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    QPO frequency derivative - frequency correlation indicates non-Keplerian boundary layer with a maximum in rotation rate

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    The correlation between the frequency and the absolute value of the frequency derivative of the kilohertz QPOs observed for the first time from 4U 1636-53 is a simple consequence and indicator of the existence of a non-Keplerian rotation rate in the accretion disk boundary layer. This paper interprets the observed correlation, showing that the observations provide strong evidence in support of the fundamental assumption of disk accretion models around slow rotators, that the boundary layer matches the Keplerian disk to the neutron star magnetosphere.Comment: Accepted for publication in MNRAS Letter