We report the discovery with the RXTE/PCA of twin kHz QPOs in the peculiar X-ray binary Circinus X-1 .
At eleven different epochs we observed two simultaneous kHz QPOs with centroid frequencies of up to \sim 225 and \sim 500 Hz and significances of up to 6.3 and 5.5 \sigma for the lower and upper kHz QPO , respectively .
The frequency range over which the twin kHz QPOs are seen is for the most part well below that of other sources , and extends down to centroid frequencies of \sim 56 Hz and \sim 230 Hz respectively for the lower and simultaneously observed upper kHz QPO .
The strongly variable QPO frequencies and their observed correlations clearly indicate that the twin peaks are the kHz QPOs such as typically seen from low-magnetic field neutron stars , and not black-hole high-frequency QPOs .
This confirms that Cir X-1 is a neutron star , as suspected since the detection of Type-I X-ray bursts from the field of the source 20 years ago .
The kHz QPO peak separation varies over the largest range yet seen , \sim 175–340 Hz , and increases as a function of kHz QPO frequency .
This is contrary to what has been observed in other sources but in good agreement with predictions of the relativistic precession model and Alfvén wave models at these low QPO frequencies .
Current beat-frequency models would require further modification in order to accommodate this .
A total of 67 observations showed only a lower kHz QPO ; this QPO can be followed down all the way to a centroid frequency of \sim 12 Hz ( Q \sim 0.5 ) .
Two observations showed only an upper kHz QPO near 450 Hz .
In addition , a strong low-frequency quasi-periodic oscillation ( LF QPO ) is seen in the 1–30 Hz range , as well as two components above and below this QPO .
The frequency-frequency correlations between the kHz QPOs and the LF QPO are in good agreement with those found previously in Z sources , confirming that Cir X-1 may be a peculiar Z source , and extend these correlations to frequencies a factor 1.5–2.3 lower .
We suggest that the low frequency range over which the kHz QPOs occur in Cir X-1 and to a lesser extent in ( other ) Z sources , might be due to a stronger radial accretion flow relative to the disk flow than in other kHz QPO sources , possibly related to the nature of the companion star .