A model for upper kHz QPO coherence of accreting neutron star

We investigate the coherence of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to the radial extent of the kHz QPO emission region associated with the transitional layer at the magnetosphere-disc boundary, which corresponds to the coherence of upper kHz QPO. The theoretical model for the Q-factor of upper kHz QPO is applied to the observational data of five Atoll and five Z sources, and the consistence is implied.