281 research outputs found
Population Synthesis of Compact Object Binaries
No full textCompact objects, such as Neutron stars (NSs) and black holes (BHs), are the remnants of massive stars and play a central role in some of the most energetic phenomena in the Universe. Through accretion and merger processes, they power X-ray binaries and gravitational-wave (GW) sources, respectively. These systems are key to probing the physics of extreme gravity, dense matter, and relativistic astrophysics. The ever-growing samples from GW observations provide an unprecedented opportunity to advance our understanding of stellar and binary evolution through population studies in the era of multi-messenger astronomy.
This thesis presents binary population synthesis (BPS) studies of compact-object binaries containing BHs and NSs, focusing on wind-fed BH high-mass X-ray binaries and merging NS--BH (NSBH) systems. Through these studies, we investigate several open questions, including the origin of BH spins, distinguishable channel-wise characteristics of merging NSBHs, and the existence of lower mass-gap BHs.
These populations are investigated using the next-generation BPS code POSYDON, which combines detailed binary evolution models, computed using the stellar evolution code MESA, with a self-consistent population framework. It includes full stellar structure, rotation, and realistic modeling of mass transfer, enabling more physically accurate simulations of binary interactions and compact object formation.
The results of this thesis highlight that accurately modeling key physical processes, using physically motivated approaches is essential for interpreting current and future multi-messenger observations, and for deepening our understanding of stellar and binary evolution. As gravitational-wave and electromagnetic observations continue to grow, their integration with advanced BPS studies will be crucial for probing the underlying physics of compact object binaries. This thesis establishes a foundation for more reliable and unified population modeling in the era of multi-messenger astronomy.</p
Unraveling the Origins of Stellar Mass Black Hole Mergers
No full textThe detections of gravitational waves from the collisions of stellar-mass black holes by the LIGO Scientific & Virgo Collaboration have opened a new window onto the Universe. Even though multiple formation mechanisms have been theorized in the literature, the question regarding their origin remains open. The first part of this thesis aims to study merging binary black holes (BBHs) formed from isolated binary evolution in galactic fields. In particular, we model the distribution of their observational properties such as black hole masses, spins, and their merger redshifts. In the second part, we consider model predictions for alternative formation channels, and illustrate how one could leverage these signatures to interpret and quantify the origin of the detected sample of BBHs. In the last part, we discuss the implications on the observable population of electromagnetic transient events of our stellar binary models, which could potentially be used to constrain further the formation mechanisms of merging BBHs
Formation and evolution of X-ray binaries: Exploring Ultra-luminous X-ray Sources
No full textX-ray binaries (XRBs) are some of the brightest X-ray sources in the Universe. The emission from XRBs is produced by the transfer of material from a star onto a compact object serving as its companion. The brightest of XRBs are ultra-luminous X-ray sources (ULXs), with X-ray luminosities greater than 1039 erg s-1 (which is about the Eddington limit of a 10Msun black hole). Observations of coherent X-ray pulsations in a subset of ULXs indicate the presence of an accreting neutron star. Considering that the Eddington limit of a neutron star is ~1038 erg s-1, it makes these pulsating ULXs highly super-Eddington. This thesis aims at studying the formation and evolution of ULXs, and by extension XRBs as a whole. We investigate ULXs in the form of XRBs undergoing super-Eddington mass-transfer phases and include super-Eddington disc models to help explain their luminosities. In doing so, we also investigate the stability of mass transfer in XRBs and formulate a method of constraining the mass-transfer stability in semi-detached binaries, using the overflow from the second Lagrange point as a constraint. We study the demographic of ULX populations at different ages of stellar populations, and the effect of different assumptions of accretion physics on ULXs. In order to carry out these studies, we used population synthesis techniques in combination with detailed stellar and binary evolutionary calculations. Given the uncertainties that currently exist in our understanding of binary interactions and assumptions that need to be made for evolutionary phases (for instance, the efficiency in the common-envelope phase), we carried out a parameter study with the aim of constraining the said physics. We base our studies of ULX populations on the results of the parameter study and draw conclusions from the inferred information, finding that observations of ULXs could be used to constrain the treatment of kicks in binaries and that NS-ULXs are almost always geometrically beamed whereas BH-ULXs are beamed in ~50% of the cases. Future studies, including a wider range of metallicities, will aid us better in our understanding of sources like ULXs that are more abundant in sub-solar metallicities.
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Data Set of PLOS Computational Paper PCOMPBIOL-D-18-02181R1
No full textFigures Data of PLOS Computational paper:Modeling of the axon plasma membrane structure and its effects on protein diffusionAuthors: Yihao Zhang, Anastasios V. Tzingounis, and George LykotrafitisCorresponding Author: George Lykotrafitis, Ph.D.University of ConnecticutStorss, CT UNITED STATES</div
The state of modern Greek language as spoken in Victoria
No full textDeposited with permission of the author. © 1986 Dr. Anastasios TamisThis thesis reports a sociolinguistic study, carried out between 1981 and 1984, of the state of the Modern Greek (MG) language in Australia, as spoken by native-speaking first-generation Greek immigrants in Victoria. Particular emphasis is given to the analysis of those characteristics of the linguistic behaviour of these Greek Australians which can be attributed to the contact with English and to other environmental, social and linguistic influence. (For complete abstract open document
The black hole spin in coalescing binary black holes and high-mass X-ray binaries
No full textDuring my PhD study, I was working on two types of Black Hole (BH) binaries, i.e. coalescing binary BHs and BH High-mass X-ray binaries (HMXBs). Most of the measured spin chi_eff are quite low, while three BH spin measurements in HMXBs show only values near the maximum. My study is focused mainly on the detailed stellar and binary evolution by tracking the evolution of angular momentum of the BH progenitor star in BH binaries. More specifically, we systematically studied the origin of the BH spin in these two different BH binaries systems. Furthermore, I have been working on a project that is related to a detailed comparison of the implementation for chemical mixing and angular momentum transport between two stellar evolution codes, namely, MESA and GENEVA codes. More specifically, we want to understand the minimum rotation that is required for chemically homogeneous evolution to occur
Aspects of Massive Binary stars' Formation and Evolution
No full textThis thesis investigate massive binary stars formation, near earth supernova from open clusters and the formation of Black hole X-ray binaries
Some Problems in Harmonic Analysis
No full textOne the most central questions in harmonic analysis of whether the Fourier series of a square integrable function on the torus converges Lebesgue a.e.\ was answered positively by L.\ Carleson in 1966 \cite{C}, by means of a weak- inequality for the maximal operator \begin{equation} \label{carleson0} \mathcal C f(x) =\sup_{N\in \mathbb Z} \Bigg| \sum_{|\xi|\leq N} \widehat f(\xi) \exp(ix\xi) \Bigg|, \qquad x\in \mathbb T. \end{equation} The argument of \cite{C} estimates pointwise as a maximal modulated Hilbert transform, outside appropriately constructed exceptional sets whose mass is controlled by almost-orthogonality. The implicit distributional estimate in \cite{C} was later exploited by Hunt \cite{H} to deduce the family of restricted weak-type bounds \begin{equation} \label{e:ch} \left\|\mathcal{C}f\right\|_{L^{p,\infty}(\mathbb T)} \leq \frac{Cp^2}{p-1}|F|^{\frac1p}, \qquad F\subset \mathbb T,\; |f|\leq \cic{1}_F, \qquad
New historical evidence for Anastasios Emm. Papas
No full textNo AbstractThe author’s attention has been drawn to the existence of this historicalevidence in the National Archives of Vienna, by his friend the writer EteoclesGregoriadis together with the numbers of the relevant files. Most of the documents were written in the old German script. Thus the author asked for the help of his friend and former colleague at the University of Thessaloniki and director of the Goethe Institute, Graf Kurt v. Posadowsky, for reading andstudying those documents. Without his help this study would have been impossible. This new evidence concerns the sojourn of Anastasios Papas·—son of Emmanuel Papas, leading figure of the Greek Revolution—in Austria andGermany between the 3rd January and 11th March 1822. There is informationabout his short imprisonment in Trieste, after his arival from Vienna. He then visits various towns in Germany and after negotiations with the Philhellene professor Fr. Thiersch in Munich, he purchases large quantities of ammunition to be despatched to Greece. He finally arrives in Greece early in 1824, and takes part—together with his three brothers who were already fighting—in the struggle for the liberation of the common great fartheland
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