Open AccessEvidence of a decrease of kHz quasi‐periodic oscillation peak separation towards low frequencies in 4U 1728–34 (GX 354–0)
Author(s) -
Migliari S.,
Van Der Klis M.,
Fender R. P.
Publication year - 2003
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2003.07186.x
Subject(s) - physics , oscillation (cell signaling) , astrophysics , quasi periodic , centroid , low frequency , astronomy , genetics , artificial intelligence , computer science , biology
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.