Discovery of twin kHz quasi-periodic oscillations (QPOs) in Cir X-1

ATel #695; S. Boutloukos (U. Tubingen/U. Amsterdam), R. Wijnands, M. van der Klis (U. Amsterdam)
on 18 Jan 2006; 16:12 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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We report the discovery with the RXTE/PCA of twin kHz QPOs in the low mass X-ray binary Circinus X-1. Out of 497 observations in the RXTE Public data base, using standard FFT power spectral techniques we selected the 119 observations that show clear low-frequency QPOs in the 1-50 Hz range. In 80 of these power spectra we detect the high frequency QPO that was previously reported (Tennant 1987 MNRAS 226, 971; Shirey et al. 1996 ApJ 469, L21; Psaltis et al. 1999, ApJ 520, 262), but up to higher frequencies (~280 Hz). In 11 of these 80 observations we find a second kHz QPO, at significance levels between 2.5 and 5.5 sigma. In all 11 cases this second QPO is at a frequency higher than the first one. The Q values of these twin QPOs are generally between 1 and 2 and their rms amplitudes between 1.25 and 4% (2-60 keV). The two QPO centroid frequencies vary together between ~55 and ~230 Hz for the lower frequency QPO and ~230 and ~500 Hz for the higher frequency QPO; the separation between the peaks varies between ~175 and ~340 Hz, generally increasing with the QPO frequencies. The relation between the two QPO frequencies is in very good accordance with that found for the kHz QPOs in low mass X-ray binaries containing a low magnetic field neutron star (e.g. van der Klis 2004, astro-ph/0410551), slightly overlapping with that of the Z sources and extending it to much lower frequencies. We conclude that we have detected twin kHz QPOs in Circinus X-1, with the immediate inference that the system contains a low-magnetic field neutron star, as previously suspected based on the detection with EXOSAT of type I X-ray bursts from the field of the source (Tennant et al. 1986, MNRAS 221, 27). We note that the QPOs occur in a low range of frequencies not previously covered by any kHz QPO pair, and that, instead of the usual decrease, we observe the peak separation to increase with QPO frequency.