Abstract
In this study, we examine the long-term torque noise fluctuations of persistent X-ray binaries Her X-1, Vela X-1, GX 301-2, Cen X-3, 4U 1538-53, OAO 1657-415, and 4U 1626-67 using the historical pulse frequency measurements provided by CGRO/BATSE and Fermi/GBM. We find that known disc-fed sources exhibit a 1/omega(2) red noise component in their power density spectra which is saturated over long time-scales. On the other hand, wind-fed sources form a clear white noise component, and the wind-fed sources with occasional transient disc formation imprint 1/omega type flicker noise. We discuss their long-term timing noise properties based on the models to describe the power density spectrum of pulse frequency derivative fluctuations in terms of monochromatic and coloured noise processes. Furthermore, we investigate the relation between measured timing noise strengths and other independently measured physical parameters. Despite the low number of sample sources, we suggest that the noise strengths of these sources are correlated with their luminosities and uncorrelated with their magnetic field strengths, implying that the dominant noise-generating mechanism is accretion.