Fractional amplitude of kilohertz quasi-periodic oscillation from 4U 1728-34: evidence of decline at higher energies

TL;DR

This study analyzes the energy dependence of kilohertz QPOs in neutron star LMXBs, revealing a decline in fractional rms amplitude at higher energies, which challenges previous assumptions of amplitude saturation.

Contribution

It provides the first evidence of a systematic decrease in kHz QPO fractional rms amplitude at high energies and explores spectral parameter oscillations as a possible explanation.

Findings

Significant decrease in fractional rms amplitude at high energies.

Oscillation of blackbody spectral parameters explains amplitude behavior.

Quality factor of kHz QPOs shows no energy dependence.

Abstract

A kilohertz quasi-periodic oscillation (kHz QPO) is an observationally robust high-frequency timing feature detected from neutron star low-mass X-ray binaries (LMXBs). This feature can be very useful to probe the superdense core matter of neutron stars, and the strong gravity regime. However, although many models exist in the literature, the physical origin of kHz QPO is not known, and hence this feature cannot be used as a tool yet. The energy dependence of kHz QPO fractional rms amplitude is an important piece of the jigsaw puzzle to understand the physical origin of this timing feature. It is known that the fractional rms amplitude increases with energy at lower energies. At higher energies, the amplitude is usually believed to saturate, although this is not established. We combine tens of lower kHz QPOs from a neutron star LMXB 4U 1728-34 in order to improve the signal-to-noise-ratio. Consequently, we, for the first time to the best of our knowledge, find a significant and systematic decrease of the fractional rms amplitude with energy at higher photon energies. Assuming an energy spectrum model, blackbody+powerlaw, we explore if the sinusoidal variation of a single spectral parameter can reproduce the above mentioned fractional rms amplitude behavior. Our analysis suggests that the oscillation of any single blackbody parameter is favored over the oscillation of any single powerlaw parameter, in order to explain the measured amplitude behavior. We also find that the quality factor of a lower kHz QPO does not plausibly depend on photon energy.