Pulse Frequency Fluctuations of Persistent Accretion Powered Pulsars

In this study, we examine the long term torque noise fluctuations of persistent Xray 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 disk-fed sources exhibit $1/{\omega }^2$ red noise component in their power density spectra which is saturated over long timescales. On the other hand, wind-fed sources form a clear white noise component and the wind-fed sources with occasional transient disk 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 colored 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 noise strengths of these sources are correlated with their luminosities and uncorrelated with their magnetic fields strengths, implying that the dominant noise generating mechanism is accretion