We investigate the quality factor and RMS amplitude of the lower kHz QPOs from XTE J1701–462 a unique X-ray source which was observed in both the so-called Z and atoll states .
Correcting for the frequency drift of the QPO , we show that , as in all sources for which such a correction can be applied , the quality factor and RMS amplitude drops sharply above above a critical frequency .
For XTE J1701–462 this frequency is estimated to be \sim 800 Hz , where the quality factor reaches a maximum of \sim 200 ( e.g .
a value consistent with the one observed from more classical systems , such as 4U 1636–536 ) .
Such a drop has been interpreted as the signature of the innermost stable circular orbit , and that interpretation is consistent with the observations we report here .
The kHz QPOs in the Z state are much less coherent and lower amplitude than they are in the atoll state .
We argue that the change of the QPO properties between the two source states is related to the change of the scale height of the accretion disk ; a prediction of the toy model proposed by Barret et al .
( 5 ) .
As a by-product of our analysis , we also increased the significance of the upper kHz QPO detected in the atoll phase up to 4.8 \sigma ( single trial significance ) , and show that the frequency separation ( 266.5 \pm 13.1 Hz ) is comparable with the one measured from simultaneous twin QPOs the Z phase .