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

Valentina Peirano,Mariano Méndez

doi:10.1093/mnras/stab1905

Valentina Peirano, Mariano Méndez

Open Access

https://doi.org/10.1093/mnras/stab1905

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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.

Highlights

Quasi-periodic oscillations (QPOs) are variability features observed in the power spectra of neutron-star and black-hole Low-Mass X-ray Binaries (LMXBs; Strohmayer et al 1996; van der Klis et al 1996; Morgan et al 1997; Strohmayer 2001)
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
As with the rms fractional amplitude, the energydependent time lags of the lower kHz QPOs depend upon luminosity in 8 neutron-star LMXBs

Summary

INTRODUCTION

Quasi-periodic oscillations (QPOs) are variability features observed in the power spectra of neutron-star and black-hole Low-Mass X-ray Binaries (LMXBs; Strohmayer et al 1996; van der Klis et al 1996; Morgan et al 1997; Strohmayer 2001). Barret et al 2006; Méndez 2006; Ribeiro et al 2019), whereas the rms amplitude of the upper kHz QPO generally decreases with increasing QPO frequency (van Straaten et al 2002; Méndez 2006; Altamirano et al 2008; Troyer et al 2018) Another property of the QPO signal is the energy-dependent time (or phase) lag. Méndez (2006) studied the behaviour of the maximum quality factor and fractional rms amplitude of the kHz QPOs in 12 different LMXBs, and found that for both the lower and upper kHz QPOs the maximum fractional rms amplitude decreases exponentially with increasing luminosity of the source, while for the lower kHz QPOs the maximum coherence increases up to a certain luminosity and decreases exponentially Motivated by these results, in this paper we explore the dependence of both the fractional rms amplitude and the time lags of the lower kHz QPOs in the LMXBs studied by Troyer et al (2018) upon luminosity.

DATA SELECTION AND ANALYSIS

RESULTS

DISCUSSION