KHz QPOS AND THEIR RELATIONS TO SPIN FREQUENCIES OF NEUTRON STARS

We give a brief review on the neutron star spin frequency and its relation to the kHz QPOs. A simple power law relation can be applicable for the Atoll and Z sources with the slightly different power index, e.g. ν1 ~ 700(Hz)(ν2/1000)^1.8; However, for the two accretion induced millisecond X-ray pulsars, Sax J 1808.4-3658 and XTE 1807-294, their low values of twin kHz QPO separations make them different from the other QPO sources. In addition, there are not found the clear relation between a spin frequency and the twin kHz QPO separation, which seems to hint that the beat model for kHz QPOs confronts the difficulty. The kHz QPO separations are not constant, varied up and down with accretion rate, so the models that follow this tendency are the relativistic precession model by Stella & Vietri and the Alfvën wave oscillation model by Zhang. Averagely, the ratio of upper kHz QPO frequency to the lower one is very close to 1.5, then for Cir X-1 this ratio (~ 3) is unusual higher than those of the other sources.