Understanding the physics of accretion onto black holes, and the role these cosmic engines play in shaping the universe, is one of the major challenges of contemporary astrophysics research. In this thesis I present several studies, aiming to better understand the variable emission we observe in accreting black holes. The focus of these studies are highly variable Narrow Line Seyfert 1 galaxies in the nearby universe. I start by summarising the current field of research in an introductory chapter. This is followed by a more technical chapter, outlining the analysis methods used in any subsequent chapters. This thesis contains five science chapters which are summarised below. I end with some concluding remarks and a discussion about the future prospects of the field. In chapter 3 we present a study of correlated UV and X-ray variability in the NGC 4051, where we found evidence for the reprocessing of X-ray photons in the disc. In chapter 4 we present a study of the time delays between a soft and hard X-ray band as a function of source flux in NGC 4051. We found a strong flux dependence on the time delays, and through analytical modelling we found reprocessing of the primary power law emission close to the central source. In chapter 5 we study the X-ray time delays in a sample of objects, and found a significant flux dependence in only one other source, Ark 564. In chapter 6 we study the energy and flux dependence of the X-ray timing properties in NGC 4051. A strong flux dependence on the lag energy is found and simple modelling performed. In chapter 7 we study the frequency and energy dependent lags in PG 1244+026, and found high frequency lags from both reflection and thermal reprocessing.

The prediction by van de Hulst, (1945) and Shklovsky (1949) and the subsequent discovery by Ewen and Purcell, Muller and Oort and Pawsey in 1951 of 21 cm emission from interstellar neutral hydrogen have opened up many new fields of research in astronomy. Half of the mass of our Galaxy, in t he form of neutral hydrogen and previously undetectable, is now accessible to detailed study. Moreover, since there is negligible interstellar obscuration at radio frequencies, even the most distant parts of the Galaxy are observable. An investigation of the Doppler displacement of the line enables the emission from neutral hydrogen to be mapped in a third dimension - depth.

The current state of the art concerning measurements of the Zeeman splitting of hydrogen line spectral features is described. An early claim, that a magnetic field of 2.5 x 10-5 Gauss had been deteoted in a cloud in the direction of Taurus A, is shown to be unfounded, The present evidence indicates that only an upper limit to the magnetic field within a cloud can be set. This is of the order of 5 x 10-6 Gauss. ~le experimental methods used, and the difficulties involved, in tryine to measure this Zeeman splitting are discussed in detail. The work done in determining more accurately the parameters of the clouds being searched for magnetic fields is outlined. Measurements of the linear polarization of radio sources at 2lcm are described and the earlier results are shown to have been subject to severe errors as a result of the method of analysis. The results of more accurate measurements are presented and compared with values obtained elsewhere. Rotation measure data, obtained elsewhere, is discussed in relation to the magnetic field existing within the galaxy. All the data mentioned above is finally combined in an attempt to obtain a picture of the magnetic field configuration in the solar neighbourhood.

The evidence for the existence of a magnetic field in the Galaxy is very briefly summarised, end the importance and principle of the measurement of the Zeeman effect in interstellar neutral hydrogen clouds is discussed. The results and limiting factors of the earlier Jodrell Bank Zeeman experiments (1959 to 1961) are revieved, and the equip rental, observational and analytical techniques used in the Fourth Zeeman experiment and associated continuum and polarisation measurements, (January 1962) are described in some detail. By combining the results of the first four experiments, significant results are obtained for the magnetic field components in three absorptior. features, enabling tentative conclusions to be drawn with regard to the magnitude and direction of the magnetic field in the vicinity of the Sun. It is shown that in order to obtain mole significant results, the receiver noise performance required improvement. The progress of the low noise work at Jodrell Bank is traced, and the expected improvement in the sensitivity of the Zeeman experiment when applied to various spectral features, is calculated. The continuum, polarisation and Zeeman observations of August/ September 1962, employing a parametric amplifier at the 'front end' of the receiver, are described in detail, and the general magnetic field in the 'Orion' and 'Perseus' spiral arms is estimated. It is shown that the observations indicate a magnetic field highly variable in magnitude and/or direction, with a net field ~ 5 x 10-6 oersted directed clockwise around the Galaxy when viewed from the North Galactic Pole. other measurements and estimates of the Galactic magnetic field are reviewed and compared with the Jodrell Bank measurements. The thesis concludes with a brief discussion of the possible further work required to elucidate the problem.

Interactions and mergers play an important role in the evolution of galaxy systems, with very few systems thought to exist that have not been shaped or formed through a collision or interaction with another galaxy. One of the main areas of contention in this field is the formation of elliptical galaxies, with one theory predicting that some of these systems form through the merger of two spiral galaxies. To test the validity of this theory, merging systems at different stages of evolution must be observed, to determine the nature of these interactions as these systems evolve. Here, we present a sample of merging galaxy systems, observed with the Chandra X-ray observatory. These systems have been ordered in a chronological sequence, to represent the evolution of a pair of merging galaxies. In this thesis, we first look at an early stage merger pair, charactering both the system's point source population and the nature of its hot diffuse gas. This galaxy pair is then included in the merger sample we compile; starting with systems in their first perigalactic passage, through to systems at the stage of nuclear coalescence, and we finally present relaxed merger remnant systems, with the oldest thought to be rv 3 Gyr after coalescence has taken place. In the final chapter, an analysis of the radio galaxy Centaurus A, as observed by Chandra, is presented. This galaxy is a system that has recently undergone a minor merger, and, as a consequence of this, has heavy dust lanes crossing the nucleus of the galaxy. In this work, the hot diffuse gas in the system is investigated, and compared to the complex regions of dust obscuration, therefore providing information on correlations between X-ray properties of the ISM and these optically extinct regions.

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