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

ABSTRA C T 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.

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