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The frequencies of quasi-periodic oscillations around neutron stars are believed to be related to characteristic frequencies in the gravitational fields of the compact objects. In different variability models, these include the Keplerian,…

Astrophysics · Physics 2007-05-23 Simon DeDeo , Dimitrios Psaltis

In this review article, we present the main results from our most recent research concerning the oscillations of fast rotating neutron stars. We derive a set of time evolution equations for the investigation of non-axisymmetric oscillations…

General Relativity and Quantum Cosmology · Physics 2021-10-04 Christian J. Krüger , Kostas D. Kokkotas , Praveen Manoharan , Sebastian H. Völkel

According to various models, the orbital and the epicyclic frequencies of particles moving on a circular orbit around compact objects are related to the quasi-periodic oscillations observed in the X-ray flux of some pulsars or black hole…

General Relativity and Quantum Cosmology · Physics 2016-01-27 Kalin V. Staykov , Daniela D. Doneva , Stoytcho S. Yazadjiev

We are investigating universal relations between different normalisations of the moment of inertia and the compactness of neutron and strange stars. Slowly rotating as well as rapidly rotating models are studied in General Relativity,…

General Relativity and Quantum Cosmology · Physics 2016-04-13 Kalin V. Staykov , Daniela D. Doneva , Stoytcho S. Yazadjiev

We study the orbital and epicyclic frequencies of particles orbiting around rapidly rotating neutron stars and strange stars in a particular scalar-tensor theory of gravity. We find very large deviations of these frequencies, when compared…

High Energy Astrophysical Phenomena · Physics 2014-08-06 Daniela D. Doneva , Stoytcho S. Yazadjiev , Nikolaos Stergioulas , Kostas D. Kokkotas , Tilemachos M. Athanasiadis

Testing modified theories of gravity with direct observations of the parameters of a neutron star is not the optimal way of testing gravitational theories. However, observing electromagnetic signals originating from the close vicinity of…

General Relativity and Quantum Cosmology · Physics 2019-11-06 Kalin V. Staykov , Daniela D. Doneva , Stoytcho S. Yazadjiev

We are investigating universal relations between different normalisations of the moment of inertia and the compactness of neutron stars in slow rotation approximation. We study the relations in particular class of massive scalar-sensor…

General Relativity and Quantum Cosmology · Physics 2019-03-27 Dimitar Popchev , Kalin V. Staykov , Daniela D. Doneva , Stoytcho S. Yazadjiev

In the present paper we investigate self-consistently slowly rotating neutron and strange stars in R-squared gravity. For this purpose we first derive the equations describing the structure of the slowly rotating compact stars in…

General Relativity and Quantum Cosmology · Physics 2015-06-22 Kalin V. Staykov , Daniela D. Doneva , Stoytcho S. Yazadjiev , Kostas D. Kokkotas

In the last few decades, lots of universal relations between different global physical quantities of neutron stars have been proposed to constrain the unobservable or hard to be observed properties of neutron stars. But few of them are…

General Relativity and Quantum Cosmology · Physics 2020-08-10 Wenjie Sun , Dehua Wen , Jue Wang

We study oscillations and instabilities of relativistic stars using perturbation theory in general relativity and take into account the contribution of a dynamic spacetime. We present the oscillation spectrum as well as the critical values…

General Relativity and Quantum Cosmology · Physics 2020-09-16 C. J. Krüger , K. D. Kokkotas

The innermost stable circular orbits (ISCOs) around rapidly rotating neutron stars are studied in dilatonic Einstein-Gauss-Bonnet theory. Universal relations for properly scaled ISCO properties are extended from General Relativity to…

General Relativity and Quantum Cosmology · Physics 2019-02-06 Guillermo A. Gonzalez , Burkhard Kleihaus , Jutta Kunz , Sindy Mojica

Recently it was shown that slowly rotating neutron stars exhibit an interesting correlation between their moment of inertia $I$, their quadrupole moment $Q$, and their tidal deformation Love number $\lambda$ (the I-Love-Q relations),…

General Relativity and Quantum Cosmology · Physics 2014-03-31 George Pappas , Theocharis A. Apostolatos

We calculate moment of inertia of neutron star with different exotic constituents such as hyperons and (anti)kaon condensates and study its variation with mass and spin frequency. The sets of equation of state, generated within the…

High Energy Astrophysical Phenomena · Physics 2017-06-14 S. S. Lenka , P. Char , S. Banik

A full-relativistic approach is used to compute the radius of the innermost stable circular orbit (ISCO), the Keplerian, frame-dragging, precession and oscillation frequencies of the radial and vertical motions of neutral test particles…

Solar and Stellar Astrophysics · Physics 2014-01-14 Andres F. Gutierrez-Ruiz , Cesar A. Valenzuela-Toledo , Leonardo A. Pachon

Universal relations that are insensitive to the equation of state (EoS) are useful in reducing the parameter space when measuring global quantities of neutron stars (NSs). In this paper, we reveal a new universal relation that connects the…

High Energy Astrophysical Phenomena · Physics 2023-08-22 Yong Gao , Lijing Shao , Jan Steinhoff

This thesis explores compact objects, particularly neutron stars, focusing on their properties, classification, and stability within the framework of general relativity. Two distinct studies are presented. The first study examines the…

General Relativity and Quantum Cosmology · Physics 2024-10-15 A. C. Khunt

Neutron stars are highly compact astrophysical objects and therefore of utmost relevance to learn about theories of gravity. Whereas the proper equation of state of the nuclear matter inside neutron stars is not yet known, and a wide range…

General Relativity and Quantum Cosmology · Physics 2022-04-28 Jutta Kunz

$f(R)$ theories of gravity are one of the most popular alternative explanations for dark energy and therefore studying the possible astrophysical implications of these theories is an important task. In the present paper we make a…

General Relativity and Quantum Cosmology · Physics 2015-06-23 Stoytcho S. Yazadjiev , Daniela D. Doneva , Kostas D. Kokkotas

In the present paper we examine gravitational wave asteroseismology relations for f-modes of rapidly rotating neutron stars. An approach different to the previous studies is employed - first, the moment of inertia is used instead of the…

Solar and Stellar Astrophysics · Physics 2015-12-09 Daniela D. Doneva , Kostas D. Kokkotas

We construct rapidly rotating neutron star models in scalar-tensor theories with a massive scalar field. The fact that the scalar field has nonzero mass leads to very interesting results since the allowed range of values of the coupling…

General Relativity and Quantum Cosmology · Physics 2016-11-15 Daniela D. Doneva , Stoytcho S. Yazadjiev
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