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تسجيل مستخدم جديدThe kilohertz quasi-periodic oscillations during the Z and atoll phases of the unique transient XTE J1701--462
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A. Sanna
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We analysed 866 observations of the neutron-star low-mass X-ray binary XTE J1701-462 during its 2006-2007 outburst. XTE J1701-462 is the only example so far of a source that during an outburst showed, beyond any doubt, spectral and timing characteristics both of the Z and atoll type. We found that the lower kHz QPO in the atoll phase has a significantly higher coherence and fractional rms amplitude than any of the kHz QPOs seen during the Z phase, and that in the same frequency range, atoll lower kHz QPOs show coherence and fractional rms amplitude, respectively, 2 and 3 times larger than the Z kHz QPOs. Out of the 707 observations in the Z phase, there is no single observation in which the kHz QPOs have a coherence or rms amplitude similar to those seen when XTE J1701-462 was in the atoll phase, even though the total exposure time was about 5 times longer in the Z than in the atoll phase. Since it is observed in the same source, the difference in QPO coherence and rms amplitude between the Z and atoll phase cannot be due to neutron-star mass, magnetic field, spin, inclination of the accretion disk, etc. If the QPO frequency is a function of the radius in the accretion disk in which it is produced, our results suggest that in XTE J1701-462 the coherence and rms amplitude are not uniquely related to this radius. Here we argue that this difference is instead due to a change in the properties of the accretion flow around the neutron star. Regardless of the precise mechanism, our result shows that effects other than the geometry of space time around the neutron star have a strong influence on the coherence and rms amplitude of the kHz QPOs, and therefore the coherence and rms amplitude of the kHz QPOs cannot be simply used to deduce the existence of the innermost stable circular orbit around a neutron star.
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The neutron-star X-ray transient XTE J1701-462 was observed for $sim$3 Ms with xte during its 2006-2007 outburst. Here we report on the discovery of three type-I X-ray bursts from XTE J1701-462. They occurred as the source was in transition from the
typical Z-source behavior to the typical atoll-source behavior, at $sim10%$ of the Eddington luminosity. The first burst was detected in the Z-source flaring branch; the second in the vertex between the flaring and normal branches; and the third in the atoll-source soft state. The detection of the burst in the flaring branch cast doubts on earlier speculations that the flaring branch is due to unstable nuclear burning of accreted matter. The last two of the three bursts show photospheric radius expansion, from which we estimate the distance to the source to be 8.8 kpc with a 15% uncertainty. No significant burst oscillations in the range 30 to 4000 Hz were found during these three bursts.
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 so
urces 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.
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