arXiv subjects

Daniela D. Doneva

Publications and source records attributed to Daniela D. Doneva.

At least 73 records · Page 4Linked to original sources

arXiv2020

Multi-scalar Gauss-Bonnet gravity -- hairy black holes and scalarization

In the present paper we consider multi-scalar extension of Einstein-Gauss-Bonnet gravity. We focus on multi-scalar Einstein-Gauss-Bonnet models whose target space is a three-dimensional maximally symmetric space, namely either $\mathbb{S}^3$, $\mathbb{H}^3$ or $\mathbb{R}^3$, and in the case when the map $\text{\it spacetime} \to \text{\it target space}$ is nontrivial. We prove numerically the existence of black holes in this class of models for several Gauss-Bonnet coupling functions, including the case of scalarization. We also perform systematic study of a variety of black hole characteristics and the space-time around them, such as the area of the horizon, the entropy and the radius of the photon sphere. One of the most important properties of the obtained solutions is that the scalar charge is zero and thus the scalar dipole radiation is suppressed which leads to much weaker observational constraints compared to the majority of modified theories possessing a scalar degree of freedom. For one of the coupling functions we could find branches of scalarized black holes which have a nontrivial structure -- there is non-uniqueness of the scalarized solutions belonging to a single branch and there is a region of the parameter space where most probably stable scalarized black holes coexist with the stable Schwarzschild black holes. Such a phenomena can have a clear observational signature.

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arXiv2020

Polar quasinormal modes of the scalarized Einstein-Gauss-Bonnet black holes

We study the polar quasinormal modes of spontaneously scalarized black holes in Einstein-Gauss-Bonnet theory. In previous works we showed that a set of nodeless solutions of the fundamental branch of the model studied in [1] are stable under both radial [2] and axial perturbations [3]. Here we calculate the polar quasinormal modes and show that this set of solutions is stable against the polar perturbations as well. Thus for a certain region of the parameter space the scalarized black holes are potentially stable physically interesting objects. The spectrum of the polar quasinormal modes differs both quantitatively and qualitatively from the Schwarzschild one which offers the possibility to test the Gauss-Bonnet theory via the future gravitational wave observations.

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arXiv2020

Stability of topological neutron stars

Tensor-multi-scalar theories (TMST) are among the most natural generalizations of Einstein's theory, they are mathematically self-consistent and free from pathologies. They pass through all the known observations but contrary to standard scalar-tensor theories, TMST offer extremely rich spectrum of solutions and allow for large deviations from general relativity. One of the most interesting objects in these theories are the topological neutron stars. The main goal in the present paper is to study their radial stability since many branches of solutions can exist even for a fixed value of the topological charge. It turns out that only one of these branches is stable for each value of the topological charge while all the rest are unstable. The stable branch is exactly the one that spans from small to large neutron star masses having as well moderate values of the radii in agreement with the observations. As expected, it becomes unstable at the maximum of the mass. The frequencies of the radial modes are examined and it turns out that in most cases the mode frequencies are larger than the general relativistic ones and they increase with the increase of the topological charge. All this shows that the topological neutron stars are viable astrophysical objects that should be explored further in order to determine their observational manifestations.

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arXiv2020

Non-topological spontaneously scalarized neutron stars in tensor-multi-scalar theories of gravity

In the present paper, we numerically construct new non-topological, spontaneously scalarized neutron stars in the tensor-multi-scalar theories of gravity whose target space is a three-dimensional maximally symmetric space, namely either $\mathbb{S}^3$, $\mathbb{H}^3$ or $\mathbb{R}^3$, and in the case of a nontrivial map $φ: \text{\it spacetime} \to \text{\it target space}$. The theories of gravity admitting scalarization are characterized by the fact that the field equations always admit the general relativistic solution but for certain ranges of the parameters space it loses stability and nonlinear development of a scalar field is observed. Thus, in order to determine the values of the parameters where such scalarization is possible we studied the stability of the general relativistic solution within the framework of the considered tensor-multi-scalar theories. Based on these results we could obtain a family of scalarized branches characterized by the number of the scalar field nodes. These branches bifurcate from the general relativistic solution at the points where new unstable modes appear and they are energetically more favorable over the pure Einstein solutions. Interestingly, in certain parameter ranges, we could obtain non-uniqueness within a single branch of scalarized solutions.

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arXiv2020

Relativistic stars in 4D Einstein-Gauss-Bonnet gravity

In the present paper we investigate the structure of relativistic stars in 4D Einstein-Gauss-Bonnet gravity. The mass-radius relations are obtained for realistic hadronic and for strange quark star equations of state, and for a wide range of the Gauss-Bonnet coupling parameter $α$. Even though the deviations from general relativity for nonzero values of $α$ can be large enough, they are still comparable with the variations due to different modern realistic equations of state if we restrict ourselves to moderate value of $α$. That is why the current observations of the neutron star masses and radii alone can not impose stringent constraints on the value of the parameter $α$. Nevertheless some rough constraints on $α$ can be put. The existence of stable stellar mass black holes imposes $\sqrtα\lesssim 2.6 {\rm km}$ for $α>0$ while the requirement that the maximum neutron star mass should be greater than two solar masses gives $\sqrt{|α|}\lesssim 3.9 {\rm km}$ for $α<0$. We also present an exact solution describing the structure of relativistic stars with uniform energy density in 4D Einstein-Gauss-Bonnet gravity.

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arXiv2020

Axial perturbations of the scalarized Einstein-Gauss-Bonnet black holes

We study the axial perturbations of spontaneously scalarized black holes in Einstein-Gauss-Bonnet (EGB) theories. We consider the nodeless solutions of the fundamental branch of the model studied in [1], which possesses a region of radially stable configurations, as shown in [2]. Here we show that almost all of the radially stable black holes are also stable under axial perturbations. When the axial potential is no longer strictly positive, we make use of the S-deformation method to show stability. As for the radial perturbations, hyperbolicity is lost below a certain critical horizon size for a fixed coupling constant. In the stable region, we determine the spectrum of the quasinormal modes by time evolution and by solving the associated time-independent eigenvalue problem.

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arXiv2019

Rotating tensor-multi-scalar solitons

In the context of a special class of tensor-multi-scalar theories of gravity for which the target-space metric admits an isometry under which the theory is invariant, we present rotating vacuum solutions, namely with no matter fields. These objects behave like nontopological solitons, whose primary stability is due to the conserved charge arising from the global symmetry. We consider theories with two and three scalar fields, different Gauss curvatures and quartic interaction coefficients. As it occurs for boson stars, their angular momentum is quantized.

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arXiv2019

Topological neutron stars in tensor-multi-scalar theories of gravity

In the present paper we demonstrate that there exists a new type of neutron stars in the tensor-multi-scalar theories of gravity. We call this new type of neutron stars topological neutron stars. In addition to the standard characteristics of the usual neutron stars the topological neutron stars are also characterized by a topological charge. We numerical construct explicit examples of topological neutron stars in tensor-multi-scalar theories whose target space is $\mathbb{S}^3$. Besides the topological charge the topological neutron stars also exhibit other attractive features which can place them among the realistic compact objects that could exist in Nature. For example they possess zero scalar charge and thus evades the strong binary pulsar constraints on the dipole scalar radiation.

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arXiv2019

Mixed configurations of tensor-multi-scalar solitons and neutron stars

In the present paper we consider special classes of massive tensor-multi-scalar theories of gravity whose target space metric admits Killing field(s) with a periodic flow. For such tensor-multi-scalar theories we show that there exist mixed configurations of tenor-multi-scalar solitons and relativistic (neutron) stars. The influence of the curvature of the target space on the structure of the mixed configurations is studied for two explicit scalar-tensor theories. The stability of the obtained compact objects is examined and it turns out that the stability region spans to larger central energy densities and central values of the scalar field compared to the pure neutron stars or the pure tensor-multi-scalar solitons.

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arXiv2019

Gauss-Bonnet black holes with a massive scalar field

In the present paper we consider the extended scalar-tensor-Gauss-Bonnet gravity with a massive scalar field. We prove numerically the existence of Gauss-Bonnet black holes for three different forms of the coupling function including the case of spontaneous scalarization. We have performed a systematic study of the black hole characteristics such as the area of the horizon, the entropy and the temperature for these coupling functions and compared them to the Schwarzschild solutions. The introduction of scalar field mass leads to a suppression of the scalar field and the increase of this mass brings the black holes closer to the Schwarzschild case. For linear and exponential coupling, a nonzero scalar field mass expands the domain of existence of black holes solutions. Larger deviations from the GR case are observed only for small masses and these differences decrease with the increase of the scalar field mass. In the case of a coupling function which leads to scalarization the scalar field mass has a significant influence on the bifurcation points where the scalarized black holes branch out of the Schwarzschild solution. The largest deviation from the case with a massless scalar field are observed for black hole masses close to the bifurcation point.

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arXiv2019

Orbital and epicyclic frequencies in massive scalar-tensor theory with self-interaction

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 the compact object my turn out an excellent way of probing spacetime in strong field regime. A promising candidate for doing so are the so-called quasi-periodic oscillations, observed in the X-ray light curves of some pulsars. Although the origin of those oscillations is unknown, one thing most of the models describing them have in common is that in one way or another they incorporate the radius of the innermost stable circular obit, and the orbital and the epicyclic frequencies of particles moving around the compact object. In this paper we study the aforementioned quantities in the context of massive scalar-tensor theory and massive scalar-tensor theory with self-interaction, both of which in strong regime allow for significant deviations from General relativity for values for the free parameters of the theory in correlation with the observations.

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arXiv2019

Axial quasi-normal modes of scalarized neutron stars with massive self-interacting scalar field

We study the axial quasi-normal modes of neutron stars in scalar-tensor theories with massive scalar field including a self-interacting term in the potential. Various realistic equations of state including nuclear, hyperonic and hybrid matter are employed. Although the effect of spontaneous scalarization of neutron stars can be very large, binary pulsar observations and gravitational wave detections significantly constrain the massless scalar-tensor theories. If we consider a properly chosen nonzero mass for the scalar field, though, the scalar-tensor parameters cannot be restricted by the observations, resulting in large deviation from pure general relativity. With this motivation in mind, we extend the universal relations for axial quasi-normal modes known in general relativity to neutron stars in massive scalar-tensor theories with self-interaction using a wide range of realistic EOS. We confirm the universality of the scaled frequency and damping time in terms of the compactness and scaled moment of inertia for neutron stars with and without massive scalarization.

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arXiv2019

A new type of dark compact objects in massive tensor-multi-scalar theories of gravity

In the present paper we consider special classes of tensor-multi-scalar theories of gravity whose target-space metric admits Killing field(s) with a periodic flow. For such tensor-multi-scalar theories we show that if the dynamics of the scalar fields is confined on the periodic orbits of the Killing field(s) then there exists a new type of compact objects - the tensor-multi-scalar solitons formed by a condensation of the gravitational scalars. The existence of the tensor-multi-scalar solitons is proven by solving the fully non-linear eigenvalue problem which follows from the dimensional reduction of the field equations of tensor-multi-scalar theories of gravity. The mass of the tensor-multi-scalar solitons can range at least from the mass of a neutron star to the mass of dark objects in the center of the galaxies in dependence of mass(es) of the gravitational scalars and which sector of massive scalars is excited. These facts show that the tensor-multi-scalar solitons could have important implications for the dark matter problem. The existence of the tensor-multi-scalar solitons points towards the possibility that the dark matter, or part of it, is made of condensed gravitational scalars.

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arXiv2018

Multipole moments and universal relations for scalarized neutron stars

In recent years there has been a surge of interest in what has come to be known as the `universal relations' between various global properties of neutron stars. These universal relations are equation of state independent relations between quantities such as the moment of inertia $I$, the tidal deformability or Love number $λ$, and the quadrupole $Q$ (I-Love-Q relations), or the relativistic multipole moments (3-hair relations). While I-Love-Q relations have been studied extensively in both general relativity and various alternatives, 3-hair relations have been studied only in general relativity. Recent progress on the definition of the multipole moments of a compact object in the case of scalar-tensor theories allows for the study of 3-hair relations in modified theories of gravity. Specifically, the aim of this work is to study them for scalarized stars in scalar-tensor theories with a massless scalar field that admit spontaneous scalarization. We find that the 3-hair relations between the mass and angular momentum moments that hold in general relativity hold for scalarized stars as well. The scalar moments also exhibit a universal behaviour, which is equation of state independent within one specific theory, but differs between different theories. Combining astrophysical observations one can in principle measure the different properties of scalarized neutron star and tell different theories apart.

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arXiv2018

Moment of inertia - mass universal relations for neutron stars in scalar-tensor theory with self-interacting massive scalar field

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 theories with self-interaction, for which significant deviations from General Relativity are allowed for values of the parameters that are in agreement with the observations. Moment of inertia-compactness relations are examined for different normalisation of the moment of inertia. It is shown that for all studied cases the deviations from EOS universality are small for the examined equations of state. On the other hand the scalarization can lead to large deviations from the general relativistic universal relations for values of the parameters that are in agreement with the current observations that can be potentially used to set further test the scalar-tensor theories.

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arXiv2018

Quasinormal modes of compact objects in alternative theories of gravity

We address quasinormal modes of compact objects in several alternative theories of gravity. In particular, we focus on black holes and neutron stars with scalar hair. We consider black holes in dilaton-Einstein-Gauß-Bonnet theory, and in a generalized scalar-Einstein-Gauß-Bonnet theory. In the latter case scalarized black holes arise, and we study the stability of the different branches of solutions. In particular, we discuss how the spectrum of quasinormal modes is changed by the presence of a non-trivial scalar field outside the black hole horizon. We discuss the existence of an (effective) minimum mass in these models, and how the spectrum of modes becomes richer as compared to general relativity, when a scalar field is present. Subsequently we discuss the effect of scalar hair for realistic neutron star models. Here we consider $R^2$ gravity, scalar-tensor theory, a particular subsector of Horndeski theory with a non-minimal derivative coupling, and again dilatonic-Einstein-Gauß-Bonnet theory. Because of the current lack of knowledge on the internal composition of the neutron stars, we focus on universal relations for the quasinormal modes, that are largely independent of the equations of state and thus the matter content of the stars.

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arXiv2018

Charged Gauss-Bonnet black holes with curvature induced scalarization in the extended scalar-tensor theories

Recently new scalarized black hole solutions were constructed in the extended scalar-tensor-Gauss-Bonnet gravity, where the scalar field is sourced by the curvature of the spacetime via the Gauss-Bonnet invariant. A natural extension of these results is to consider the case of nonzero black hole charge. In addition we have explored a large set of coupling functions between the Gauss-Bonnet invariant and the scalar field, that was not done until now even in the uncharged case, in order to understand better the behavior of the solutions and the deviations from pure general relativity. The results show that in the case of nonzero black hole charge two bifurcation points can exist - one at larger masses where the scalarized solutions bifurcated from the Reissner-Nordstrom one, and one at smaller masses where the scalar charge of the solutions decreases again to zero and the branch merges again with the GR one. All of the constructed scalarized branches do not reach an extremal limit. We have examined the entropy of the black holes with nontrivial scalar field and it turns out, that similar to the uncharged case, the fundamental branch which possesses scalar field with no nodes is thermodynamically favorable over the Reissner-Nordstrom one for the considered coupling functions, while the rest of the branches possessing scalar field with one or more zeros have lower entropy compared to the GR case and they are supposed to be unstable.

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arXiv2018

Differentially rotating neutron stars in scalar-tensor theories of gravity

We present the first numerical models of differentially rotating stars in alternative theories of gravity. We chose a particular class of scalar-tensor theories of gravity that is indistinguishable from GR in the weak field regime but can lead to significant deviations when strong fields are considered. We show that the maximum mass that a differentially rotating neutron star can sustain increases significantly for scalarized solutions and such stars can reach larger angular momenta. In addition, the presence of a nontrivial scalar field has the effect of increasing the required axis ratio for reaching a given value of angular momentum, when compared to a corresponding model of same rest mass in general relativity. We find that the scalar field also makes rapidly rotating models less quasi-toroidal than their general-relativistic counterparts. For large values of the angular momentum and values of the coupling parameter that are in agreement with the observations, we find a second turning point for scalarized models along constant angular momentum sequences, which could have interesting implications for the stability of remnants created in a binary neutron star merger.

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