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We investigate the frequency and amplitude of the millihertz quasi-periodicoscillations (mHz QPOs) in the neutron-star low-mass X-ray binary (NS LMXB) 4U1636-53 using Rossi X-ray Timing Explorer observations. We find that no mHzQPOs appear when the source is in the hard spectral state. We also find thatthere is no significant correlation between the frequency and the fractionalrms amplitude of the mHz QPOs. Notwithstanding, for the first time, we findthat the absolute RMS amplitude of the mHz QPOs is insensitive to the parameterSa, which measures the position of the source in the colour-colour diagram andis usually assumed to be an increasing function of mass accretion rate. Thisfinding indicates that the transition from marginally stable burning to stableburning or unstable burning could happen very rapidly since, before thetransition, the mHz QPOs do not gradually decay as the rate further changes.

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