Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 1636$-$53

TL;DR

This study explores how spectral parameters relate to the frequencies and amplitudes of kHz QPOs in 4U 1636$-$53, revealing that frequencies follow similar relations while amplitudes differ, suggesting different radiative mechanisms.

Contribution

It demonstrates that kHz QPO frequencies are linked to spectral parameters with a systematic shift, while their amplitudes are governed by different relations, providing insights into their underlying mechanisms.

Findings

QPO frequencies follow similar spectral relations with a systematic shift.

QPO amplitudes follow different spectral relations for lower and upper kHz QPOs.

Implication that dynamical and radiative mechanisms differ for QPO frequencies and amplitudes.

Abstract

We investigate the relation between the parameters of the energy spectrum and the frequency and amplitude of the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binary 4U 1636$-$53. We fit the $3-180$-keV spectrum of this source with a model that includes a thermal Comptonisation component. We show that the frequencies of both kHz QPOs follow the same relation as a function of the parameters of this spectral component, except for a systematic frequency shift, whereas the rms fractional amplitude of each QPO follows a different relation with respect to those same parameters. This implies that, while the dynamical mechanism that sets the frequencies of the QPO can be the same for both kHz QPOs, the radiative mechanisms that set the amplitudes of the lower and the upper kHz QPO are likely different. We discuss the implications of these results to the modelling of the kHz QPOs and the possibility that the lower kHz QPO reflects a resonance between the Comptonising medium and the photons from the accretion disc and/or the neutron star surface.