The drop of the coherence of the lower kHz QPOs is also observed in XTE J1701-462

Abstract

We investigate the quality factor and RMS amplitude of the lower kHz QPOs from XTE J1701-462, a unique X-ray source which was observed in both the so-called Z and atoll states. Correcting for the frequency drift of the QPO, we show that, as in all sources for which such a correction can be applied, the quality factor and RMS amplitude drops sharply above above a critical frequency. For XTE J1701-462 this frequency is estimated to be ~800 Hz, where the quality factor reaches a maximum of ~200 (e.g. a value consistent with the one observed from more classical systems, such as 4U~1636-536). Such a drop has been interpreted as the signature of the innermost stable circular orbit, and that interpretation is consistent with the observations we report here. The kHz QPOs in the Z state are much less coherent and lower amplitude than they are in the atoll state. We argue that the change of the QPO properties between the two source states is related to the change of the scale height of the accretion disk; a prediction of the toy model proposed by barret et al. (2007). As a by-product of our analysis, we also increased the significance of the upper kHz QPO detected in the atoll phase up to 4.8 sigma (single trial significance), and show that the frequency separation (266.5+/-13.1 Hz) is comparable with the one measured from simultaneous twin QPOs the Z phase.

Accepted for publication in the Astrophysical Journal

References in corpus (5)

• RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs
• The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars
• On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries
• Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U1636-536
• Type I X-ray Bursts from the Neutron-star Transient XTE J1701-462

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• High-frequency variability in neutron-star low-mass X-ray binaries
• Statistical properties of twin kHz QPO in neutron star LMXBs
• Highly coherent kilohertz quasi-periodic oscillations from a neutron star X-ray binary EXO 1745-248
• Evidences of an innermost stable bound orbit predicted by general relativity from the amplitude of the twin-peak quasi-periodic oscillations
• The kHz QPOs of Neutron Stars and Millisecond Pulsars and Implications
• Oscillations of the Boundary Layer and High-frequency QPOs
• Lags of the KiloHertz Quasi-Periodic Oscillations in the transient source XTE J1701$-$462