XMM-Newton and NICER measurement of the rms spectrum of the millihertz quasi-periodic oscillations in the neutron-star low-mass X-ray binary 4U 1636-53

TL;DR

This study measures the energy-dependent rms amplitude of mHz QPOs in neutron-star binary 4U 1636-53, revealing a novel increasing trend below 3 keV, challenging existing models and suggesting a possible nuclear burning origin.

Contribution

First measurement of mHz QPO rms amplitude versus energy down to 0.2 keV in 4U 1636-53, revealing a new increasing trend below 3 keV not predicted by current models.

Findings

Rms amplitude increases from 0.2 to 3 keV

Contradicts previous decreasing trend above 3 keV

Provides observational evidence for nuclear burning hypothesis

Abstract

We used two XMM-Newton and six Neutron Star Interior Composition Explorer (NICER) observations to investigate the fractional rms amplitude of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636-53. We studied, for the first time, the fractional rms amplitude of the mHz QPOs vs. energy in 4U 1636-53 down to 0.2 keV. We find that, as the energy increases from 0.2 keV to 3 keV, the rms amplitude of the mHz QPOs increases, different from the decreasing trend that has been previously observed above 3 keV. This finding has not yet been predicted by any current theoretical model, however, it provides an important observational feature to speculate whether a newly discovered mHz oscillation originates from the marginally stable nuclear burning process on the neutron star surface.