Khz Qpo Pairs Expose the Neutron Star of Circinus X-1

We discovered kHz QPOs in 80 archived RXTE observations from the peculiar low-mass X-ray binary (LMXB) Circinus X-1. In 11 cases these appear in pairs in the frequency range of ∼230 Hz to ∼500 Hz for the upper kHz QPO and ∼56 Hz to ∼225 Hz for the lower kHz QPO. Their correlation with each other, which is similar to that of frequencies of kHz QPO pairs in other LMXBs containing a neutron star, and their variation by a factor two confirm that the central object is a neutron star. These are the lowest frequencies of kHz QPO pairs discovered so far and extend the above correlation over a frequency range of factor four. In this new frequency range the frequency difference of the two kHz QPOs increases monotonically by more than ∼170 Hz with increasing kHz QPO frequency, challenging theoretical models.