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

Abstract

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. We find that for the lower kHz QPO both the slope of the best-fitting linear model to the time-lag spectrum and the total rms amplitude integrated over the 2–25 keV energy band 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 Comptonizing cloud or corona that is coupled to the innermost regions of the accretion disc, close to the neutron star.