arXiv subjects

Jutta Kunz

Publications and source records attributed to Jutta Kunz.

At least 73 records · Page 4Linked to original sources

arXiv2021

(Un)balanced holographic superconductors with electric and spin motive force coupling

We study holographic phase transitions in (2+1) dimensions that possess interacting phases which result from a direct coupling between the two U(1) gauge fields. This can be interpreted as a non-minimal interaction between the electric and spin motive forces of the dual model. We first present a new analytical solution of the Einstein-Maxwell equations that describes a black hole with charge non-equivalent to the sum of the asymptotic charges of the two U(1) gauge fields and briefly discuss formation of uncharged scalar hair on this solution. We then study the formation of charged scalar hair on an uncharged black hole background and discuss the dual description of balanced as well as unbalanced superconductors.

hep-th↗

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

Wormhole solutions with NUT charge in higher curvature theories

We present wormholes with a Newman-Unti-Tamburino (NUT) charge that arise in certain higher curvature theories, where a scalar field is coupled to a higher curvature invariant. For the invariants we employ i) a Gauss-Bonnet term and ii) a Chern-Simons term, which then act as source terms for the scalar field. We map out the domain of existence of wormhole solutions by varying the coupling parameter and the scalar charge for a set of fixed values of the NUT charge. The domain of existence for a given NUT charge is then delimited by the set of scalarized nutty black holes, a set of wormhole solutions with a degenerate throat and a set of singular solutions.

gr-qc↗

arXiv2021

Polish Doughnuts around Scalarized Kerr Black Holes

In this work we aim to investigate non-mainstream thick tori configurations around Kerr Black Holes with Scalar Hair (KBHsSH). For that goal, we provide a first approach using constant specific angular momentum non-self-gravitating Polish doughnuts. Through a series of examples, we show the feasibility of new topologies, such as double-centered tori with two cusps as well as similar structures as the ones found for rotating Boson Stars (BSs), namely tori endowed with two centers and a single cusp. These KBHsSH' solutions are also shown to possibly house static surfaces, associated to the static rings present in these spacetimes. Through this report we highlight the differences between these fluid configurations when housed by some KBHsSH examples, standard Kerr black holes and rotating BSs.

gr-qc↗

arXiv2021

Polar Quasinormal Modes of Neutron Stars in Massive Scalar-Tensor Theories

We study polar quasinormal modes of relativistic stars in scalar-tensor theories, where we include a massive gravitational scalar field and employ the standard Brans-Dicke coupling function. For the potential of the scalar field we consider a simple mass term as well as a potential associated with $R^2$ gravity. The presence of the scalar field makes the spectrum of quasinormal modes much richer than the spectrum in General Relativity. We here investigate radial modes ($l=0$) and quadrupole modes ($l=2$). The general relativistic $l=0$ normal modes turn into quasinormal modes in scalar-tensor theories, that are able to propagate outside of the stars. In addition to the pressure-led modes new scalar-led $ϕ$-modes arise. We analyze the dependence of the quasinormal mode frequencies and decay times on the scalar field mass.

gr-qc↗

arXiv2021

Quasi-periodic Oscillations in Rotating Ellis Wormhole Spacetimes

We analyze the properties of the circular orbits for massive particles in the equatorial plane of symmetric rotating Ellis wormholes. In particular, we obtain the orbital frequencies and the radial and vertical epicyclic frequencies, and consider their lowest parametric, forced and Keplerian resonances. These show that quasi-periodic oscillations in accretion disks around symmetric rotating Ellis wormholes have many distinct properties as compared to quasi-periodic oscillations in accretion disks around rotating Teo wormholes and the Kerr black hole. Still we can distinguish some common features which appear in wormhole spacetimes as opposed to black holes. The most significant ones include the possibility of excitation of stronger resonances such as lower order parametric and forced resonances and the localization of these resonances deep in the region of strong gravitational interaction near the wormhole throat, which will lead to further amplification of the signal.

gr-qc↗

arXiv2021

Scalarized Black Holes

Black holes represent outstanding astrophysical laboratories to test the strong gravity regime, since alternative theories of gravity may predict black hole solutions whose may differ distinctly from those of General Relativity. When higher curvature terms are included in the gravitational action as, for instance, in the form of the Gauss-Bonnet term coupled to a scalar field, scalarized black holes result. Here we discuss several types of scalarized black holes and some of their properties.

gr-qc↗

arXiv2021

Tidal effects in the motion of gas clouds around boson stars

We report simulations regarding tidal disruption clouds orbiting spherically symmetric compact boson stars in two different regimes. First we consider clouds in three different bound orbits close to the boson star and analyze the mechanisms of debris formation for these. We infer from the simulations that the lifetimes of these hot-spots are longer for circularly orbiting clouds than for clouds on eccentric orbits. Next we compare the evolution of more extended and less dense clouds on circular orbits around a boson star and a Schwarzschild black hole. As an outcome of the simulations we observe the formation of a ring-like structure around the boson star endowed with a spiralling shock structure and a constant thermal bremsstrahlung total luminosity. This final configuration contrasts strongly with the black hole scenario where the gas is totally captured behind the event horizon.

astro-ph.HE↗

arXiv2021

Retrograde Polish Doughnuts around Boson Stars

We investigate Polish doughnuts with a uniform constant specific angular momentum distribution in the space-times of rotating boson stars. In such space-times thick tori can exhibit unique features not present in Kerr space-times. For instance, in the context of retrograde tori, they may possess two centers connected or not by a cusp. Rotating boson stars also feature a static ring, neither present in Kerr space-times. This static ring consists of static orbits, where particles are at rest with respect to a zero angular momentum observer at infinity. Here we show that the presence of a static ring allows for an associated static surface of a retrograde thick torus, where inside the static surface the fluid moves in prograde direction. We classify the retrograde Polish doughnuts and present several specific examples.

gr-qc↗

arXiv2021

Quasi-periodic oscillations from the accretion disk around rotating traversable wormholes

We study the quasi-periodic oscillations from the accretion disk around rotating traversable wormholes by means of the resonance models. We investigate the linear stability of the circular geodesic orbits in the equatorial plane for a general class of wormhole geometries deriving analytical expressions for the epicyclic frequencies. Since wormholes can often mimic black holes in the astrophysical observations, we analyze the properties of the quasi-circular oscillatory motion in comparison with the Kerr black hole. We demonstrate that wormholes possess distinctive features, which can be observationally significant. It is characteristic for the Kerr black hole that the orbital and the epicyclic frequencies obey a constant ordering in the whole range of the spin parameter. In contrast, for wormhole spacetimes we can have various types of orderings between the frequencies in the different regions of the parametric space. This enables the excitation of much more diverse types of resonances including parametric and forced resonances of lower order, which could lead to stronger observable signals. In addition, for co-rotating orbits the resonances can be excited in a very close neighbourhood of the wormhole throat for a wide range of values of the angular momentum, making wormholes a valuable laboratory for testing strong gravity.

gr-qc↗

arXiv2021

Spontaneously vectorized Einstein-Gauss-Bonnet black holes

We construct spontaneously vectorized black holes where a real vector field is coupled to the Gauss-Bonnet invariant. We employ three coupling functions for the vector field, and determine the respective domains of existence of the vectorized black holes. These domains of existence are bounded by the marginally stable Schwarzschild black holes and the critical vectorized black holes. We also address the effects of a mass term. For a given black hole mass the horizon radius is smaller for the vectorized black holes than for the Schwarzschild black holes. Since the vector field vanishes at the horizon, there is no contribution from the Gauss-Bonnet term to the entropy of the vectorized black holes.

gr-qc↗

arXiv2021

Spin-induced black hole scalarization in Einstein-scalar-Gauss-Bonnet theory

We construct black hole solutions with spin-induced scalarization in a class of models where a scalar field is quadratically coupled to the topological Gauss-Bonnet term. Starting from the tachyonically unstable Kerr solutions, we obtain families of scalarized black holes such that the scalar field has either even or odd parity, and we investigate their domain of existence. The scalarized black holes can violate the Kerr rotation bound. We identify "critical" families of scalarized black hole solutions such that the expansion of the metric functions and of the scalar field at the horizon no longer allows for real coefficients. For the quadratic coupling considered here, solutions with spin-induced scalarization are entropically favored over Kerr solutions with the same mass and angular momentum.

gr-qc↗

arXiv2020

Ellis Wormholes in Anti-De Sitter Space

We construct traversable wormholes with anti-de Sitter asymptotics supported by a phantom field. These wormholes are massless and symmetric with respect to reflection of the radial coordinate $η\to - η$. Their circumferential radius decreases monotonically from radial infinity to their single throat. Analogous to their asymptotically flat counterparts, these anti-de Sitter wormholes possess an unstable radial mode.

gr-qc↗

arXiv2020

Scalarized nutty wormholes

We construct scalarized wormholes with a NUT charge in higher curvature theories. We consider both Einstein-scalar-Gauss-Bonnet and Einstein-scalar-Chern-Simons theories, following a recent paper by Brihaye et al. [1], where spontaneously scalarised Schwarzschild-NUT solutions were studied. By varying the coupling parameter and the scalar charge we determine the domain of existence of the scalarized nutty wormholes, and their dependence on the NUT charge. In the Gauss-Bonnet case the known set of scalarized wormholes [2] is reached in the limit of vanishing NUT charge. In the Chern-Simons case, however, the limit is peculiar, since with vanishing NUT charge the coupling constant diverges. We focus on scalarized nutty wormholes with a single throat and study their properties. All these scalarized nutty wormholes feature a critical polar angle, beyond which closed timelike curves are present.

gr-qc↗

arXiv2020

Perturbed black holes in Einstein-dilaton-Gauss-Bonnet gravity: Stability, ringdown, and gravitational-wave emission

Gravitational waves emitted by distorted black holes---such as those arising from the coalescence of two neutron stars or black holes---carry not only information about the corresponding spacetime but also about the underlying theory of gravity. Although general relativity remains the simplest, most elegant and viable theory of gravitation, there are generic and robust arguments indicating that it is not the ultimate description of the gravitational universe. Here we focus on a particularly appealing extension of general relativity, which corrects Einstein's theory through the addition of terms which are second order in curvature: the topological Gauss-Bonnet invariant coupled to a dilaton. We study gravitational-wave emission from black holes in this theory, and {\bf(i)} find strong evidence that black holes are linearly (mode) stable against both axial and polar perturbations; {\bf(ii)} discuss how the quasinormal modes of black holes can be excited during collisions involving black holes, and finally {\bf(iii)} show that future ringdown detections with large signal-to-noise ratio would improve current constraints on the coupling parameter of the theory.

gr-qc↗

arXiv2020

Critical solutions of scalarized black holes

We consider charged black holes with scalar hair obtained in a class of Einstein-Maxwell-scalar models, where the scalar field is coupled to the Maxwell invariant with a quartic coupling function. Besides the Reissner-Nordström black holes, these models allow for black holes with scalar hair. Scrutinizing the domain of existence of these hairy black holes, we observe a critical behavior: A limiting configuration is encountered at a critical value of the charge, where spacetime splits into two parts: an inner spacetime with a finite scalar field and an outer extremal Reissner-Nordström spacetime. Such a pattern was first observed in the context of gravitating non-Abelian magnetic monopoles and their hairy black holes.

gr-qc↗

arXiv2020

Quasinormal modes of hot, cold and bald Einstein-Maxwell-scalar black holes

Einstein-Maxwell-scalar models allow for different classes of black hole solutions, depending on the non-minimal coupling function $f(ϕ)$ employed, between the scalar field and the Maxwell invariant. Here, we address the linear mode stability of the black hole solutions obtained recently for a quartic coupling function, $f(ϕ)=1+αϕ^4$ [1]. Besides the bald Reissner-Nordström solutions, this coupling allows for two branches of scalarized black holes, termed cold and hot, respectively. For these three branches of black holes we calculate the spectrum of quasinormal modes. It consists of polar scalar-led modes, polar and axial electromagnetic-led modes, and polar and axial gravitational-led modes. We demonstrate that the only unstable mode present is the radial scalar-led mode of the cold branch. Consequently, the bald Reissner-Nordström branch and the hot scalarized branch are both mode-stable. The non-trivial scalar field in the scalarized background solutions leads to the breaking of the degeneracy between axial and polar modes present for Reissner-Nordström solutions. This isospectrality is only slightly broken on the cold branch, but it is strongly broken on the hot branch.

gr-qc↗

arXiv2020

Ultra long lived quasinormal modes of neutron stars in massive scalar-tensor gravity

The spectrum of frequencies and characteristic times that compose the ringdown phase of gravitational waves emitted by neutron stars carries information about the matter content (the equation of state) and the underlying theory of gravity. Typically, modified theories of gravity introduce additional degrees of freedom/fields, such as scalars, which result in new families of modes composing the ringdown spectrum. Simple but physically promising candidates are scalar-tensor theories, which effectively introduce an additional massive scalar field (i.e. an ultra-light boson) that couples non-minimally to gravity, resulting in scalarized neutron stars. Here we present the first calculation of the full ringdown spectrum in such theories. We show that the ringdown spectrum of neutron stars with ultra-light bosons is much richer and fundamentally different from the spectrum in general relativity and that it possesses propagating ultra long lived modes.

gr-qc↗