High Mass X-Ray Binary Stars (HMXBs) are stars that contain one early-type main sequence or giant star and one of a black hole, neutron star or white dwarf. HMXBs in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are instructive to study because both galaxies are metal poor in compari- son to the Milky Way and they are fairly transparent to both optical and X-ray radiation. This allows a more complete study of the whole population, without the biasing effects of gas and dust that occur in our own Galaxy. The objective of this study was to find the periods of HMXBs in the LMC and SMC with known optical counterparts in the dataset acquired by the Robotic Optical Transient Search Ex- periment telescope. Two possible orbital periods were found for the objects XTE J0055-724 and RX J0101.3-7211 of 1724 days and 478 days, respectively. Continued observations are recommended to conrm the two periods.

High Mass X-Ray Binary Stars (HMXBs) are stars that contain one early-type main sequence or giant star and one of a black hole, neutron star or white dwarf. HMXBs in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are instructive to study because both galaxies are metal poor in compari- son to the Milky Way and they are fairly transparent to both optical and X-ray radiation. This allows a more complete study of the whole population, without the biasing effects of gas and dust that occur in our own Galaxy. The objective of this study was to find the periods of HMXBs in the LMC and SMC with known optical counterparts in the dataset acquired by the Robotic Optical Transient Search Ex- periment telescope. Two possible orbital periods were found for the objects XTE J0055-724 and RX J0101.3-7211 of 1724 days and 478 days, respectively. Continued observations are recommended to conrm the two periods.

In this work present the broad-band noise structure in the 2-60 keV data of Cataclysmic Variables (CVs) with Anomalous X-Ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs). We analyzed Rossi X-ray Timing Explorer (RXTE) PCA data and derived time series from 27 CVs, 4 AXPs and 1 SGR using the RXTE archive. In general, CVs of different types all show broad band noise which can be fitted with power laws, using exponentional cut-offs, and Lorentzians in a similar way to power spectral (noise) characteristics of X-ray Binaries (XRBs). In general terms the power spectra show a power law index of (-)1.2-2. A rather large scale flattening of the power spectra exits in nonmagnetic systems in the low to very low frequency range. We observe that in low and high states/outbursts the noise in the high frequency range and low frequency range is changed. CVs show considerably low frequency noise. In addition, we recovered several possible QPOs in the X-ray wavelengths from CVs mainly from Intermediate Polar systems. AXP and SGR sources which are thought to be powered by either magnetic decay or accretion show band limited noise in their low frequencies. We also correlated their equal time interval noise characteristic with their burst states and discovered that in the two AXPs (1E 2259+586, 1E 1048.1-5937) noise correlates with their bursts.

In this thesis we analyse X-ray data of accretion powered low mass and high mass X-ray binaries to understand the nature of their accretion mechanisms by searching for some clues of viscous time-scales of their accretion discs, if they have, in their low frequency power density spectra created from their long-term X-ray observations, or by doing pulse timing analysis with much shorter X-ray data to detect the effects of torque fluctuations caused by the accreting material on the pulsar. The low mass and high mass X-ray binaries we analysed have breaks in their power density spectra, which are attributed to the role of viscosity in the formation of accretion discs. Although, the time-scales corresponding to these break frequencies are smaller than the predictions of the Standard theory of accretion discs, the sources give consistent results among themselves by displaying the expected correlation between their break and orbital frequencies. The correlation curve of LMXBs implies thicker appearing accretion discs than those assumed by the theory. The dichotomy of the HMXBs on this curve points out the different origins of accretion that these sources may have, and offers a way to distinguish the stellar-wind fed systems from the Roche-lobe overflow systems. The timing and spectral analysis of Swift J1626.6-5156 reveal a correlation between the spin-up rate and the luminosity of the source implying that the pulsar is accretion-powered. This correlation together with the characteristics of the X-ray spectra enables us to estimate the magnetic field and the distance of the source. The AXP 1E 2259+586 does not display any signs of viscous time-scale in its low frequency power density spectra, and its pulse timing analysis gives a much smaller torque noise value than that expected from accretion powered pulsars. In addition, the analysis results presented in this thesis reveal magnetar-like glitches which differ than those of radio pulsars, due to the presence of the strong magnetic field of the pulsar. These results eliminate the possibility that the AXP is an accretion-powered pulsar.

In the present thesis, “Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars”, the hydrodynamic effects, instabilities and superfluid turbulence in binary immiscible ultracold gases, and hydro-elastic macroscopic coupled modes and microscopic structure of the inner layers of the crust of neutron stars, are studied. The ultracold gas dynamics can be realized in the laboratory. The excitation modes of the inner crust determine a number of observable properties such as elasticity, thermal properties and mass transport properties. Here we focus on expanding the details, rather than repeating the results presented in the published articles. In the part of the thesis related to atomic ultracold gases, we utilize the physical parameters in the experimentally realizable parameter region. We numerically simulate the coupled non-linear Schrödinger equations, and calculate observable quantities, such as phase and modulus of the order parameter, conditions needed for observation of the Rayleigh-Taylor instability and for turbulence generation. The numerical calculations are accompanied by analytical description of the processes. The dispersion relation for capillary-gravitational waves at the interface between two ultracold gases, is derived straightforwardly from the superfluid Lagrangian. The equations of motion for centre-of-mass of the superfluids are derived, and then used in our model of the quantum swapping of immiscible superfluids pressed by a strong external force. By numerical simulation, we find that the Kelvin-Helmholtz instability which occurs at the non-linear stage of the Rayleigh-Taylor instability, can generate quantum turbulence with peculiar properties. We find that two-dimensional superfluid systems with weak inter-component repulsion are different from previously studied strongly repulsive binary superfluids, because the quantum Kelvin-Helmholtz instability in weakly repulsive superfluids rolls up the whole interface forming a vortex bundle, similarly to dynamics of the shear fluid layers in the classical hydrodynamics. Production of vortex bundles favours the Kolmogorov spectrum of turbulence, and we find that the Kolmogorov scaling indeed is present in a freely decaying turbulence. In the part of the thesis related to neutron stars, we study the inner crust of neutron stars, where the fully ionized atomic nuclei coexist with a superfluid of neutrons. The interaction between superfluid neutrons and the crystallized Coulomb plasma is due to the interaction between density perturbations (interaction of the scalar type), and between the current - the non-dissipative entrainment effect (interaction of the vector type). We calculate velocities of the collective modes of the crystal coupled to superfluid neutrons. As an input we use the results of microscopic nuclear calculations in the framework of the compressible liquid drop model (the Lattimer and Swesty equation of state), and more recent effective Thomas-Fermi calculations with shell corrections (N. Chamel, and the Brussels theoretical nuclear physics group). Knowledge of velocities as functions of the matter density in the inner crust is important for calculation of a number of dynamic and transport properties. The heat transport properties of the inner crust are directly observable in accreting binary systems (low-mass x-ray binaries). The mass transport properties of the inner crust are directly linked to the rotational evolution, being a key physical ingredient of the pulsar glitch phenomenon. The elastic properties are related to the vibrational modes of the star, and to the breaking stress of the crust. In the second part of our work on neutron stars we investigate the microscopic structure of the inner crust treating the structure as an anisotropic crystal coupled to s-wave superfluid neutron liquid. As we analyse dynamics of the elementary excitations at higher wavenumbers (smaller scales), we reach the edge of the first Brillouin zone. The Lattimer-Swesty data is applicable for wavenumbers much smaller than the edge of the first Brillouin zone. We extrapolate the data through the whole first Brillouin zone to calculate the fastest growth rate of the unstable modes. The crucial step is to calculate the mode velocities in anisotropic crystal incorporating both the induced neutron-proton interactions, and the electron screening properties. We find that the combined influence of these two effects leads to softening of the longitudinal phonon of the lattice above about the Thomas-Fermi screening wavenumber of the electrons. The critical wavenumber when the frequency becomes purely imaginary is about  1/5 - 2/3  of the reciprocal lattice vector, thus validating our assumption. The imaginary mode frequency implies instability at finite wavenumbers. Our calculations suggest that the mode at the first Brillouin zone edge is the most unstable, and thus the structure experiences a displacive phase transition when the central ion of a unit cell of the body-cubic-centred lattice, is displaced to the cube face. Thus, the electronic structure of matter at densities above the neutron drip [1], is richer than previously appreciated, and new microscopic calculations of nuclear structure are necessary which take into account the high-wavenumber physics. Such calculations will provide crucial input to models interpreting the quasi-periodic oscillations in Soft Gamma Repeaters as magnetar x-ray flares, and to the theory of glitches of neutron stars. [1] The neutron drip density is ~3×1011 g cm-3.

In this thesis, RXTE observations of 4U 1907+09 are presented. Timing analysis of these data sets have yielded quasi periodic oscillations (QPOs) at orbital phases corresponding to the two flares in every orbital period. Known continuous spin down trend and QPO behaviour at the flares strongly suggest that a transient accretion disk occurs at the flares. Our findings strongly suggested that neutron star passes through the equatorial wind of Be companion star. During these passages a transient disk forms around Be neutron star.

The high time-resolution radio sky represents unexplored astronomical territory where the discovery potential is high. In this thesis I have studied the transient radio sky, focusing on millisecond scales. As such, this work is concerned primarily with neutron stars, the most populous member of the radio transient parameter space. In particular, I have studied the well known radio pulsars and the recently identified group of neutron stars which show erratic radio emission, known as RRATs, which show radio bursts every few minutes to every few hours. When RRATs burst onto the scene in 2006, it was thought that they represented a previously unknown, distinct class of sporadically emitting sources. The difficulty in their identification implies a large underlying population, perhaps larger than the radio pulsars. The first question investigated in this thesis was whether the large projected population of RRATs posed a problem, i.e. could the observed supernova rate account for so many sources. In addition to pulsars and RRATs, the various other known neutron star manifestations were considered, leading to the conclusion that distinct populations would result in a 'birthrate problem'. Evolution between the classes could solve this problem - the RRATs are not a distinct population of neutron stars. Alternatively, perhaps the large projected population of RRATs is an overestimate. To obtain an improved estimate, the best approach is to find more sources. The Parkes Multi-beam Pulsar Survey, wherein the RRATs were initially identified, offered an opportunity to do just this. About half of the RRATs showing bursts during the survey were thought to have been missed, due to the deleterious effects of impulsive terrestrial interference signals. To remove these unwanted signals, so that we could identify the previously shrouded RRATs, we developed new interference mitigation software and processing techniques. Having done this, the survey was completely re-processed, resulting in the discovery of 19 new sources. Of these, 12 have been re-detected on multiple occasions, whereas the others have not been seen to re-emit since the initial discovery observations, and may be very low burst-rate RRATs, or, isolated burst events. These discoveries suggest that the initial population estimate was not over-estimated - RRATs, though not a distinct population, are indeed numerous. In addition to finding new sources, characterisation of their properties is vital. To this end, a campaign of regular radio observations of the newly discovered sources, was mounted, at the Parkes Observatory, in Australia. In addition, some of the initially identified RRATs were observed with the Lovell Telescope at Jodrell Bank. These have revealed glitches in J1819-1458, with anomalous post-glitch recovery of the spin-down rate. If such glitches were common, it would imply that the source was once a magnetar, neutron stars with the strongest known magnetic fields of up to 10¹⁵ gauss. The observations have also been used to perform 'timing' observations of RRATs, i.e. determination of their spin-down characteristics. At the beginning of this thesis, 3 of the original sources had 'timing solutions' determined. This has since risen to 7, and furthermore, 7 of the newly discovered sources now also have timing solutions. With this knowledge, we can see where RRATs lie in period-period derivative space. The Parkes RRATs seem to be roughly classifiable into three groupings, with high observed nulling fractions - normal pulsars, high magnetic field pulsars and old, 'dying' pulsars. It seems that RRATs and pulsars are one and the same. When a pulsar is more easily detected in searches for single bright pulses, as opposed to in periodicity searches, we label it a RRAT. Such searches impart a selection effect on the parameter space of possible sources, in both nulling fraction and rotation period. In this sense, an observational setup could be designed to make any pulsar appear as a RRAT. For realistic survey parameters however, this is not the case, and the groups mentioned above seem to be the most likely to appear as RRATs. In fact, we can utilise RRAT searches to identify neutron stars, difficult to find by other means, in particular high-magnetic field pulsars, and pulsars approaching the pulsar "death valley". Some of the RRATs are well explained as being distant/weak pulsars with a high modulation index, others seem to be nulling pulsars. This highlights the incomplete knowledge of nulling behaviour in the pulsar population. It seems that there may be a continuum of nulling durations, under a number of guises, from 'nulling pulsars' to 'RRATs' to 'intermittent pulsars'. In fact this nulling may fit into the emerging picture, whereby pulsar magnetospheres switch between stable configurations.

This thesis explores the topic of maximum mass stability of neutron stars. The outer structure is detailed and explores nuclear pasta phases, the neutron drip line, and density transitions of matter in the crust and atmosphere layers. Other discussion points include superfluids in the crust and core, vortex roles in neutron stars, and magnetic field effects on the EOS in neutron stars. The inner core is studied in much more detail due to its significant role in EOS. The variety of stars include pion condensate stars, kaon condensate stars, npeu stars, npeu stars with the inclusion of hyperons, quark-hybrid stars, and strange stars. Included with these is a description of nucleon-nucleon, nucleon-nucleon-nucleon interactions, the appearance factors that affect hyperon species, and the formation process of kaons, pions, quarks, and hyperons. The ending EOS are compared with their maximum mass values to determine which ones are likely to limit the mass of neutron stars.

Este trabalho visa o estudo das perturbações de sistemas gravitacionais altamente compactos, como buracos negros e estrelas de nêutrons e de quarks. As perturbações em questão podem ser de diversas naturezas (escalar, eletromagnética ou gravitacional), sendo que detivemo-nos mais atentamente naquelas de natureza gravitacional, pois estas têm despertado mais interesse por serem astronomicamente mais fáceis de detectar. Além de estudarmos tais perturbações, procedemos a uma comparação dos resultados para buracos negros, estrelas de nêutrons e de quarks. Tal comparação justifica-se pelo fato de que a confrontação de previsões teóricas com resultados experimentais pode ajudar-nos a identificar objetos astronômicos de interesse, distingüi-los e, no caso de estrelas, aprender algo sobre sua estrutura interna, particularmente sobre a equação de estado (EDE) do fluido estelar. No que se segue, dividimos o trabalho em sete partes. Em primeiro lugar, damos uma introdução bastante sucinta ao mesmo (Capítulo 1). Depois, falamos sobre a física de estrelas de nêutrons e de quarks (Capítulo 2). A seguir, fazemos um estudo comparativo dos dois tipos de estrelas (Capítulo 3). Mais adiante, discutimos as perturbações de diversos tipos, além de definir os MQNs (Modos Quasi-Normais) de buracos negros de Schwarzschild (Capítulo 4). Prosseguimos a discussão com a métrica de Reissner-Nordström-de Sitter (Capítulo 5) e introduzimos, na seqüência, os MQNs estelares, de forma muito breve (Capítulo 6). Por fim, apresentamos nossas conclusões (Capítulo 7). A parte inédita deste trabalho está concentrada nos capítulos 5 e 6, os quatro anteriores servindo de preparativo e de base comparativa para estes dois. / This work aims the detailed study of the perturbations of highly compact gravitational systems, such as black holes and both neutron and quark stars. Such perturbations may have several different characters, such as scalar and electromagnetic fields as well as gravitational (either axial or polar) disturbances. We have focused more closely on the latter kind of perturbation, since they offer better possibilities of detection in the near future, in the form of gravitational waves. Besides studying the aforementioned perturbations, we have proceeded to a comparison between black holes and neutron and quark stars , when it comes to the outcomes of the perturbations, usually called QNMs (quasi-normal modes). Such a comparison is actually in order, since a direct comparison of theoretical and observational data may help us identify astronomical objects and, in the case of compact stars, may provide valuable insights into these stars' inner structure, particularly when it comes to their equation of state (EOS). In what follows, we have subdivided this work in seven parts. We begin with a brief introduction (Chapter 1), then proceed to a description of the physics of both neutron and quark stars (Chapter 2) and, in the sequence, to a comparative study of both kinds of star (Chapter 3). Subsequently, we develop the perturbation theory of the Schwarzschild black holes, discussing their QNMs (Chapter 4) and doing the same, later, for a more general Reissner-Nordström-de Sitter geometry (chapter 5). After that, we provide a very brief introduction to the stellar MQNs (Chapter 6). Finally, we present our conclusions (Chapter 7). The chapters 5 and 6 carry the inedit part of this work, and the chapters from 1 to 4 pave the way and provide a comparative basis for them.

O evento estraordinário de SN1987A vem sendo investigado há mais de vinte e cinco anos. O fascínio que cerca tal evento astronômico está relacionado com a observação em tempo real da explosão à luz da Física de neutrinos. Detectores espalhados pelo mundo observaram um surto neutrinos que dias mais tarde foi confirmado como sendo a SN1987A. Kamiokande, IMB e Baksan apresentaram os eventos detectados que permitiu o estudo de modelos para a explosão e resfriamento da hipotética estrela de nêutrons remanescente. Até hoje não há um consenso a origem do progenitor e a natureza do objeto compacto remanescente. O trabalho se divide em duas partes: estudo dos neutrinos de SN1987A através de Análise Estatística Bayesiana através de um modelo proposto com duas temperaturas que evidenciam dois surtos de neutrinos. A motivação está na hipótese do segundo surto como resultado da formação de matéria estranha no objeto compacto. A metodologia empregada foi a desenvolvida por um trabalho interessante de Loredo (2002) que permite modelar e testar hipóteses sobre os modelos via Bayesian Information Criterion (BIC). A segunda parte do trabalho, a mesma metodologia estatística é usada no estudo da distribuição de massas das estrelas de nêutrons usando a base de dados disponível (http://stellarcollapse.org). A base de dados foi analisada utilizando somente o valor do objeto e seu desvio padrão. Construindo uma função de verossimilhança e utilizando distribuições ``a priori\'\' com hipótese de bimodalidade da distribuição das massas contra uma distribuição unimodal sobre todas as massas dos objetos. O teste BIC indica forte tendência favorável à existência da bimodalidade com valores centrados em 1.37M para objetos de baixa massa e 1.73M para objetos de alta massa e a confirmação da fraca evidência de um terceiro pico esperado em 1.25M. / The extraordinary event of supernova has been investigated twenty five years ago. The fascination surrounds such astronomical event is on the real time observation the explosion at light to neutrino Physics. Detectors spread for the world had observed one burst neutrinos that days later it was confirmed as being of SN1987A. Kamiokande, IMB and Baksan had presented the detected events that allowed to the study of models for the explosion and cooling of hypothetical neutron star remain. Until today it does not have a consensus the origin of the progenitor and the nature of the remaining compact object. The work is divided in two parts: study of the neutrinos of SN1987A through Analysis Bayesiana Statistics through a model considered with two temperatures that two evidence bursts of neutrinos. The motivation is in the hypothesis of as burst as resulted of the formation of strange matter in the compact object. The employed methodology was developed for an interesting work of Loredo & Lamb (2002) that it allows shape and to test hypotheses on the models saw Bayesian Information Criterion (BIC). The second part of the work, the same methodology statistics is used in the study of the distribution of masses of the neutron stars using the available database http://stellarcollapse.org/. The database was analyzed only using the value of the object and its shunting line standard. Constructing to a a priori function likelihood and using distributions with hypothesis of bimodal distribution of the masses against a unimodal distribution on all the masses of objects. Test BIC indicates fort favorable trend the existence of the bimodality with values centered in 1.37M for objects of low mass and 1.73M for objects of high mass and week evidence of one third peak around 1.25M.