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This work explores the connection between the spectral properties of the X-ray emission produced by black hole X-ray binaries (BHXRBs) and the variability properties of these sources that are embedded in lightcurves. While BHXRBs are mostly studied using ‘static observables‘ such as the average energy spectrum, a variability timescale-resolved study (in terms of Fourier-frequencies) provides a much stronger and more reliable tool for understanding the intricate physics and geometrical properties of accretion flows around stellar-mass black holes. In the present Thesis, we first show how the geometrical properties of the accretion disc can be mapped by combining spectra and frequency-resolved time-lags. A time-scale-dependent reflection model is developed in terms of Fourier-frequencies and compared to observational hard-state data of GX 339–4 obtainedwithXMM-Newton. Secondly, we explore the variability properties of the accretion disc in SWIFT J1753.5–0127 using a combination of novel spectraltiming methods and the soft X-ray coverage of XMM-Newton. Finally, we show the development of a novel technique for fitting correlated signals in the Fourier domain as a function of energy and Fourier-frequency that can prove particularly useful with high signal-to-noise datasets fromfuture X-ray missions.

This thesis presents new spectroscopic observations of the donor star in several High Mass X-ray Binary systems. The various types of X-ray Binary systems that exist are outlined and the methods and limitations of mass determination of the stellar components are discussed. The masses of the compact objects are determined for the systems SMC X-1 and LMC X-3. In both cases the heating of the donor star by the X-ray source is taken into account and its importance demonstrated. In the case of SMC X-1, we determine the radial velocity semi-amplitude of Sk 160 to be 24.7 ± 1.9 km s-1. Assuming Sk 160 fills its Roche-lobe we obtain upper limits for the mass of the neutron star as Mx = 1.36 ± 0.10, and by assuming that the system is seen edge on we obtain lower limits for the masses of the neutron star as Mx = 1.05 ± 0.09 M⊙. However, based on the model predictions of Timmes et al. (1996), it is concluded that the mass lies in the upper end of the heating corrected mass determinations. In the case of LMC X-3 the spectral type of the donor star is observed to change with phase due to irradiation by the X-ray source. We find the spectral type is likely to be B5V, and only appears as B3V when viewing the heated side of the donor. Combining our measurements with those previously published, and taking into account the effects of X-ray irradiation, results in a value for the donor star radial velocity semi-amplitude of Ko = 256.7±4.9 km s-1. Assuming an inclination of 50° ≤ i ≤ 70° we find the mass of the black hole lies in the range 9.5 ± 1 M⊙ ≤ Mx ≤ 13.2 ± 1 M⊙. The idea of using these mass determination techniques as a tool for detecting potential black hole/Be star XRB systems is presented and the radial velocities for seven such systems are determined. Of these, one system, LS 4356 shows a radial velocity variation large enough (i.e. ~ 95 km s-1) to indicate the presence of a black hole. If the compact companion is confirmed as a black hole, it will make LS 4356 the first known black hole/Be XRB. Finally the emission lines observed in the spectra of two Be XRBs, X Per and A0535+26 are analysed and compared to the previous studies of Clark et al. (2001) and Grundstrom et al. (2007). The Hα emission line profiles are discussed in terms of their evolution with time and the changing state of the circumstellar disc, thereby confirming and adding to the long term data set. The emission line profiles of the other spectral lines, namely Hβ, H-γ, He I 5876, He I 6678 and He I 7065, are presented and compared with that of Hα.

This thesis presents results on fast transient gamma-ray sources detected and studied with INTEGRAL. Fast transient events occur at unpredictable locations and times and it is very difficult to observe them using traditional narrow field of view (FOV) X-ray instruments. The IBIS instrument onboard INTEGRAL is particularly suited to the detection of fast transient events thanks to its large FOV and good sensitivity. As result of a detailed study of a 3 year dataset of IBIS observations, designed to search for fast transient sources, this thesis shows that INTEGRAL is playing a key role in unveiling a new class of Supergiant High Mass X-ray Binaries (SGXBs) which have been named as Supergiant Fast X-ray Transients (SFXTs). Most of the time they are undetectable, with quiescent X-ray luminosities ≤10³²-10³³ erg s⁻¹. Then occasionally they undergo fast X-ray transient activity lasting less than a day, typically a few hours, reaching peak luminosities of ~ 10³⁶-10³⁷ erg s⁻¹. Their outbursts show complex time structures, being characterized by several fast flares with a duration of typically a few tens of minutes. This kind of X-ray behaviour is very different from that seen in classical SGXBs, which are known to be persistent bright X-ray sources with X-ray luminosities always detectable in the range 10³⁶-10³⁸ erg s⁻¹. INTEGRAL results on 8 firm SFXTs as well as 6 candidates are presented in chapter 4 and chapter 5, respectively. The number of SFXTs discovered in only a few years is already comparable to that of persistent classical SGXBs discovered in ~ 40 years of X-ray astronomy. The class of SFXTs could be much larger than the 14 objects reported in this thesis. An entire population of still undetected SFXTs could be hidden in the Galaxy.

Apart from their intrinsic interest as high energy radiation sources, high mass X-ray binaries are integral to the understanding of binary evolution and star formation. By studying them as individual sources at high energies we broaden our understanding of the complex physical processes at work in these systems. Detailed X-ray spectral analysis undertaken in this work reveals the presence of cyclotron features in two sources, enabling a direct measurement of the neutron star magnetic fields. γ-ray timing and spectral analysis of high energy sources in the Small Magellanic Cloud detects a transient X-ray pulsar and provides a measurement of the accretion-induced spin-up of SMC X-1. Another aspect of this work studies the collective properties of Be/X-ray binaries as a population in the Small Magellanic Cloud. This work provides spectral identification of 27 Be/X-ray binary counterparts in the Small Magellanic Cloud and discusses the results in the context of star formation and binary evolution in a low metallicity environment.

This thesis presents the results of monitoring of 14 Be/X-ray binary systems in the Galaxy over ~9 years with the BATSE instrument on-board CGRO, and 46 in the SMC over ~8 years using PCA-RXTEdata from a weekly survey program. The reduction pipeline for these data we perfected and new code was written to analyse the products. A technique is presented for subtracting the contribution of pulsations to a light curve, enabling the cleaning of power spectra with many sources and the precise measurement of power for all the harmonic components of a pulsar's profile. application of these techniques has permitted the production of pulse amplitude light curves for the SMC systems, timing analysis of which has returned a large number of orbital ephemeredes, which in turn has allowed for a better estimate of the spin/orbit relationship parameters. Two new quantities are defined for transient outburst systems: δorb is the difference between the predicted and actual orbital periods expressed as a percentage, and is found to be inversely proportional to the magnetic fields of the pulsars in the SMC, but no such relation is seen for the Galactic systems. Xod is the number of outbursts per orbit, which we find to be strongly correlated with the orbital period for both galaxies. Based on the distribution of Xod we propose a classification scheme that would categorize Be/X-ray binary systems as Class Ia, Ib or II. A dependence of the X-ray luminosity on Xod is also found for SMC systems, but is weaker for Galactic ones. We present 21 possible new pulsars in the SMC for which orbital ephemerides have also been calculated. A multi-set technique for folding light curves is discussed and used to create pulse profiles for all the SMC pulsars, qualitatively showing that in most cases both fan and pencil beams are required together in order to explain the observed shapes. Lastly, two pulsars: SXP46.4 and SXP165 were found to be misidentifications of SXP46.6 and SXP169 respectively.

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.

Cataclysmic variable stars (CVs) are binary systems in which matter is transferred from a low mass star to a white dwarf via an accretion disc. My thesis is a spectroscopic study of three of these objects: U Gem, GD 552 and GY Cnc. I present high-resolution optical spectra of U Gem taken during quiescent. For U Gem, the radial velocity semi-amplitude of the white dwarf, K₁, is accurately known thanks to a direct observation by Long et al. (1999), I find that even with these data the optical measurements are seriously distorted compared to the known value, which is not recovered to better than ˜ 20%. Doppler tomograms show emission at low velocity, close to the centre of mass, and a transient and sharp absorption feature is seen in the Balmer lines close to eclipse. I suggest that stellar prominences may explain part of these features. I study two features detected in He II4686.75 Å. They seem to be produced in the bright spot. The narrower feature has a velocity close to that of the accretion disc in the impact region. I present evidence of weak spiral structure, which may support explanations for "spiral shocks " based upon 3-body effects. I apply a method of isophote fitting to search for evidence of stream-disc overflow, but fail to uncover any. I detect evidence of irradiation of the mass donor with shielding by the disc: I estimate an H/R ratio between 0.15 and 0.20. For GD 552 I present spectroscopy taken with the aim of detecting emission from the mass donor. I fail to do so at a level which allows me to rule out the presence of a near-main-sequence star donor. Given GD 552's orbital period of 103 minutes, this suggests instead that it may be a system that has evolved through the ˜ 80-minute orbital period minimum of CVs and now has a brown dwarf mass donor. Finally, I give a first look at high-resolution data for GY Cnc whose dynamical parameters make it a near-perfect twin of U Gem. I find several surprising features: the bright spot is completely absent from the Balmer lines, although visible in other lines; emission from the secondary star seems to arise from the whole of its Roche lobe; and low velocity emission is detected near the centre of mass as in U Gem. I argue that GY Cnc provides further evidence of the presence of prominence-like structures on CVs.

The disc instability model successfully explains the outbursts of dwarf novae, but modifications are required to account for the differences observed in the outbursts of Low Mass X-ray Binaries. I investigate the effects of X-ray irradiation on the accretion disc during outburst and find that the shape of the decaying lightcurve can be simply accounted for. Fitting the decay provides a new way to determine system parameters based on the accretion disc radius, supplementing existing methods. The effects of X-rays in the period following outburst allow echo outbursts to occur on timescales much longer than those of dwarf novae, as observed. In developing this work I determine a new form for the surface density profile of the disc during the decay from outburst. I examine observations of XTE J 1118+480 taken at UKIRT to find the magnitude and spectrum of the system in its near-quiescent state. From the photometry I determine a new, more accurate orbital period. In the spectrum produced hydrogen and helium emission lines from the disc are apparent. I also searched for the optical counterparts of two newly detected X-ray sources. Although no strong candidate was found for either, a list of less probable candidates is provided for each star.