Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity

TL;DR

This study investigates how the energy-dependent time lags and rms amplitudes of kHz QPOs in neutron-star X-ray binaries vary with luminosity, revealing exponential decreases and correlations that inform models of the accretion environment.

Contribution

It presents the first detailed analysis of how kHz QPO timing properties depend on luminosity, highlighting a linear correlation and potential radiative mechanisms.

Findings

Both the slope of the lag spectrum and rms amplitude decrease exponentially with luminosity.

The slope of the lag spectrum for the lower kHz QPO correlates linearly with the rms amplitude.

The upper kHz QPO's lag spectrum slope is consistent with zero across sources.

Abstract

We study the energy-dependent time lags and rms fractional amplitude of the kilohertz quasi-periodic oscillations (kHz QPOs) of a group of neutron-star low mass X-ray binaries (LMXBs). We find that for the lower kHz QPO the slope of the best-fitting linear model to the time-lag spectrum and the total rms amplitude integrated over the 2 to 25 keV energy band both decrease exponentially with the luminosity of the source. For the upper kHz QPO the slope of the time-lag spectrum is consistent with zero, while the total rms amplitude decreases exponentially with the luminosity of the source. We show that both the slope of the time-lag spectrum and the total rms amplitude of the lower kHz QPO are linearly correlated with a slope of ~1. Finally, we discuss the mechanism that could be responsible for the radiative properties of the kHz QPOs, with the variability originating in a Comptonising cloud or corona that is coupled to the innermost regions of the accretion disc, close to the neutron star.