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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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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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MIGLIARI, Simone, VAN DER KLIS, M., FENDER, R. p.. Evidence of a decrease of kHz quasi-periodic oscillation peak separation towards low frequencies in 4U 1728-34 (GX 354-0). _Monthly Notices of the Royal Astronomical Society_. 2003. Vol. 345, núm. 3, pàgs. L35-L39. [consulta: 20 de gener de 2026]. ISSN: 0035-8711. [Disponible a: https://hdl.handle.net/2445/99391]