Resonant switch model of twin peak HF QPOs applied to the atoll source   4U 1636-53
  We present a Resonant Switch (RS) model of twin peak high-frequency quasi-periodic oscillations (HF QPOs), assuming switch of twin oscillations at a resonant point, where frequencies of the upper and lower oscillations $\nu_U$ and $\nu_L$ become to be commensurable and the twin oscillations change from one pair of the oscillating modes (corresponding to a specific model of HF QPOs) to some other pair due to non-linear resonant phenomena. The RS model enables to determine range of allowed values of spin $a$ and mass $M$ of the neutron star located at the atoll source 4U 1636-53 where two resonant points are observed at frequency ratios $\nu_U:\nu_L$ = 3:2, 5:4. 
Zdenek Stuchlik, Andrea Kotrlova and Gabriel Torok
astro-ph.HE