Low- and high-frequency variability as a function of spectral properties in the bright X-ray binary GX 5–1.

We report on a detailed analysis of data obtained over nearly four years with the Rossi X-ray Timing Explorer of the Z source GX 5–1. From a spectral analysis using a hardness–intensity diagram it was found that the source traced out the typical Z-shaped pattern. The study of the power spectral properties showed that when the source moved on the horizontal branch towards the normal branch the fractional rms amplitudes and time-scales of all variability decreased, while their FWHMs increased. The frequency separation of the two kHz QPO peaks decreased from 344±12 to 232±13 Hz , while the frequency of the lower and upper kHz QPO increased from 172±10 to 608±6 Hz and from 516±10 to 840±12 Hz , respectively. At low frequencies, besides the horizontal branch oscillation (HBO) and its second harmonic, two additional broad Lorentzian components were needed to obtain acceptable fits. These broad Lorentzians have Q-values of ∼1–2 and frequencies 0.5 and 1.5 times the HBO frequency. When interpreted as being related to the HBO, they seem to favour disc models for the HBO over the magnetic beat-frequency model. The frequency of the normal branch oscillations changed slightly and non-monotonically, while on the normal branch between ∼6 Hz at both ends and 5.25±0.05 Hz near the middle of the branch. It evolved into a flat-topped noise component on the flaring branch. We compared the timing properties of the some of the Z sources. We also compare the timing properties and colour–colour diagrams (CDs) of GX 5–1 with those of the back hole candidate XTE J1550–564 and the atoll source 4U 1608–52 . The CDs are strikingly similar when a colour scheme commonly employed in back hole studies is used. However, this may be a degeneracy as the CDs turn out to be more complicated when colours common in neutron star studies are employed. Apart from some remarkable similarities between the CD of XTE J1550–564 and that of 4U 1608–52 , several differences can be seen between these CDs and that of GX 5–1. Conclusions on spectral states or properties based solely on the use of CDs using the ‘black hole scheme’ should be regarded with caution. [ABSTRACT FROM AUTHOR]