Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0)

Simone Migliari; Michiel van der Klis; Rob Fender

doi:10.1046/j.1365-8711.2003.07186.x

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arXiv · astro-ph/0309288

Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0)

Simone Migliari·Michiel van der Klis·Rob Fender

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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 274+/-11 Hz, significantly smaller than the constant value of ~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 briefly compare the result to theoretical models for kHz QPO production.

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Simone Migliari, Michiel van der Klis, Rob Fender. 2003-09-10. Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0). https://doi.org/10.1046/j.1365-8711.2003.07186.x

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