Discovery of an accretion-rate independent absolute RMS amplitude of millihertz quasi-periodic oscillations in 4U 1636-53

TL;DR

This study reveals that the absolute RMS amplitude of mHz QPOs in 4U 1636-53 remains constant regardless of accretion rate, challenging previous assumptions about their dependence on source state and accretion parameters.

Contribution

It is the first to demonstrate that the absolute RMS amplitude of mHz QPOs is independent of the accretion rate parameter Sa in a neutron-star low-mass X-ray binary.

Findings

No mHz QPOs in the hard spectral state.

Absolute RMS amplitude is insensitive to Sa.

Rapid transition from marginally to stable burning inferred.

Abstract

We investigate the frequency and amplitude of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary (NS LMXB) 4U 1636-53 using Rossi X-ray Timing Explorer observations. We find that no mHz QPOs appear when the source is in the hard spectral state. We also find that there is no significant correlation between the frequency and the fractional rms amplitude of the mHz QPOs. Notwithstanding, for the first time, we find that the absolute RMS amplitude of the mHz QPOs is insensitive to the parameter Sa, which measures the position of the source in the colour-colour diagram and is usually assumed to be an increasing function of mass accretion rate. This finding indicates that the transition from marginally stable burning to stable burning or unstable burning could happen very rapidly since, before the transition, the mHz QPOs do not gradually decay as the rate further changes.