  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.
