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

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 �ν = 274 ± 11 Hz, significantly smaller than the constant value of �ν ∼ 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.