Slowly rotating topological neutron stars: universal relations and epicyclic frequencies
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Regular Article - Theoretical Physics
Slowly rotating topological neutron stars: universal relations and epicyclic frequencies Victor I. Danchev1,a , Daniela D. Doneva2,3 , Stoytcho S. Yazadjiev1,4 1
Department of Theoretical Physics, Faculty of Physics, Sofia University, 1164 Sofia, Bulgaria Theoretical Astrophysics, Eberhard Karls University of Tübingen, 72076 Tübingen, Germany 3 INRNE, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria 4 Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev Street 8, 1113 Sofia, Bulgaria
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Received: 15 July 2020 / Accepted: 13 September 2020 © The Author(s) 2020
Abstract In the modern era of abundant X-ray detections and the increasing momentum of gravitational waves astronomy, tests of general relativity in strong field regime become increasingly feasible and their importance for probing gravity cannot be understated. To this end, we study the characteristics of slowly rotating topological neutron stars in the tensormulti-scalar theories of gravity following the static study of this new type of compact objects by two of the authors. We explore the moment of inertia and verify that universal relations known from general relativity hold for this new class of compact objects. Furthermore, we study the properties of their innermost stable circular orbits and the epicyclic frequencies due to the latter’s hinted link to observational quantities such as quasi-periodic X-ray spectrum features.
1 Introduction Some of the most promising extensions of general relativity (GR) are based on the existence of additional fields which may contribute to Gravity either as additional energymomentum sources or even through direct coupling to it. While weak-field tests place strong restrictions to such extensions [1], these fields may have a strong impact on the properties of compact objects such as neutron stars and black holes, which places these objects among the best “laboratories” for experimental tests of the strong field regime of gravity [2–5]. The tensor-multi-scalar theories (TMST) are among the most promising and natural such extensions of General Relativity which are mathematically self-consistent and pass all known experimental and observational constraints [6,7]. TMST have a solid theoretical background – they are motivated for example by more fundamental theories, such as a e-mail:
theories trying to unify all physical interactions. They provide a broader theoretical framework as compared to scalartensor theories and posses richer phenomenology. A new type of neutron stars called topological neutron star was shown to exist in their framework [8] (see also [9] for the case of scalarization in such theories). In addition to the properties of classical neutron stars in GR, these compact objects carry an integer topological charge and exhibit new quantitative and qualitative properties. One of the most important properties of these solutions is that the scalar charge is zero and thus there is no scalar dipole radiation. This is in contrast with t
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