Irregular variability related to accretion discs in X-ray binaries

In this thesis I describe observational and analytical work on several X-ray binaries which are in quite different categories: X-ray transient, LMXBs, and HMXB X-ray pulsars. They all exhibit irregular variability in their X-ray and/or optical fluxes on time scales from seconds to months. In particular, with observations from both ground and space, we have found long-term variations in a number of luminous X-ray sources that cannot be orbital in nature. That these are present in such a wide variety of objects, indicates that must therefore be an inherent property of the accretion discs. 4U 1636-536 has been a persistent LMXB over at least ~ 40 years. However, it has been gradually declining in X-rays, whilst undergoing short outburst cycles. GX 339-4 is a black hole X-ray transient which has exhibited many outbursts in the last decades. I investigated the relationship between its X-ray flux and its optical counterpart during its previous X-ray outburst in 2002. Optical photometric and spectroscopic data were obtained to solve the LMC X-2’s binary dynamics and our optical data also show that the system has an unstable long term variation. SMC X-1 has shown a correlation between the quadratic fit to the pulse periods and the third period variation in the RXTE/ASM and CGRO/BATSE light curves. It is therefore very important to investigate this phenomenon further, as it is related to the physical interaction between the inner disc and pulsar.