When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response coming from states containing two quasiparticle-quasihole pairs may be evaluated in terms of Landau theory if one allows for the effect of collisions in the Landau kinetic equation. We consider the case of asymmetric nuclear matter, and our work goes beyond earlier works in that they contain the effects of collisions in addition to those of the mean field.

Thesis (Ph.D.)--University of Melbourne, School of Physics, 2007. / Typescript. Includes bibliographical references (leaves 275-308).

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 139-145).

Στην παρούσα εργασία μελετώνται οι ταλαντώσεις των αστέρων νετρονίων με ιδιαίτερη έμφαση στις ακτινικές ταλαντώσεις τους. Σκοπός αυτής της μελέτης είναι ο υπολογισμός των συχνοτήτων των ακτινικών ταλαντώσεων των αστέρων νετρονίων. Στο πρώτο, κεφάλαιο κάνουμε μία μικρή εισαγωγή για τους αστέρες νετρονίων και τους ταχέως περιστρεφόμενους αστέρες νετρονίων (pulsars) καθώς και για τον ρόλο, που διαδραματίζουν αυτοί και τα διπλά συστήματα που σχηματίζουν, στην σύγχρονη Αστροφυσική. Ακόμα αναφερόμαστε στην εσωτερική δομή των αστέρων νετρονίων και σε κάποιες από τις καταστατικές εξισώσεις, που μπορεί να περιγράφουν την ύλη στο εσωτερικό του, δίνοντας έμφαση στην πολυτροπική καταστατική εξίσωση την οποία και υιοθετούμε στην παρούσα εργασία. Στο δεύτερο κεφάλαιο, παραθέτουμε τις εξισώσεις Oppenheimer–Volkoff(OV) που περιγράφουν την ισορροπία ενός αδιατάρακτου αστέρα νετρονίων. Στη συνέ- χεια, θεωρώντας τις ακτινικές ταλαντώσεις 1) ως απειροστού πλάτους αδιαβατικές ταλαντώσεις που διατηρούν τον βαρυονικό αριθμό και 2) ως αποτέλεσμα της αργής περιστροφής του αστέρα, καταλήγουμε σε μία δεύτερης τάξης διαφορική εξίσωση που διέπει τις ακτινικές ταλαντώσεις των αστέρων νετρονίων. Η εξί- σωση αυτή γράφεται στη μορφή Sturm– Liouville. Επιπροσθέτως, συνεχίζουμε παραθέτοντας τον διορθωτικό όρο, λόγω περιστροφής, για την τιμή της συχνότη- τας και τις εξισώσεις που διέπουν τις μη ακτινικές ταλαντώσεις. Τέλος κλείνουμε το κεφάλαιο αυτό με μία ανάλυση των διαφόρων τρόπων ταλάντωσης. Στο τρίτο κεφάλαιο, αρχικά επιλύουμε, με τη χρήση ενός πρωτότυπου επα- ναληπτικού αλγορίθμου, το σύστημα διαφορικών εξισώσεων OV για την εύρεση των φυσικών παραμέτρων του αστέρα. Στη συνέχεια, αφού αρχικά αναλύσουμε τις βασικότερες μεθόδους επίλυσης της διαφορικής εξίσωσης των ακτινικών ταλα- ντώσεων, που εμφανίζονται στην βιβλιογραφία, μετατρέπουμε τη μορφή Sturm– Liouville σε ένα σύστημα δύο διαφορικών εξισώσεων πρώτης τάξης, το οποίο επιλύουμε με την βοήθεια της μεθόδου σκόπευσης (shooting method). Στη βιβλιογραφία, υπάρχουν δύο διαφορετικές τάσεις αντιμετώπισης της πο- λυτροπικής καταστατικής εξίσωσης, ανάλογα με το αν στην θέση της πυκνότητας εισέρχεται η πυκνότητα μάζας ηρεμίας ή η πυκνότητα της ολικής μάζας–ενέργειας. Ακόμα, δύο είναι και οι διαφορετικοί τρόποι αντιμετώπισης του αδιαβατικού δεί- κτη, ο οποίος εισέρχεται στην εξίσωση που περιγράφει τις ακτινικές ταλαντώσεις, ανάλογα με το αν είναι σταθερός ή μεταβάλλεται. Από τις τέσσερις αυτές βασικές υποθέσεις για την πολυτροπική καταστατική εξίσωση και τον αδιαβατικό δείκτη, προκύπτουν τέσσερα διαφορετικά πρωτότυπα μοντέλα για τις ακτινικές ταλαντώ- σεις, τα οποία και επιλύουμε. Στο τελευταίο κεφάλαιο, υπολογίζουμε και παρουσιάζουμε τις τρεις πρώτες συχνότητες των τεσσάρων πρωτότυπων μοντέλων για τις ακτινικές ταλαντώσεις των αστέρων νετρονίων για τρεις διαφορετικές τιμές του πολυτροπικού δείκτη και αναλύουμε τις αριθμητικές μεθόδους, τις οποίες χρησιμοποιούμε, καθώς και τις αντίστοιχες υπορουτίνες της βιβλιοθήκης SLATEC. Εν κατακλείδι, τα αποτελέσματα αυτής της εργασίας είναι η ανάπτυξη ενός πρωτότυπου επαναληπτικού αλγορίθμου για την εύρεση της ακτίνας του αστέρα με μεγάλη ακρίβεια και η παρουσίαση αποτελεσμάτων για τέσσερα πρωτότυπα μοντέλα που περιγράφουν τις ακτινικές ταλαντώσεις των αστέρων νετρονίων. / In the present Thesis we study the oscillations of neutron stars emphasizing on the radial oscillations. The Thesis is organized in four chapters. In the first chapter, we introduce the theoretical background of neutron stars and pulsars. We then discuss the importance of the role that the binary neutron stars play in modern Astrophysics. Next, we refer to the structure of these stars and introduce some of the equations of state (EOS) which try to describe the matter occupying the inner layers of neutron stars, emphasizing on the polytropic EOS which is adopted here. In the second chapter we, first introduce the Oppenheimer–Volkoff (OV) system of differential equations, describing the hydrostatic equilibrium of a non rotating, non pulsating neutron star, and considering the radial oscillations 1) as infinitesimal, baryon-number conserving, adiabatic oscillations 2) as the result of the slow rotation of the neutron star, we derive the second order differential equation governing the radial oscillations of a neutron star. We then rewrite this equation in the Sturm–Liouville form. The expression of the change of frequency of the radial oscillations due to slow rotation and the equations of state is obtained. Finally, we conclude this chapter with a mode analysis of oscillations of neutron stars in general. In the third chapter, we first solve the OV system of differential equations, implementing an original iterative algorithm, and thus calculate the physical parameters of the star. Next, some of the methods used for solving the equations describing the radial oscillations are discussed. Finally, we transform the Sturme–Liouville form to a set of two first order differential equations, which are computed by implementation of the shooting method. In the bibliography, the polytropic EOS is considered in two different ways, depending on which density (rest mass or total mass–energy) is involved in the polytropic EOS. In a similar manner, we have two different ways for considering the adiabatic exponent which enters the equation describing the radial oscillations (constant or variable). Considering these four different assumptions for the polytropic EOS and the adiabatic exponent, we construct four different models of pulsating neutron stars. In the final chapter, we compute and present the first three frequencies of each basic model concerning radial oscillations of neutron stars for three values of the polyropic index. We discuss the numerical methods implemented here and the involved subroutines, which can be found in the SLATEC Library. The main issues of the present Thesis are the development of an iterative algorithm for accurately computing the radius of the star and the computation of the frequencies for the four basic models describing th radial oscillations of neutron stars.

We will consider the scenario of the co-rotation of a fluid star (in specific, a neutron star) and a black hole. The neutron star (or primary)is assumed to have constant angular velocity. The tidal effects on the primary are investigated. First, the centrally condensed approximation is applied, where both bodies are considered as point sources. In the second treatment, the primary is treated as an incompressible and homogeneous fluid mass, which in addition to its own gravity is subject to centrifugal and Coriolis forces, derived from fluid motions. The black hole (or secondary) is treated as a rigid sphere and can be regarded as a point mass. The equilibrium figure is derived. The problem is then adapted to include vorticity and a pseudo-Newtonian potential. The coalescence of neutron star - black hole binaries and their importance to gravitational wave detection is also discussed. / Mathematical Sciences / M. Sc. (Applied Mathematics)

It is now widely known that most of the large galaxies we observe (e.g. the Milky Way) host in their center a supermassive black hole ($10^{6}-10^{9}$ $M_\odot$). Several relationships between the central black hole mass and the properties of the stars in the central part of the galaxy have been established in the past 3 decades indicating that the central black hole is able to efficiently structure the matter around it due to episodes of accretion of matter onto the black hole. Recent infrared and optical sky surveys have detected supermassive black holes with masses around $10^{8-9}$ $M_\odot$ when the universe was less than a tenth of its current age and current theories have difficulties explaining how such massive objects could have formed over such short timescales. The goal of the present work is to shed light on the properties of a still largely unknown extreme accretion regime, the so called super-Eddington accretion regime. If such accretion regime could be sustained over sufficient timescales, it could play an important role in both the rapid growth of supermassive black holes as well as its co-evolution with its host galaxy. The aim of this work is therefore to apply high resolution spectroscopy to Ultraluminous X-ray sources in order to identify narrow spectral features to derive constrains on the outflows expected from super-Eddington accreting sources using data from the XMM-Newton observatory. For this purpose I developed a framework to analyse low count background dominated spectra that uses a Monte Carlo approach to detect these narrow features. After analysis of the source Holmberg II X-1, I identify 7 unresolved discrete features with a 3$\sigma$ confidence level that can be tentatively identified with ionic species. Furthermore, the instrumental resolution allows us to put upper limits on the broadening of the lines. This findings will allow us to probe the properties of the outflows of the super-Eddington regime and by extending the analysis to other sources we will able to characterize the observational properties of this accretion regime.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / As estrelas de nêutrons nascem com altas temperaturas (~ 1011 K) e durante alguns segundos sofrem um rápido resfriamento por emissão de neutrinos. O processo Urca direto é o principal mecanismo para explicar essa perda de energia. O problema do resfriamento das estrelas de nêutrons é um problema de grande interesse porque seu entendimento pode fornecer informações importantes sobre a constituição do interior da estrela. Na literatura existente até o momento, a emissividade de neutrinos é calculada considerando os núcleons como partículas não relativísticas quando considerados todos os níveis de Landau das partículas carregadas. Por outro lado, a emissividade de neutrinos para núcleons relativísticos é calculada considerando somente o primeiro nível de Landau (para campo magnético forte). Para campos magnéticos fracos, onde mais de um nível de Landau é ocupado, é usada a emissividade correspondente à do campo nulo. Neste trabalho aplicamos a teoria de Weinberg-Salan para interações fracas no cálculo da emissividade de neutrinos com e sem campo magnético presente, num cálculo totalmente relativístico para os núcleons e considerando todos os níveis de Landau. Esta é a contribuição original do trabalho. Para descrever a matéria a altas densidades, utilizamos uma teoria relativística de campo médio a temperatura zero que inclui apenas o octeto bariônico e os léptons mais leves. São apresentados os resultados para a emissividade de neutrinos, onde é evidente a ocupação dos diferentes níveis de Landau como função do campo magnético.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / As estrelas de nêutrons nascem com altas temperaturas (~ 1011 K) e durante alguns segundos sofrem um rápido resfriamento por emissão de neutrinos. O processo Urca direto é o principal mecanismo para explicar essa perda de energia. O problema do resfriamento das estrelas de nêutrons é um problema de grande interesse porque seu entendimento pode fornecer informações importantes sobre a constituição do interior da estrela. Na literatura existente até o momento, a emissividade de neutrinos é calculada considerando os núcleons como partículas não relativísticas quando considerados todos os níveis de Landau das partículas carregadas. Por outro lado, a emissividade de neutrinos para núcleons relativísticos é calculada considerando somente o primeiro nível de Landau (para campo magnético forte). Para campos magnéticos fracos, onde mais de um nível de Landau é ocupado, é usada a emissividade correspondente à do campo nulo. Neste trabalho aplicamos a teoria de Weinberg-Salan para interações fracas no cálculo da emissividade de neutrinos com e sem campo magnético presente, num cálculo totalmente relativístico para os núcleons e considerando todos os níveis de Landau. Esta é a contribuição original do trabalho. Para descrever a matéria a altas densidades, utilizamos uma teoria relativística de campo médio a temperatura zero que inclui apenas o octeto bariônico e os léptons mais leves. São apresentados os resultados para a emissividade de neutrinos, onde é evidente a ocupação dos diferentes níveis de Landau como função do campo magnético.

Ultraluminous X-ray sources (ULXs) are extragalactic, non-nuclear, point-like X-ray sources whose luminosity supersedes that of the Eddington limit of an accreting stellar mass black hole (L&gt; 10 ^ 39 erg / s). Most of them are powered by black holes and neutron stars undergoing genuine super-Eddington accretion, with a small handful of candidates being consistent with sub-Eddington accretion on an intermediate mass black hole. In this thesis, we explore the populations of ULXs in the sky surveys of ESA's X-ray satellite, XMM-Newton, and the MPE's newly launched X-ray telescope, eROSITA. We do so by correlating them with the HECATE list of galaxiesto build two X-ray non-nuclear catalogs, and comparing the yields with very expensive surveys and previous works. To build a catalog, we useother reference lists of contaminant objects, such as the Gaia data releases, the SIMBAD database or the SDSS survey to look for contaminating objects of diverse nature, such as foreground stars or background quasars, in order to make sure that our resulting ULX samples are as clean as possiblewith catalog data only. Our results include the attestation that the XMM-Newton ninth data release provides an improvement in quantity and quality with respect to older data releases used in previous works, and that the eROSITA survey is currently in a very preliminary stage. The two new catalogs contain 12,952 and 3,720 non-nuclear X-ray sources, out of which 914 and 132 are ULX candidates with an expected ~ 25% fraction of undetected contaminants. This constitutes a very significant contribution to the already known 300 ULX candidates. Since the sky coverage and depth of the XMM-Newton and eROSITA surveys are vastly different, only 19 of the ULX candidates are shared between the catalogs. ULX candidates are preferentially found in star-forming galaxies, but a subset of very bright objects (L&gt; 5x10 ^ 40 erg / s) try to be more common in elliptical galaxies, in contradiction to what has been established in the literature. / <p>This thesis was written under the joint supervision of Erin O'Sullivan at Uppsala University and Axel Schwope at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 circumstances.</p> / Master Thesis

The work of this dissertation will study various aspects of the dynamics of compact objects using numerical simulations.We consider BH dynamics within two modified or alternative theories of gravity. Within a family of Einstein-Maxwell-Dilaton-Axion theories, we find that the GW waveforms from binary black hole (BBH) mergers differ from the standard GW waveform prediction of GR for especially large axion values. For more astrophysically realistic (i.e. smaller) values, the differences become negligible and undetectable. Weestablish the existence of a well-posed initial value problem for a second alternative theory fo gravity (quadratic gravity) and demonstrate in spherical symmetry that a linear instability is effectively removed on consideration of the full nonlinear theory.We describe the key components and development of a code for studying BBH mergers for which the mass ratio of the binaries is not close to one. Such intermediate mass ratio inspirals (IMRIs) are much more difficult to simulate and present greater demands on resolution, distributed computing, accuracy and efficiency. To this end, we present a highly-scalable framework that combines a parallel octree-refined adaptive mesh with a wavelet adaptive multiresolution approach. We give results for IMRIs with mass ratios up to 100:1. We study the ejecta from BNS in Newtonian gravity. Using smoothed particle hydrodynamics we develop and present the highly scalable FleCSPH code to simulate such mergers. As part of the ejecta analysis, we consider these mergers and their aftermath as prime candidates for heavy element creation and calculate r-process nucleosynthesis within the post-merger ejecta. Lastly we consider a non-standard, yet increasingly explored, interaction between a BH and a NS that serves as a toy model for primordial black holes (PBH) and their possible role as dark matter candidates. We present results from a study of such systems in which a small BH forms at the center of a NS. Evolving the spherically symmetric system in full GR, we follow the complete dynamics as the small BH consumes the NS from within. Using numerical simulations, we examine the time scale for the NS to collapse into the PBH and show that essentially nothing remains behind. As a result, and in contradiction to other claims in the literature, we conclude that thisis an unlikely site for ejecta and nucleosynthesis, at least in spherical symmetry.