPO-related structures on spectral fits, highlighting potential inaccuracies in disk parameter measurements during strong QPO states.

Findings

QPO presence affects the inner disk radius and temperature relation.

Simulated spectra with QPO-like structures reproduce observed spectral deviations.

Increased QPO strength leads to unreliable disk parameter estimates.

Abstract

While nobody would deny the presence of Quasi-Periodic Oscillations in the power density spectrum of Black hole binaries nor their importance in the understanding of the mechanisms powering the X-ray emissions, the possible impact on the time-averaged disk energy spectrum from the phenomenon responsible for the Quasi-Periodic Oscillations is largely ignored in models of sources emission. Here we investigate the potential impact of such structure on the resultant energy spectrum. Using data from the well documented outbursts of XTE J$1550$-$564$ we looked at possible hints that the presence of Quasi-Periodic Oscillations actually impacts the energy spectrum emitted by the source. In particular we look at the evolution of the relation between the inner disc radius and the inner disc temperature obtained from fits to the spectral data. We then test this further by developing a simple model in order to simulate spectra of a disk with a structure mimicking an increasing in strength Quasi-Periodic Oscillation and compare the simulated results to those obtained from real data. We detect a similar departure in the inner radius - inner temperature curve coming from the standard fit of our simulated observations as is seen in XTE J$1550$-$564$ data. We interpret our results as evidence that the structure at the origin of the Quasi-Periodic Oscillation impact the energy spectrum. Furthermore, in states with a significant disk emission the inaccuracy of the determination of the disk parameters increases with the strength of the Quasi-Periodic Oscillation, an increase which then renders the value given by the fit unreliable for strong Quasi-Periodic Oscillation.