Discovery of twin kHz QPOs in the peculiar X-ray binary Circinus X-1

We report the discovery with the RXTE PCA of twin kHz QPOs in the peculiar X-ray binary Circinus X-1. Eleven cases of simultaneous double QPOs occurred, with significances of up to 6.3 and 5.5 sigma and centroid frequencies ranging between approximately 56 and 225 and 230 and 500 Hz for the two QPO peaks, respectively, i.e., for the most part at frequencies well below those of other sources. The QPO properties clearly indicate that these double peaks are the kHz QPOs known from low magnetic field neutron stars, and not black hole high-frequency QPOs, confirming 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 a wide range, similar to 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 Alfven wave models; beat-frequency models require modification to accommodate this. In other observations, single kHz QPOs can be seen down to frequencies as low as similar to 12Hz, as well as a strong low-frequency quasi-periodic oscillation (LF QPO) between 1 and 30 Hz. The relations 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. 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.