Evidence of a decrease of kHz quasi-periodic oscillation peak separation towards low frequencies in 4U 1728–34 (GX 354–0) |
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| Authors: | S Migliari M van der Klis R P Fender |
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| Institution: | Astronomical Institute 'Anton Pannekoek', Universityof Amsterdam, and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ, Amsterdam, the Netherlands |
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| Abstract: | We have produced the colour–colour diagram of all the observations of 4U 1728–34 available in the Rossi X-ray Timing Explorer public archive (from 1996 to 2002) and found observations filling in a previously reported 'gap' between the island and the banana X-ray states. We have made timing analysis of these gap observations and found, in one observation, two simultaneous kHz quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit in the overall trend of the source. The 'lower' kHz QPO has a centroid frequency of ～308 Hz. This is the lowest 'lower' kHz QPO frequency ever observed in 4U 1728–34. The peak frequency separation between the 'upper' and the 'lower' kHz QPO is Δν= 274 ± 11 Hz , significantly smaller than the constant value of Δν～ 350 Hz found when the 'lower' kHz QPO frequency is between ～500 and 800 Hz. This is the first indication in this source for a significant decrease of kHz QPO peak separation towards low frequencies. We compare the result briefly to theoretical models for kHz QPO production. |
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| Keywords: | accretion accretion discs binaries: close stars: individual: 4U 1728 34 stars: neutron |
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