Includes abstract. Includes bibliographical references.

Bibliography: p. 243-248. / An intensive photometric study in the Johnson B band of ten β Cephei stars in the open cluster NGC 3293 is presented. High sampling rates of the stars' light curves allow the identification of many formerly unknown pulsation frequencies in these stars, by means of Fourier periodogram analysis. All of the stars are found to be multiperiodic, with up to five frequencies identified for individual stars. Physical parameters of the stars are determined from previous photometric results in the literature. These parameters are used to make a comparison of the observed frequencies with theoretical predictions, including the effects of rotation. The results imply a preference for first and second overtone quadrupole (l=2) pulsation in these stars, while a weak relationship between stellar mass and pulsation overtone is identified. One of the stars is identified as a member of an eclipsing binary system, only the second β Cephei star to be identified as such. Physical parameters of the system are determined. The effects of rapid rotation (specifically, the distortion of the stellar profile from a perfect sphere) on alternative methods of pulsation mode identification are investigated for a model β Cephei star. Ratios between light and velocity amplitudes show extreme differences between rotationally distorted and undistorted models, but these ratios show high sensitivity to other parameters as well. Thermally broadened spectral line profiles are calculated for the same model, for modes with l = 0 to 4 and equatorial rotation velocities up to 400 km/s. Neglect of rotational distortion is shown to lead to underestimation of true equatorial rotation velocities by as much as 253. This result confirms that observed B stars are probably rotating at break-up velocities. In summary, various aspects of the behaviour of the β Cephei stars in NGC 3293 in particular, and of β Cephei stars in general, including rapid rotators, are illuminated and augmented by this study.

I present an analysis of the dispersion measure (DM) variations for 68 pulsars. The observations were taken using six International LOFAR Stations in Europe over the period of 3.5 years (between June 2014 and November 2017) at the centre frequency of 150 MHz with 80 MHz of bandwidth. During this time each pulsar was observed on weekly basis resulting in an average of 160 observations per source. I show that, the variations of the DM measurements show various trends along the span of the observation: increasing or decreasing, and in some cases more changes from one trend to another. I perform the structure function analysis for each of observed pulsar included in the study, in order to check if the DM variations follow the Kolmogorov power spectrum which describes the turbulence structure of the interstellar medium (ISM). I find that for a number of pulsars results show consistency with the Kolmogorov distribution (e.g. PSRs J1913−0440 and J2157+4017) while other sources show significant difference (e.g. PSRs J0108+6608 and J0614+2229). I also obtain the DM derivatives (i.e. dDM/dt) for each pulsar, in order to examine the correlation between the DM and its derivative. The result of this correlation shows a best-fit with a square-root dependence of 0.6±0.2, which is comparable with the result that was previously obtained by Hobbs et al. (2004), who shows a dependence of square-root between the DM and its derivative; with a gradient of 0.57±0.09. Also, one of the major results of this study that, thanks to the timing analysis, allowed me to produce a new timing solution for three pulsars: PSRs J0613+3731, J0815+4611 and J1740+27. This study concludes in that: i) the DM variations can be used to understand the general properties of the ISM ii) the low-frequency observations can enable us to study the dispersion effect on pulsar signals, which can be very useful for the effort of the pulsar timing array (PTA) project iii) IISM studies using pulsar timing is a powerful technique requiring careful approach to data reduction and analysis due to characteristic of the pulsars.

Bibliography: leaves 75-82. / The purpose of this thesis is a discussion of the generation and influence of magnetic fields in gaseous disks. It builds mainly on the work of Vainstein and Ruzmaiken (1972) on the generation of galactic magnetic fields. Emphasis here is on application to accretion disks in the cataclysmic variables (CVs). These include novae, dwarf novae and nova-like variables. Disks are maintained by mass transferred from a Roche-lobe filling secondary star to a white dwarf. More information can be found in Warner (1976) and Robinson (1976). As is pointed out in the final chapter, the basic theory should have wider applicability.

Doppler tomography has revolutionised the interpretation of phase-resolved spectra of interacting binaries. The standard technique extracts emission and kinematic information contained in such spectra and projects it onto a two-dimensional velocity coordinate frame. This thesis describes my investigation of constructing Doppler tomograms in 'inside-out' coordinates by reversing the velocity axis. The aim was to determine if the emission distribution in the inside-out tomogram is more intuitive to interpret than the standard tomogram, and if it reveals indiscernible or enhances less discernible details. The inside-out tomogram is constructed by projecting the spectra onto the inside-out coordinate frame with zero velocity transposed to the outer circumference and the maximum velocities to the centre of the tomogram. In addition, this thesis describes a new flux modulation mapping technique applied to the standard and inside-out Doppler tomography of magnetic cataclysmic variables. I developed this technique with the aim to obtain more information from their observed spectra and present it in a useful format. It extracts the flux modulation from consecutive half-phase tomograms and constructs maps of the amplitude and phasing characteristics of the modulation in these systems. My investigation, involving the spectra of non-magnetic and magnetic cataclysmic variables, shows that the inside-out projection redistributes the relative contrast levels in and amongst the emission components. The inside-out projection exposes low-velocity emission details which are overly compacted and enhances high-velocity emission details which are overly tenuous in the standard projection. In addition, the flux modulation mapping technique gives a significant improvement in reproducing the input spectra adding more confidence in the interpretation of the modulation maps. Notable results were obtained for the polars where the blob-like low-velocity emission in their standard tomograms is more exposed in their inside-out tomograms, making it easier to distinguish between the ballistic and magnetic accretion flows that are evident in their trailed spectra. Also, for all the polars investigated the high-velocity magnetic accretion flows not seen in their standard tomograms are revealed in their inside-out tomograms. This extra information is extremely valuable to form a more complete picture of the emission components, broadening our knowledge of the accretion dynamics in these systems.

In this thesis we study a structure associated with the Perseus-Pisces Supercluster (PPS; cz ~ 6000 km s⁻¹), where it crosses the Zone of Avoidance (ZoA) at the Galactic longitude of ℓ ≈ 160ᵒ. This largely unexplored region contains the X-ray emitting 3C 129 cluster which hosts two strong radio sources with bent morphologies, thus indicative of a rich intra-cluster medium (ICM). Due to its low Galactic latitude of b = 0.27, where Galactic extinction is severe at optical wavelengths (AB = 1.8 − 8.0), the details of its galaxy population and dynamics had not been investigated in detail before. Additionally, the diverse and dynamic cosmic environments of the PPS with the embedded galaxy cluster make this region an ideal laboratory to study the effects on these environments on the galaxy properties and galaxy transformation processes therein. We used the Westerbork Synthesis Radio Telescope (WSRT) to blindly map this ZoA crossing of the PPS in the 21 cm HI-wavelength. This was conducted through 35 individual pointings observed for a total of 420 hours, covering a large area of about 9.6 sq.deg and a velocity range of approximately cz ~ 2000− 16000 km s⁻¹, thus mapping the immediate surrounding regions to enable investigations of the various cosmic environmental effects on the HI properties of galaxies. With the resulting spectral line data cubes of an angular resolution of 23" x 16" and a velocity resolution of 16.5 km s⁻¹ with a rms sensitivity of ~ 0.4 mJy/beam, we detect 211 galaxies within the entire surveyed volume. These new HI detections reveal a total of four distinct overdensities. Of these overdensities, the two major ones are located at the distance of the PPS at cz ~ 4000 − 8000 km s⁻¹ and behind it at cz ~ 8000 − 12000 km s−1. The galaxy overdensity associated with the PPS is used to demonstrate how this supercluster connects across the ZoA and confirms earlier indications of a filamentary connection between Perseus, Pisces and the A569 clusters through this region of the sky. Galaxies in the background of the PPS seem to be part of the CID15 structure that had been earlier predicted in the reconstructed density and velocity maps from the 2MASS Redshift Survey. We carry out a detailed census of the galaxy population in the 3C129 cluster by combining the HI-data of the gas-rich galaxies with the near-infrared (NIR) images of the gas-poor galaxies. The NIR galaxies are identified from the high resolution (0.2"/pix, seeing ~0.8") images of the UKIDSS Galactic Plane Survey. We obtain photometry in the J,H and K bands for about 9700 galaxies identified in the NIR images within the WSRT HI surveyed area. These measurements are used to derive the red-sequence of this cluster through the (J − K) vs K colour-magnitude diagram to identify the gas-poor cluster member candidates. Within the spatial extent of the cluster of radius ~ 1.7 Mpc, a total of 261 galaxies are identified as cluster members, with 23 detected in HI. An assessment of the morphologies of these galaxy members reveals a clear morphological segregation, with E and E/S0 galaxies dominating the inner regions of the 3C129 cluster and the late-type spirals found in the cluster outskirts. We also examine the richness of the cluster by comparing it to two well-known clusters at similar redshifts. One being a massive cluster in the Great Attractor region, namely the Norma cluster, also located in the ZoA but in the South, and the other being the Coma cluster. The comparison shows that the 3C129 cluster is quite rich as was suspected. The galaxy density in the core of the 3C129 cluster is similar to that in the Norma cluster and slightly less than in the Coma cluster. Furthermore, an assessment of the spatial distribution of galaxies in the core shows a slight asymmetry aligned with the irregular distribution of the X-ray emission, thus consistent with the results from the X-ray analysis which surmised that the 3C129 cluster seems to have undergone a merger and has not yet reached a dynamically relaxed state. In support of this we find a large substructure dominated by gas-rich galaxies North of the main cluster at a slightly higher recession velocity which could be falling into the cluster. All these findings seem to support a scenario where the 3C129 cluster is still growing through accretion of galaxies from the PPS filament. The wealth of HI data allows an analysis of the environmental effects on the galaxy properties within the WSRT surveyed volume. We first characterise the cosmic environments found within the two major overdensities as outlined by the HI-detected galaxies. This is conducted by perfoming tests to search for substructures. A total of four distinct substructures are found within the PPS ZoA overdensity (cz ~ 4000 − 8000 km s−1 ) and three in the background galaxy overdensity (cz ~ 8000 − 12000 km s−1). Our analysis of the HI properties of galaxies in these varying cosmic environments shows that highly disturbed HI-disks are found in larger and tightly bound groups, due to the higher incidence of tidal interactions between the galaxies. Furthermore, an indication of HI-gas deficiency is found in the core of the 3C129 cluster. We assess the cause of this deficiency through models of the ICM of the 3C129 cluster and find ram-pressure stripping to be the dominant gas removal process in this region. Additionally, highly HI-gas deficient galaxies are also found in the cluster outskirts. Most of these galaxies are located within galaxy groups where galaxy-galaxy interactions and mergers are prevalent, thus highlighting the importance of these processes in removing gas from galaxies in groups.

Gamma-ray binary stars are intriguing interacting stellar systems that exhibit emission across the whole electromagnetic spectrum. This thesis is focused on optical and gamma-ray emission of gamma-ray binary stars (GRBis). I have performed radial velocity measurements of 1FGL J1018.6-5856 to derive the orbital parameters of the system. In particular, I derive, for the first time, the orbital eccentricity which, together with other parameters, enables us to attain the orbital geometry of the system. I also use the derived parameters to put constraints on the mass of the compact object. For a large range of orbital inclination angles, a neutron star is favoured, while a black hole is allowed for very low inclination angles. I have also explored the long term variability of the Be disc in LS I +61 303. Using the measured parameters of the Hα emission line together with a semi-analytical model which describes the motions of particles in the disc, I examine long term geometric changes of the Be disc in LS I +61 303. I have explored different possible scenarios for the Be disc changes by studying the variability of the eccentricity and argument of pericenter on the superorbital timescale. One of the effects we have examined from the implication of our results is the Kozai Lidov mechanism, which has so far only been explored theoretically. I have explored the Bethe-Heitler mechanism for the production of gamma-ray emission, where energetic protons collide with stellar photons to produce energetic electron-positron pairs which upscatter photons to high energies. This mechanism has been neglected in studies of photo-hadronic processes due to the larger radiative efficiency of pion production. I demonstrate, however, that the BetheHeitler process can dominate the radiative output at lower proton energies where pairs can form but not pions. I calculate the spectrum and modulated flux in the context of the pulsar wind scenario, with photon-photon absorption and cascading effects taken into account. The effects of the ambient magnetic field around the massive star are also considered, which result in quasi-isotropic gamma-ray emission, and the orbital modulation of the flux resulting in photon-photon absorption. The work presented in this thesis encompasses different observational and theoretical aspects of gamma-ray binaries in which I use data and modeling to address some of the puzzling features of these systems. These include understanding the nature of the compact object, and thereby the physical mechanism driving the multiwavelength emission, the variability of the Be disc and its long term interaction with the compact object, and the origin of high energy emission. The results and methods presented have potential implications not only for understanding these objects, but also for other classes of interacting binaries.

Classical novae are transient astronomical events resulting from an eruption on the surface of an accreting white dwarf in interacting binary stars, namely cataclysmic variables. These eruptions and their progenitors are ideal astronomical events/objects to study a variety of interesting topics, such as mass-transfer mechanisms, binary evolution, and thermonuclear and radiative emission processes. Novae share some general observational characteristics during their eruption phases, yet each nova is new and can show unique light-curve and spectral developments. Therefore, carrying out detailed multiwavelength studies of individual novae is important to provide an holistic picture of the eruption development with the ultimate aim of a general understanding of the physical mechanisms involved in nova eruptions, as well as the structure and evolutionary condition of their progenitors. This dissertation presents detailed multiwavelength studies of three nova events, namely V5852 Sgr, SMCN 2016-10, and V407 Lup, as a series of chapters (papers). Our data were collected using several ground-based and space telescopes including: high- and mediumresolution optical spectroscopy from SALT, SOAR, and FLOYDS, optical and near-infrared photometry from IRSF, OGLE and SMARTS, UV and X-ray data from Swift, Chandra, and XMM-Newton. In the second chapter, we present an optical and infrared study of the unusual nova V5852 Sgr. This nova is a moderately fast nova showing features of the Fe II spectroscopic type. The light-curve development is unique, showing a combination of several light-curve classes. Estimates of the distance and measurements of the line velocities suggest that the nova might be associated with the Sagittarius stream. If so, it would be the first nova to be discovered in a dwarf spheroidal galaxy. The third chapter presents a multiwavelength study of the very fast nova SMCN 2016-10a. The fast light-curve development suggests that the system hosts a massive white dwarf (& 1.25 M), in good agreement with the high temperature of the super-soft X-ray emission and the turn-on/turn-off time of the super-soft state. At the distance of the Small Magellanic Cloud our measurements suggest that SMCN 2016-10a is the brightest nova in the Small Magellanic Cloud and one of the brightest novae on record, with an absolute maximum magnitude of ∼ −10.5 in the V -band. Chapter four is a multiwavelength study of nova V407 Lup. With a light-curve decline time t2 ≤ 2.9 d, this is one of the fastest known examples and the white dwarf is possibly more massive than 1.25 M. Our set of optical, UV, and X-ray data suggest that this system is an intermediate-polar cataclysmic variable based essentially on the presence of two periodicities (3.57 h and 565 s) in the light-curves. These periodicities are attributed to the orbital period of the binary and rotational period of the white dwarf. The late optical spectra (taken from day 165 post-eruption) show narrow and moving lines of He II and O VI, possibly associated with accretion regions within the binary system. This, along with the X-ray light-curves and spectra, suggest that the accretion probably resumed around 168 days post-eruption. These studies are modest, yet essential steps in the quest for a better understanding of nova eruptions. They also demonstrate the importance of multiwavelength follow-up of novae for constraining the physical parameters of the eruption, the ejecta, and the properties of the progenitor. In the era of large all-sky surveys, such as the All-Sky Automated Survey for Supernovae and eventually the Large Synoptic Survey Telescope which have been and are ex- pected to find a large number of optical transients and classical novae, similar multiwavelength follow-up will play a crucial role for initially identifying these transient events and further understanding their physical behaviour.

This thesis aims to test the reliability of the Wide-Field Infrared Space Explorer (WISE) star formation indicators centred at 12 and 22 µm. To accomplish this, the total infrared (TIR) luminosity was used as a calibrator to understand the behaviour of the two indicators. Our sample consists of Spitzer galaxies from the SINGS and KINGFISH surveys, with a select few excluded. The TIR calibration also necessitates that most of our galaxies have FIR data from Herschel. The photometric analysis done on the raw WISE and Spitzer images of our galaxies is explained, including how the isophotes and background sky level were determined for each band/image. Once the reliability of the WISE indicators was established, new star formation rate relations were formulated. The new relations are comparable to previous relations in the literature, but improve on them in stellar mass-, metallicity-, and luminosity range. We also find that metallicity of the galaxies has minimal effect on our relations. In our comparison of emission from PAH molecules, the two bands in question, IRAC4 and W 3 are strongly correlated. This may suggest that the W3 band is dominated by emission from the 11.3 µm PAH molecule, or it might be an effect of the band overlap between IRAC4 and W3.

The evolution of galaxies in the cluster environment is a complex process, with many outstanding questions. A wide range of galaxy morphologies, colours, sizes and luminosities are found in clusters, the least studied of which are the faint galaxy populations. Studying the faint end of the galaxy luminosity distribution may provide a valuable insight into the evolution of galaxies in cluster environments. The largest of these faint galaxies are classified as Ultra-Diffuse galaxies (UDGs). UDGs are low surface brightness galaxies with a very low stellar mass component, however they have sizes comparable to the Milky Way. These galaxies are hard to detect and classify as they are very faint. To survive in the cluster environments, where they have been observed, these galaxies must contain significant amounts of dark matter as the strong tidal fields would normally tear diffuse low-mass galaxies apart. The high abundance of UDGs in clusters has only recently been recognized, therefore identifying and measuring their properties is key to understanding how they are formed and continue to exist. In this thesis, I search for low surface brightness galaxies, spanning from typical dwarf galaxies to UDGs, in 16 low redshift (z< b/a >= 0.52. The number of faint galaxies in clusters follows a power-law with respect to the cluster halo mass, N ∝ M1.05±0.45, determined through bootstrap resampling. This shows that the number of UDG candidates increases as the cluster halo mass increases.