The harmonic component of the millihertz quasi-periodic oscillations in 4U 1636-53

TL;DR

This study analyzes the harmonic components of millihertz quasi-periodic oscillations in the neutron star binary 4U 1636-53, revealing energy-dependent amplitude behaviors and their occurrence in different spectral states.

Contribution

It is the first to examine the rms amplitude of QPO harmonics versus energy in 4U 1636-53, providing new insights into their properties and occurrence.

Findings

Harmonics detected in 73 data intervals, mostly in transitional states.

RMS amplitude ratio of harmonic to fundamental remains constant across frequencies.

RMS amplitude decreases with energy in the 2-5 keV range, unlike black hole QPOs.

Abstract

We studied the harmonics of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636-53 using the Rossi X-ray Timing Explorer observations. We detected the harmonics of the mHz QPOs in 73 data intervals, with most of them in the transitional spectra state. We found that the ratio between the rms amplitude of the harmonic and that of the fundamental remains constant in a wide range of the fundamental frequency. More importantly, we studied, for the first time, the rms amplitude of the harmonics vs. energy in 4U 1636-53 in the 2-5 keV range. We found that the rms amplitude of both the harmonic and the fundamental shows a decreasing trend as the energy increases, which is different from the behaviors reported in QPOs in certain black hole systems. Furthermore, our results suggest that not all observations with mHz QPOs have the harmonic component, although the reason behind this is still unclear.