arXiv subjects

Daniela D. Doneva

Publications and source records attributed to Daniela D. Doneva.

At least 55 records · Page 3Linked to original sources

arXiv2022

Dynamical descalarization with a jump during black hole merger

Black holes in scalar-Gauss-Bonnet gravity are prone to scalarization, that is a spontaneous development of scalar hair for strong enough spacetime curvature while the weak field regime of the theory coincides with general relativity. Since large spacetime curvature is associated with smaller black hole masses, the merging of black holes can lead to dynamical descalarization. This is a spontaneous release of the scalar hair of the newly formed black hole in case its mass is above the scalarization threshold. Depending on the exact form of the Gauss-Bonnet coupling function, the stable scalarized solutions can be either continuously connected to the Schwarzschild black hole, or the transitions between the two can happen with a jump. By performing simulations of black hole head-on collisions in scalar-Gauss-Bonnet gravity prone to dynamical descalization, we have demonstrated that such a jump can be clearly observed in the accumulated gravitational wave data of multiple merger events with different masses. The distinct signature in the gravitational wave signal will share similarities with the effects expected from first order matter phase transitions happening during neutron star binary mergers.

gr-qc↗

arXiv2022

Tensor-Multi-Scalar Gravity: Equations of Motion to 2.5 post-Newtonian Order

In the present paper we take a step towards the generalization of the post-Newtonian formalism to tensor-multi-scalar theories. These are theories where we have more than one scalar field being mediators of the gravitational interaction in addition to the spacetime metric. They are very natural extensions of Einstein's gravity allowing for the existence of new classes of compact objects and offering interesting phenomenology reaching far beyond the single scalar field theories. We calculate the expansion up to 2.5 post-Newtonian order in the near zone using the so-called direct integration of the relaxed Einstein equations formalism and derive the equation of motion. This work is the first step towards the calculation of gravitational waveforms in tensor-multi-scalar theories.

gr-qc↗

arXiv2022

Radial perturbations of scalar-Gauss-Bonnet black holes beyond spontaneous scalarization

Spontaneous scalarization of black holes in scalar-Gauss-Bonnet (sGB) gravity is a very interesting phenomenon allowing black holes to circumvent the no-hair theorem and acquire scalar hair while leaving the weak field regime of the theory practically unaltered. It was recently shown that if we allow for a different form of the coupling between the scalar field and the Gauss-Bonnet invariant, a new type of scalarization is possible beyond the standard one that can be excited only nonlinearly. In this case the spectrum of black hole solutions can be more complicated, naturally opening the question about their stability. The goal of the present paper is to study this question including the possible loss of hyperbolicity of the radial perturbation equation. We show that one of the nonlinearly scalarized phases of the Schwarzschild black hole can be stable and hyperbolic for large enough black hole masses. These results establish the studied hairy black holes as viable astrophysical candidates.

gr-qc↗

arXiv2022

Equatorial extreme-mass-ratio inspirals in Kerr black holes with scalar hair spacetimes

In this work we analyze some judiciously chosen solutions of Kerr Black Holes with Scalar Hair (KBHsSH) of special interest for Gravitational Wave (GW) events originated from Extreme Mass Ratio Inspirals (EMRIs). Because of the off-center distribution of energy density, these spacetimes are warped in such a way that not all metric functions behave monotonically on the equatorial plane as in the exterior region of Kerr black holes (KBHs). This has great impact on the orbital parameters, which in turn affects the imprints on signals descendant from EMRIs in a adiabatic evolution. By investigating circular obit parameters, we unveil what qualitative features could be present in the signals that are new and distinct compared to KBHs, and we evolve some inspirals by employing the usual quadrupole formula approximation. We show that the frequencies of the emitted signals behave nonmonotonically, i.e. they can backward chirp, and for some particular cases they can become arbitrarily small, falling below LISA's sensibility range. Finally, we present two sets of waveforms produced by a noncircular EMRI in which the compact object (CO) follows a type of geodesic motion typically present in spacetimes with a static ring (SR), in which the compact object is periodically momentarily at rest.

gr-qc↗

arXiv2021

Axial perturbations of hairy Gauss-Bonnet black holes with massive self-interacting scalar field

We study the axial quasinormal modes of hairy black holes in Gauss-Bonnet gravity with massive self-interacting scalar field. Two coupling functions of the scalar field to the Gauss-Bonnet invariant are adopted with one of them leading to black hole scalarization. The axial perturbations are studied via time evolution of the perturbation equation, and the effect of the scalar field mass and the self-interaction constant on the oscillation frequency and damping time is examined. We study as well the effect of nonzero scalar field potential on the critical point at which the perturbation equation loses hyperbolicity in the case of black hole scalarization. The results show that the non-zero scalar field potential extends the range of parameters where such loss of hyperbolicity is observed thus shrinking the region of stable black hole existence. This will have an important effect on the nonlinear dynamical simulation studies in massive scalar Gauss-Bonnet gravity.

gr-qc↗

arXiv2021

Beyond the spontaneous scalarization: New fully nonlinear dynamical mechanism for formation of scalarized black holes

In the present letter we show the existence of a fully nonlinear dynamical mechanism for the formation of scalarized black holes which is different from the spontaneous scalarization. We consider a class of scalar-Gauss-Bonnet gravity theories within which no tachyonic instability can occur. Although the Schwarzschild black holes are linearly stable against scalar perturbations, we show dynamically that for certain choices of the coupling function they are unstable against nonlinear scalar perturbations. This nonlinear instability leads to the formation of new black holes with scalar hair. The fully nonlinear and self-consistent study of the equilibrium black holes reveals that the spectrum of solutions is more complicated and more than one scalarized branch can exist. We have also considered classes of scalar-Gauss-Bonnet theories where both the standard and the nonlinear scalarization can be present, and they are smoothly connected that completes in an interesting way the picture of black hole scalarization. The fully nonlinear (de)scalarization of a Schwarzschild black hole will always happen with a jump because the stable "scalarized branch" is not continuously connected to the Schwarzschild one that can leave clear observational signatures.

gr-qc↗

arXiv2021

Dynamical Formation of Scalarized Black Holes and Neutron Stars through Stellar Core Collapse

In a certain class of scalar-Gauss-Bonnet gravity, the black holes and the neutron stars can undergo spontaneous scalarization -- a strong gravity phase transition triggered by a tachyonic instability due to the nonminimal coupling between the scalar field and the spacetime curvature. Studies of this phenomenon have, so far, been restricted mainly to the study of the tachyonic instability and stationary scalarized black holes and neutron stars. To date, no realistic physical mechanism for the formation of isolated scalarized black holes and neutron stars has been proposed. We study, for the first time, the spherically symmetric fully nonlinear stellar core collapse to a black hole and a neutron star in scalar-Gauss-Bonnet theories allowing for a spontaneous scalarization. We show that the core collapse can produce scalarized black holes and scalarized neutron stars starting with a nonscalarized progenitor star. The possible paths to reach the end (non)scalarized state are quite rich leading to interesting possibilities for observational manifestations.

gr-qc↗

arXiv2021

Oscillation dynamics of scalarized neutron stars

Scalar-tensor theories are well studied extensions of general relativity that offer deviations which are yet within observational boundaries. We present the time evolution equations governing the perturbations of a nonrotating scalarized neutron star, including a dynamic spacetime as well as scalar field within the framework of such scalar-tensor theories. We employ a theory that allows for a massive scalar field or a self-interaction term and we study the impact of those parameters on the non-axisymmetric $f$-mode. The time evolution approach allows for a comparatively simple implementation of the boundary conditions. We find that the $f$-mode frequency is no longer a simple function of the star's average density when a scalar field is present. We also evaluate the accuracy of different variants of the Cowling approximation commonly used in previous studies of neutron star oscillation modes in alternative theories of gravity and demonstrate that it can give us not only qualitatively correct results, but in some cases also good quantitative estimates of the oscillations frequencies.

gr-qc↗

arXiv2021

Nonlinear evolution and non-uniqueness of scalarized neutron stars

It was recently shown, that in a class of tensor-multi-scalar theories of gravity with a nontrivial target space metric, there exist scalarized neutron star solutions. An important property of these compact objects is that the scalar charge is zero and therefore, the binary pulsar experiments can not impose constraints based on the absence of scalar dipole radiation. Moreover, the structure of the solutions is very complicated. For a fixed central energy density up to three neutron star solutions can exist -- one general relativistic and two scalarized, that is quite different from the scalarization in other alternative theories of gravity. In the present paper we address the stability of these solutions using two independent approaches -- solving the linearized radial perturbation equations and performing nonlinear simulations in spherical symmetry. The results show that the change of stability occurs at the maximum mass models and all solutions before that point are stable. This leads to the interesting consequence that there exists a stable part of the scalarized branch close to the bifurcation point where the mass of the star increases with the decrease of the central energy density.

gr-qc↗

arXiv2021

Spontaneously scalarized black holes in dynamical Chern-Simons gravity: dynamics and equilibrium solutions

In the present paper, we construct spontaneously scalarized rotating black hole solutions in dynamical Chern-Simons (dCS) gravity by following the scalar field evolution in the decoupling limit. For the range of parameters where the Kerr black hole becomes unstable within dCS gravity the scalar field grows exponentially until it reaches an equilibrium configuration that is independent of the initial perturbation. Interestingly, the $\mathbb{Z}_2$ symmetry of the scalar field is broken and a strong maximum around only one of the rotational axes can be observed. The black hole scalar charge is calculated for two coupling functions suggesting that the main observations would remain qualitatively correct even if one considers coupling functions/coupling parameters producing large deviations from the Kerr solution beyond the decoupling limit approximation.

gr-qc↗

arXiv2021

Circular Orbits Structure and Thin Accretion Disks around Kerr Black Holes with Scalar Hair

In this paper we first investigate the equatorial circular orbit structure of Kerr black holes with scalar hair (KBHsSH) and highlight their most prominent features which are quite distinct from the exterior region of ordinary bald Kerr black holes, i.e. peculiarities that arise from the combined bound system of a hole with an off-center, self-gravitating distribution of scalar matter. Some of these traits are incompatible with the thin disk approach, thus we identify and map out various regions in the parameter space respectively. All the solutions for which the stable circular orbital velocity (and angular momentum) curve is continuous are used for building thin and optically thick disks around them, from which we extract the radiant energy fluxes, luminosities and efficiencies. We compare the results in batches with the same spin parameter $j$ but different normalized charges, and the profiles are richly diverse. Because of the existence of a conserved scalar charge, $Q$, these solutions are non-unique in the $(M, J)$ parameter space. Furthermore, $Q$ cannot be extracted asymptotically from the metric functions. Nevertheless, by constraining the parameters through different observations, the luminosity profile could in turn be used to constrain the Noether charge and characterize the spacetime, should KBHsSH exist.

astro-ph.HE↗

arXiv2021

On the dynamics of the nonrotating and rotating black hole scalarization

Even though black hole scalarization is extensively studied recently, little has been done in the direction of understanding the dynamics of this process, especially in the rapidly rotating regime. In the present paper, we focus exactly on this problem by considering the nonlinear dynamics of the scalar field while neglecting the backreaction on the spacetime metric. This approach has proven to give good results in various scenarios and we have explicitly demonstrated its accuracy for nonrotating black holes especially close to the bifurcation point. We have followed the evolution of a black hole from a small initial perturbation, throughout the exponential growth of the scalar field followed by a subsequent saturation to an equilibrium configuration. As expected, even though the emitted signal and the time required to scalarize the black hole are dependent on the initial perturbation, the final stationary state that is reached is independent on the initial data.

gr-qc↗

arXiv2020

Non-linear stability of soliton solutions for massive tensor-multi-scalar-theories

The aim of this paper is to study the stability of soliton-like static solutions via non-linear simulations in the context of a special class of massive tensor-multi-scalar-theories of gravity whose target space metric admits Killing field(s) with a periodic flow. We focused on the case with two scalar fields and maximally symmetric target space metric, as the simplest configuration where solitonic solutions can exist. In the limit of zero curvature of the target space $κ= 0$ these solutions reduce to the standard boson stars, while for $κ\ne 0$ significant deviations can be observed, both qualitative and quantitative. By evolving these solitonic solutions in time, we show that they are stable for low values of the central scalar field $ψ_c$ while instability kicks in with the increase of $ψ_c$. Specifically, in the stable region, the models oscillate with a characteristic frequency related to the fundamental mode. Such frequency tends to zero with the approach of the unstable models and eventually becomes imaginary when the solitonic solutions lose stability. As expected from the study of the equilibrium models, the change of stability occurs exactly at the maximum mass point, which was checked numerically with a very good accuracy.

gr-qc↗

arXiv2020

Rotating tensor-multiscalar black holes with two scalars

We construct hairy black hole solutions in a particular set of tensor-multi-scalar theories of gravity for which the target-space admits a Killing vector field with periodic flow that is furthermore the generator of a one-parameter family of point transformations which leave the whole theory invariant. As for black holes with scalar hair in general relativity, these must be rotating objects as the scalarization arises from superradiant instability. We analyze the most fundamental rotating solutions (winding number $m=1$) in five theories which differ from each other by the value of the Gauss curvature of the target-space. The domain of existence of these solutions is investigated, as well as their hairyness, horizon area and deformation factor.

gr-qc↗

arXiv2020

Constraining the equation of state in modified gravity via universal relations

Modern multi-messenger astronomical observations and heavy ion experiments provide new insights into the structure of compact objects. Nevertheless, much ambiguity remains when it comes to super dense matter above the nuclear saturation density such as that found within neutron stars. This work explores equation of state (EOS)-independent universal relations between the physical parameters of static neutron stars at the maximum-mass point, previously proven for General Relativity (GR) and used to constraint EOS candidates within the GR framework. We explore 53 different EOS candidates and prove that these relations hold for scalarized neutron stars in massive scalar-tensor theories, exploring also the effect of non-baryonic EOS. We further show that the relation's fit parameters are highly dependent on the theory's parameters. On the basis of these relations multiple constraints on the EOS can be derived and it turns out that they can be significantly different than the GR ones. This demonstrates the importance of taking into account the modified gravity effects even when imposing constraints on the EOS.

gr-qc↗

arXiv2020

Spin-induced scalarization of Kerr black holes with a massive scalar field

In the present paper we study the onset of the spin-induced scalarization of a Kerr black hole in scalar-Gauss-Bonnet gravity with a massive scalar field. Our approach is based on a (2+1) time evolution of the relevant linearized scalar field perturbation equation. We examine the region where the Kerr black hole becomes unstable giving rise to new scalarized rotating black holes with a massive scalar field. With increasing of the scalar field mass, the minimum value of the Gauss-Bonnet coupling parameter at which scalarization is possible, increases and thus the instability region shrinks. Interestingly, the introduction of scalar field mass does not change the critical minimal value of the black hole angular momentum $a_{\rm crit}/M$ where the instability of the Kerr black hole develops.

gr-qc↗

arXiv2020

Black hole scalarization induced by the spin -- 2+1 time evolution

The scalarization is a very interesting phenomena allowing to endow a compact object with scalar hair while leaving all the predictions in the weak field limit unaltered. In Gauss-Bonnet gravity the source of the scalar field can be the curvature of the space-time. It was recently shown that for a particular type of coupling function between the scalar field and the Gauss-Bonnet invariant, spin-induced black hole scalarization is possible. In the present paper we study this phenomenon by performing a 2+1 time evolution of the relevant linearized scalar field perturbation equation and examine the region where the Kerr black hole becomes unstable giving rise to new scalarized rotating black holes. This is more direct numerical approach to study the development of spin-induced scalarization and it can serve as an independent check of the previous results.

gr-qc↗

arXiv2020

Slowly rotating topological neutron stars -- universal relations and epicyclic frequencies

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 tensor-multi-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 (ISCO) and the epicyclic frequencies due to the latter's hinted link to observational quantities such as quasi-periodic X-ray spectrum features.

gr-qc↗