**Publication date:** Dec 2012

**Abstract:**

Resonant Switch (RS) model of twin peak high-frequency quasi-periodic

oscillations (HF QPOs) assumes switch of twin oscillations at a resonant

point where frequencies of the upper and lower oscillations

νU and νL become 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 is used to determine range

of allowed values of spin a and mass M of the neutron star located in

the atoll source 4U 1636-53 where two resonant points are observed at

frequency ratios νU:νL=3:2, 5:4. We consider

the standard specific models of the twin oscillations based on the

orbital and epicyclic geodetical frequencies. The resonant points are

determined by the energy switch effect exhibited by the vanishing of the

amplitude difference of the upper and lower oscillations. The predicted

ranges of the neutron star parameters are strongly dependent on the twin

modes applied in the RS model. We demonstrate that for some of the

oscillatory modes used in the RS model the predicted parameters of the

neutron star are unacceptable. Among acceptable RS models the most

promising are those combining the Relativistic Precession and the Total

Precession frequency relations or their modifications.

**Authors:**

Stuchlík, Z.; Kotrlová, A.; Török, G.;