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.

中文翻译：

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XMM-牛顿和 NICER 测量中子星低质量 X 射线双星 4U 中毫赫兹准周期振荡的 Rms 谱 1636-53

我们使用两个 XMM-牛顿和六个中子星内部成分探测器 (NICER) 观测来研究中子星低质量 X 射线双星 4U 1636-53 中毫赫兹准周期振荡 (mHz QPO) 的分数均方根振幅. 我们首次研究了低至 0.2 keV 的 4U 1636-53 中 mHz QPO 的分数均方根振幅与能量的关系。我们发现，随着能量从 0.2 keV 增加到 3 keV，mHz QPO 的均方根幅度增加，这与之前在 3 keV 以上观察到的下降趋势不同。这一发现尚未被任何当前的理论模型预测，然而，它提供了一个重要的观测特征来推测新发现的 mHz 振荡是否源于中子星表面的边缘稳定核燃烧过程。