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kHz QPO pairs expose the neutron star of Circinus X-1

S. Boutloukos·M. van der Klis·D. Altamirano·M. Klein-Wolt·R. Wijnands
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Abstract

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.

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S. Boutloukos, M. van der Klis, D. Altamirano, M. Klein-Wolt, R. Wijnands. 2007-03-28. kHz QPO pairs expose the neutron star of Circinus X-1. https://doi.org/10.1142/9789812834300_0105

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