Marginally stable nuclear burning triggered at different depths of the neutron star surface in Low-mass X-ray binary 4U 1608-52

TL;DR

This study analyzes mHz QPOs in neutron star binary 4U 1608-52, revealing how marginally stable nuclear burning occurs at varying depths, affecting the QPO properties and energy release, with implications for understanding neutron star surface processes.

Contribution

It provides the first observational evidence that marginally stable nuclear burning ignites at different depths depending on the spectral state of the source.

Findings

QPO frequency correlates with amplitude and burning layer temperature.

Flux modulation mainly caused by blackbody temperature variations.

Burning ignites at deeper layers as the source transitions from soft to transitional state.

Abstract

We investigated the timing and spectral properties of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1608-52 using NICER observations. Our analysis reveals a correlation between the QPO frequency and its absolute amplitude, as well as between the frequency and the temperature of the burning layer. Intensity-resolved spectral analysis indicates that the flux modulation of the mHz QPOs is primarily caused by the variations in the blackbody temperature in most observations. Furthermore, for the first time, we report that as the source evolves from the soft spectral state toward the transitional state, the marginally stable burning responsible for the mHz QPOs ignites at deeper layers of the neutron-star surface. The radiation flux associated with the mHz QPOs shows a decreasing trend as the source moves into the transitional state. These two findings support a scenario in which the marginally stable nuclear burning ignites at deeper layers as the temperature decreases, releasing less energy from the nuclear reaction. Finally, we determine that the energy release rate of the marginally stable burning is around 10$^{35}$ erg/s, consistent with the theoretical predictions.