In Cardoso et al. 6 it was claimed that quasinormal modes which any stationary, spherically symmetric and asymptotically flat black hole emits in the eikonal regime are determined by the parameters ...of the circular null geodesic: the real and imaginary parts of the quasinormal mode are multiples of the frequency and instability timescale of the circular null geodesics respectively. We shall consider asymptotically flat black hole in the Einstein–Lovelock theory, find analytical expressions for gravitational quasinormal modes in the eikonal regime and analyze the null geodesics. Comparison of the both phenomena shows that the expected link between the null geodesics and quasinormal modes is violated in the Einstein–Lovelock theory. Nevertheless, the correspondence exists for a number of other cases and here we formulate its actual limits.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
It is well known that a hypothetical compact object that looks like an Einsteinian (Schwarzschild or Kerr) black hole everywhere except a small region near its surface should have the ringdown ...profile predicted by the Einstein theory at early and intermediate times, but modified by the so-called echoes at late times. A similar phenomenon appears when one considers an Einsteinian black hole and a shell of matter placed at some distance from it, so that astrophysical estimates could be made for the allowed mass of the black hole environment. While echoes for both systems have been extensively studied recently, no such analysis has been done for a system featuring phenomena simultaneously, that is, echoes due to new physics near the surface/event horizon and echoes due to matter at some distance from the black hole. Here, following Damour and Solodukhin Phys. Rev. D 76, 024016 (2007) and Cardoso et al. Phys. Rev. Lett. 116, 171101 (2016), we consider a traversable wormhole obtained by identifying two Schwarzschild metrics with the same mass M at the throat, which is near the Schwarzschild radius, and add a nonthin shell of matter at a distance. This allows us to understand how the echoes of the surface of the compact object are affected by the astrophysical environment at a distance. The straightforward calculations for the time-domain profiles of such a system support the expectations that if the echoes are observed, they should most probably be ascribed to some new physics near the event horizon rather than some “environmental” effect.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
We employ the minimal geometric deformation approach to gravitational decoupling (MGD-decoupling) in order to generate an exact anisotropic and non-uniform version of the ultracompact Schwarzschild ...star, or 'gravastar', proposed by Mazur and Mottola. This new system represents an ultracompact configuration of radius whose interior metric can be matched smoothly to a conformally deformed Schwarzschild exterior. Remarkably, the model satisfies some of the basic requirements to describe a physically acceptable stellar model, such as a positive density everywhere and decreasing monotonously from the centre, as well as a non-uniform and monotonic pressure.
Kiselev metric in the static and rotating form is widely used to test different aspects of the dark energy (
DE
) effects. We consider a
DE
Kiselev spacetime, predicting the reduction to the Kerr ...black hole (
BH
) solution under suitable conditions on the
DE
parameters and in this frame we study the effects of the dark energy on
BHs
and disks accretion. Elaborating a close comparison with the limiting vacuum Kerr spacetime, we focus on thick accretion disks around the central
BH
in the Kiselev solution, both co-rotating and counter-rotating with respect the central
BH
. We examine different aspects of
BH
accretion energetics by focusing on quantities related to the accretion rates and cusp luminosity, when considered the
DE
presence, related to the pure Kerr central
BH
. Our findings show that in these conditions heavy divergences with respect to the vacuum case are expected for the
DE
metrics. A known effect of the Kiselev metric is to lead to a false estimation the
BH
spin, we confirm this characteristic from the fluids dynamics analysis. Remarkably our results show that
DE
is affecting differently the accretion physics, and particularly the accretion rate, according to the fluid rotation orientation with respect to the central spinning attractor, leading in some cases to an under-estimation of the
BH
spin mass ratio. These contrasting aspects emerging in dependence on the fluids rotational orientation can be a distinguishing general
DE
feature which could lead to a revised observational paradigm where
DE
existence is considered.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We study the accretion flows towards a central Kerr super-spinning attractor, discussing the formation of the flow inversion points, defined by condition
u
ϕ
=
0
on the particles flow axial velocity. ...We locate two closed surfaces, defining
inversion coronas
(spherical shells), surrounding the central attractor. The coronas analysis highlights observational aspects distinguishing the central attractors and providing indications on their spin and the orbiting fluids. The inversion corona is a closed region, generally of small extension and thickness, which is for the counter-rotating flows of the order of
≲
1.4
M
(central attractor mass) on the vertical rotational axis. There are no co-rotating inversion points (from co-rotating flows). The results point to strong signatures of the Kerr super-spinars, provided in both accretion and jet flows. With very narrow thickness, and varying little with the fluid initial conditions and the emission process details, inversion coronas can have remarkable observational significance for primordial Kerr super-spinars predicted by string theory. The corona region closest to the central attractor is the most observably recognizable and active part, distinguishing black holes solutions from super-spinars. Our analysis expounds the Lense–Thirring effects and repulsive gravity effects in the super-spinning ergoregions.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We determine the class of axisymmetric and asymptotically flat black-hole spacetimes for which the test Klein-Gordon and Hamilton-Jacobi equations allow for the separation of variables. The known ...Kerr, Kerr-Newman, Kerr-Sen and some other black-hole metrics in various theories of gravity are within the class of spacetimes described here. It is shown that although the black-hole metric in the Einstein-dilaton-Gauss-Bonnet theory does not allow for the separation of variables (at least in the considered coordinates), for a number of applications it can be effectively approximated by a metric within the above class. This gives us some hope that the class of spacetimes described here may be not only generic for the known solutions allowing for the separation of variables, but also a good approximation for a broader class of metrics, which does not admit such separation. Finally, the generic form of the axisymmetric metric is expanded in the radial direction in terms of the continued fractions and the connection with other black-hole parametrizations is discussed.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Our analysis focus on the dragging effects on the accretion flows and jet emission in Kerr super-spinars. These attractors are characterized by peculiar accretion structures as double tori, or ...special dragged tori in the ergoregion, produced by the balance of the hydrodynamic and centrifugal forces and also effects of super-spinars repulsive gravity. We investigate the accretion flows, constituted by particles and photons, from toroids orbiting a central Kerr super-spinar. As results of our analysis, in both accretion and jet flows, properties characterizing these geometries, that constitute possible strong observational signatures or these attractors, are highlighted. We found that the flow is characterized by closed surfaces, defining inversion coronas (spherical shell), with null the particles flow toroidal velocity (
u
ϕ
=
0
) embedding the central singularity. We proved that this region distinguishes proto-jets and accretion driven flows, co-rotating and counter-rotating flows. Therefore in both cases the flow carries information about the accretion structures around the central attractor, demonstrating that inversion points can constitute an observational aspect capable of distinguishing the super-spinars.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We study accretion tori and proto-jets in the field of central Kerr super-spinning attractors, exposing distinctions between Kerr naked singularities (NSs) and black holes (BHs). We focus on the ...spacetime regions very close to the central singularity, interior of the ergoregion, especially for slowly spinning NSs, where repulsive gravity effects emerge, discussing the causal structure in relation to tori contained, partially contained and confined in the ergoregion. We explore particularly regions close to the attractor rotational axis and the poles, where very slowly spinning tori, with momenta ℓ≈0, and axial cusp, constrained by the light surfaces, are relevant. Proto-jets (constraining jet emission) proved to be a signature in comparison with BH case, pointing out the emergence of very fast spinning tori in the ergoregion, very slow tori, double tori with internal cusp, and the presence of an axial cusp. The accretion disks properties point out very faster spinning attractors (extremely super spinning NSs). Providing distinguishable physical characteristics of jets and accretion tori, the differences between the BH and NS accretion scenarios, emerging from this analysis, constitute astrophysical tracers distinguishing NSs from BHs, and possible strong NSs observational signature of the primordial Kerr superspinars predicted by string theory.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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