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Unlike the perturbations of massless fields, the asymptotic tails of massive fields exhibit oscillations and decay slowly, following a power-law envelope. In this work, considering various scenarios admitting (either fundamental or effective) massive scalar and gravitational fields, we demonstrate that bump deformations in the effective potential, either in the near-horizon or far-field regions, modify these asymptotic oscillatory tails. Specifically, the power-law envelope transitions to a more complex oscillatory pattern, which cannot be easily fitted to a simple formula. This behavior is qualitatively different from the echoes of massless fields, which appear mainly during the quasinormal ringing stage and are considerably suppressed at the asymptotic tails. We show that in some models echoes may considerably amplify the signal at the stage of asymptotic tails., Comment: 7 pages, 6 figures

The BL Lac Object PKS 1717+177 has been identified as potential neutrino-emitting AGN in the point source stacking analysis of IceCube data. We explore peculiarities in the morphology and kinematics of the jet and examine multi-wavelength light curves for distinctive effects which might allow to pinpoint a likely neutrino generation mechanism. We re-modeled 34 high resolution radio interferometric Very Long Baseline Array (VLBA) observations obtained at 15 GHz (between 1999/12/27 and 2023/05/03). A correlation and periodicity analysis of optical KAIT and Tuorla data, as well as for Fermi-LAT $\gamma$-ray data has been performed. The nuclear jet appears deflected and bent at about 0.5 mas distance from the radio core by an encounter with a dark, unseen object. The deviation of the jet evolves over 23.5 years from a simple apparent bend into a significantly meandering structure with increasing amplitude: a zig-zag line. To our knowledge, this is the first time that the temporal evolution of a jet deviation can be traced. The turning point shifts with time and the jet seems to brighten up almost periodically at the point of deviation. The radio core as well as the jet contribute approximately equally to the total flux-density at 15 GHz. We discuss scenarios which could explain the complex jet bending and quasi-regular flaring. We propose that the jet could either be deflected by the magnetosphere of a second massive black hole, by the pressure gradient due to a circumnuclear dense cloud, or via gravitational lensing by an intervening black hole., Comment: 22 pages, 9 figures, in print in MNRAS

We consider a counter--rotating torus orbiting a central Kerr black hole (\textbf{BH}) with dimensionless spin $a$, and its accretion flow into the \textbf{BH}, in an agglomerate of an outer counter--rotating torus and an inner co--rotating torus. This work focus is the analysis of the inter--disks inversion surfaces. Inversion surfaces are spacetime surfaces, defined by the condition $u^{\phi}=0$ on the flow torodial velocity, located out of the \textbf{BH} ergoregion, and totally embedding the \textbf{BH}. They emerge as a necessary condition, related to the spacetime frame--dragging, for the counter--rotating flows into the central Kerr \textbf{BH}. In our analysis we study the inversion surfaces of the Kerr spacetimes for the counter--rotating flow from the outer torus, impacting on the inner co--rotating disk. Being totally or partially embedded in (internal to) the inversion surfaces, the inner co--rotating torus (or jet) could be totally or in part ``shielded", respectively, from the impact with flow with $a u^{\phi}<0$. We prove that, in general, in the spacetimes with $a<0.551$ the co--rotating toroids are always external to the accretion flows inversion surfaces. For $0.5510.886$, a co--rotating torus could be entirely embedded in the inversion surface and, for larger spins, it is internal to the inversion surfaces. Tori orbiting in the \textbf{BH} outer ergoregion are a particular case. Further constraints on the \textbf{BHs} spins are discussed in the article., Comment: 28 pages, 19 figures, 3 tables. To appear in EPJC

We examine the shadow cast by a Kerr black hole, focusing on constraints on photons corresponding to different shadow boundaries. The photons are related to different orbital ranges and impact parameter values, creating a map of the shadow boundaries. Our analysis fixes also the conditions under which it is possible to observe an "imprint" of the black hole (outer) ergosurface and (outer) ergoregion on the Kerr black hole shadow boundary. The counter-rotating case resulted strongly constrained with respect to the co-rotating case, constituting a remarkable and significant difference where the counter-rotating component associated with the shadow boundary is strongly distinct from the co-rotating one. However, in this framework, even the co-rotating photons imply restrictions on conditions on the spins and planes, which are bounded by limiting values. We believe the results found here, being a tracer for the central black hole, can constitute new templates for the ongoing observations., Comment: 25 pages; 24 multi-panel figure, to appear in EPJP

Comparing different dark matter (DM) models, we explore the DM influence on black hole (BH) accretion disk physics, considering corotating and counterrotating thick accretion tori orbiting a central spinning BH. Our results identify accretion onto a central BH as a good indicator of DM presence, signaling possible DM tracers in accretion physics. We analyze accretion around a spinning BH immersed in perfect-fluid dark matter, cold dark matter and scalar field dark matter. Our investigation addresses observational evidence of distinctive DM effects on toroidal accretion disks and protojet configurations, proving that BH accretion tori immersed in DM can present characteristics, such as interdisk cusp or double tori, which have usually been considered as tracers for superspinars and naked singularity attractors. Therefore, in this context DM influence on the BH geometry could manifest as superspinar mimickers. DM also affects the central spinning attractor energetics associated with accretion physics, and its influence on accretion disks can be searched for in a variation of the central BH energetics as an increase of the mass accretion rates.

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 ($u^\phi=0$) 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., Comment: 34 pages 29 multipanels figures, 3 Tables

We study the accretion flows towards a central Kerr super-spinning attractor, discussing the formation of the flow inversion points, defined by condition $u^{\phi}=0$ on the particles flow axial velocity. We locate two closed surfaces, defining \emph{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 $\lesssim 1.4 M$ (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., Comment: 21 pages;16 figures multipanels; 1 Table. To appear in EPJC

We find accurate quasinormal frequencies of a quantum corrected black hole constructed in the renormalization group theory via the coordinate-independent iterative procedure, leading to the Dymnikova regular black hole. We show that while the fundamental mode is only slightly affected by the quantum correction, the overtones change at a much stronger rate. This outburst of overtones occurs because of the deformation of the geometry of the Schwarzschild black hole solely near the event horizon. For finding accurate values of overtones we developed a general procedure allowing one to use the Leaver method to metrics which, initially, are not expressed in terms of rational functions., Comment: 11 pages, 3 figures, 1 ancillary Mathematica file

In this work we develop a simple protocol to construct interior solutions through Gravitational Decoupling by the Minimal Geometric Deformation extended satisfying the vanishing complexity condition. The method is illustrated by using Tolman VII and Tolman IV solutions as isotropic seeds., Comment: A clarifying point not appearing in the published version regarding the mimic constraint for the mass function, has been introduced in on page 5

Asymptotically safe gravity is based on the idea that the main contribution to the Schwarzschild-like black hole spacetime is due to the value of the gravitational coupling which depends on the distance from the origin and approaches its classical value in the far zone. However, at some stage this approach has an arbitrariness of choice of some identification parameter. The two cases of identification are considered here: first, by the modified proper length (the Bonanno-Reuter metric), and second, by the Kretschmann scalar (the metric for this case coincides, up to the redefinition of constants, with the Hayward metric). Even though the quasinormal modes of these metrics have been extensively studied, a number of interesting points were missed. We have found that quasinormal modes are qualitatively similar for both types of identification. The deviation of the fundamental mode from its Schwarzschild limit may be a few times larger than it was claimed in the previous studies. The striking deviation from the Schwarzschild limit occurs for overtones, being as large as hundreds of percent even when the fundamental mode is almost coinciding with the Schwarzschild one. This happens because the above metrics are very close to the Schwarzschild one everywhere, except a small region near the event horizon, which is crucial for overtones. The spectrum of both metrics contains purely imaginary (non-oscillatory) modes, which, for some values of parameters, can appear already at the second overtone., Comment: 20 pages, 7 figures

We discuss the conditions for the existence of extended matter configurations orbiting in the ergoregion or close to the outer ergosurface of the Kerr black hole ("dragged" configurations). The corotating tori under consideration are perfect fluid configurations with barotropic equation of state, orbiting on the equatorial plane of the central Kerr black hole. The possibility of magnetized tori with a toroidal magnetic field is also discussed. Indications on the attractors where dragged tori can be observed are provided with the analysis of the fluid characteristics and geometrical features, relevant for the tori stability and their observations. QPOs emissions from the inner edges of the dragged tori are also discussed. We argue that the smaller dragged tori could be subjected to a characteristic instability, effect of the frame-dragging. This possibility is thoroughly explored. This can finally lead to the destruction of the torus (disk exfoliation) which can combine with accretion and processes present in the regions very close to the black hole horizon. Tori are characterized according to the central attractor dimensionless spin. These structures can be observed orbiting black holes with dimensionless spin a > 0.9897M., Comment: 43 pages, 40 figure multipanels, 2 tables

We consider agglomerates of misaligned tori orbiting a supermassive black hole. The aggregate of tilted tori is modeled as a single orbiting configuration by introducing a leading function governing the distribution of toroids (and maximum pressure points inside the disks) around the black hole attractor. The orbiting clusters are composed by geometrically thick, pressure supported, perfect fluid tori. This analysis places constraints on the existence and properties of tilted tori and more general aggregates of orbiting disks. We study the constraints on the tori collision emergence and the instability of the agglomerates of tori with general relative inclination angles, the possible effects of the tori geometrical thickness and on the oscillatory phenomena. Some notes are discussed on the orbiting ringed structure in dependence of the dimensionless parameter ${\xi}$ representing the (total) BH rotational energy extracted versus the mass of the BH, associating ${\xi}$ to the characteristics of the accretion processes., Comment: 15 pages; 3 multi-panels figures; 2 tables. To appear in proceedings of 16th Marcel Grossmann meeting, July 5-10 2021, World Scientific

We study the accretion flow from a counter-rotating torus orbiting a central Kerr black hole (BH). We characterize the flow properties at the turning point of the accreting matter flow from the orbiting torus, defined by the condition $u^\phi = 0$ on the flow torodial velocity. The counter-rotating accretion flow and jet-like flow turning point location along BH rotational axis is given. Some properties of the counter-rotating flow thickness and counter-rotating tori energetics are studied.The maximum amount of matter swallowed by the BH from the counter-rotating tori is determined by the background properties. The fast spinning BH energetics depends mostly on BH spin rather than on the properties of the counter-rotating fluids or the tori masses. The turning point is located in a narrow orbital corona (spherical shell), for photons and matter flow constituents, surrounding the BH stationary limit (outer ergosurface), depending on the BH spin-mass ratio and the fluid initial momentum only. The turning corona for jet-like-flow has larger thickness, it is separated from the torus flow turning corona and it is closer to the BH stationary limit. Turning points of matter accreting from torus and from jets are independent explicitly of the details of the accretion and tori model. The turning corona could be observable due to an increase of flow luminosity and temperature. The corona is larger on the BH equatorial plane, where it is the farthest from the central attractor, and narrower on the BH poles., Comment: to appear in MNRAS. 33 pages, 26 multipanel figures

We present a simple, analytic and straightforward method to elucidate the effects produced by polytropic fluids on any other gravitational source, no matter its nature, for static and spherically symmetric spacetimes. As a direct application, we study the interaction between polytropes and perfect fluids coexisting inside a self-gravitating stellar object, Comment: 10 pages, 12 figures

We constrain the BHs spin with the evaluation of the dimensionless parameter \xi, the total rotational energy extracted versus the BH mass. The energy extraction can power an outflow which can be then observed. We relate the energy extraction to the accreting configurations and the accretion processes occurring in a cluster of corotating and counter-rotating tori orbiting one central Kerr SMBH, associating \xi to the accretion processes characteristics. We relate the regions of tori parameters to the energy extraction processes, binding \xi to properties of light surfaces by using the bundles, relating measures in different regions of the spacetimes. We evaluate properties of the BH accretions disks, and correlate spacetimes prior and after their transition due to the energy extraction. Light surfaces are related to the generators of Killing horizons, proving limiting frequency of the stationary observers of the geometries. We consider the photon limiting curves of the stationary observers as constraints for various processes regulated by these frequencies, to relate different BH states, prior and after the energy extraction, investigating regions close to the BH horizons and rotational axis. From methodological view-point we used a naked singularity -BH correspondence defined with metric bundles to predict the BH--accretion disk system observational characteristics. The analysis points relevant BH spins a\approx0.94M, a\approx 0.7M and a\approx0.3M. We show the relation between the rotational law of the tori, the bundle characteristic frequency and the relativistic velocity defining the von Zeipel surfaces. The inferior limit on the formation of corotating is \ell/a\geq2, for counter-rotating tori \ell/a\leq -22/5 (\ell is the fluids specific angular momentum)., Comment: Published in: Class.Quant.Grav. 38 (2021) 14, 145014

We explore the possibility of jet collisions with accreting tori orbiting around SMBHs. The analysis provides constraints on formation and the observational evidences of the host configurations. We use a GRHD model, investigating the light surfaces contraints in aggregates of misaligned tori orbiting a central static Schwarzschild black hole. Each (toroidal) configuration of the agglomeration is a geometrically thick, pressure supported, perfect fluid torus. Aggregates include proto-jets, the open cusped solutions associated to the geometrically thick tori. Collision emergence and the stability properties of the aggregates are considered at different inclination angles relative to a fixed distant observer. We relate the constraints to the relevant frequencies of the configurations and fluid specific angular momentum, separating the constraints related to the fluids hydrodynamics and to the geometric backgrounds. We analyze existence of accreting tori supporting jet-emission. We discuss the existence of orbit-replicas that could host shadowing effects in replicas of the emissions in two regions, close and far from the BH (horizon replicas in jet shells). The investigation clarifies the role of the pressure gradients of the orbiting matter and the essential role of the radial gradient of the pressure in the determination of the disk verticality. Finally we analyze the possibility that a toroidal magnetic field could be related to the collimation of proto-jets., Comment: 30 pages; 23 figure multipanels; to appear in PASJ

An analytical solution representing traversable asymptotically flat and symmetric wormholes was obtained without adding exotic matter in two different theories independently: in the Einstein-Maxwell-Dirac theory and in the second Randall-Sundrum brane-world model. Further, a smooth normalizable asymmetric wormhole solution has been recently obtained numerically in the Einstein-Maxwell-Dirac theory. Using the time-domain integration method we study quasinormal ringing of all these wormholes with emphasis to the regime of mimicking the near extremal Reissner-Nordstr\"om black holes, which is characterised by echoes. In addition, we calculate radius of shadows cast by these wormholes., Comment: 16 pages, 3 figures

We interpret the cosmological constant as the energy of the vacuum, and under a minimum amount of assumptions, we show that it is deformed in the vicinity of a black hole. This leads us to reexamine the Kerr-de Sitter solution. We provide a new solution, simpler and geometrically richer, which shows the impact of the rotation in form of a warped curvature. We carry out a detailed and exact study on the new black hole solution, and we conclude with a conjecture regarding the possible impact of our results on alternative theories., Comment: 7 pages, 2 figures

We consider quasinormal modes and Hawking radiation of four-dimensional asymptotically flat black holes in the most general up to-cubic-order-in-curvature dimension-independent Einsteinian theory of gravity that shares its graviton spectrum with the Einstein theory on constant curvature backgrounds. We show that damping rate and real oscillation frequencies of quasinormal modes for scalar, electromagnetic and Dirac fields are suppressed once the coupling with the cubic term is on. The intensity of Hawking radiation is suppressed as well, leading to, roughly, one order longer lifetime at a sufficiently large coupling constant., Comment: 10 pages, revtex, 11 figures, the version to match the published PRD version A typo in formula 27

We investigate clusters of misaligned (inclined) tori orbiting a central static Schwarzschild black hole. To this purpose we considered a set of geometrically thick, pressure supported, perfect fluid tori analyzing purely hydrodynamic models. We study the tori collision emergence and, consequently, the stability properties of the aggregates composed by tori with different inclination angles relative to a fixed distant observer. The aggregate of tilted tori is modeled as a single orbiting configuration, by introducing a leading function governing the distribution of toroids around the black hole attractor. Eventually the tori agglomerate can be seen, depending on the tori thickness, as a (multipole) gobules of orbiting matter, with different toroidal spin orientations , covering the embedded central black hole. These systems are shown to include tori with emerging instability phase related to accretion onto the central black hole. Therefore we provide an evaluation of quantities related to tori energetics such as the mass-flux, the enthalpy-flux, and the flux thickness depending on the model parameters for polytropic fluids. Consequently this analysis places constraints on the existence and properties of tilted tori and aggregate of misaligned disks. Some notes are included on aggregates including proto-jets, represented by open cusped solutions associated to the geometrically thick tori., Comment: 46 pages; 26 multipanels figures; 1 table. Class. Quantum Grav (2020)

We consider agglomerates of misaligned, pressure supported tori orbiting a Schwarzschild black hole. A leading function is introduced, regulating the toroids distribution around the central static attractor-it enables to model the misaligned tori aggregate as a single orbiting (macro--)configuration. We first analyze the leading function for purely hydrodynamical perfect fluid toroids. Later, the function is modified for presence of a toroidal magnetic field. We study the constraints on the tori collision emergence and the instability of the agglomerates of misaligned tori with general relative inclination angles. We discuss the possibility that the twin peak high-frequency quasi-periodic oscillations (HF-QPOs) could be related to the agglomerate inner ringed structure. The discrete geometry of the system is related to HF-QPOs considering several oscillation geodesic models associated to the toroids inner edges. We also study possible effect of the tori geometrical thickness on the oscillatory phenomena., Comment: 28 pages, 15 multipanels figures, 1 Table, Monthly Notices of the Royal Astronomical Society, staa503, https://doi.org/10.1093/mnras/staa503 Published: 19 February 2020 Key words: black hole physics-accretion, accretion discs-hydrodynamics-(magnetohydrodynamics) MHD--galaxies: active--galaxies: jets

We present a special case of the Stephani solution with spherical symmetry while considering different values of spatial curvature. We investigate the dynamics of the universe evolution in our model, build the R--T-regions for the resulting spacetime and analyze the behavior of the deceleration parameter. The singularities of the model are also discussed. The geometry of the spatial part of the obtained solution is explored., Comment: 4 pages, 2 figures

A simple one-parameter generalization of the Schwarzschild spacetime was recently suggested by A. Simpson and M. Visser [JCAP 1902, 042 (2019)] as a toy model describing the regular black hole and traversable wormhole states separated by the border (one-way wormhole) state. We study quasinormal modes of all the three states and show that the black-hole/wormhole transition is characterized by echoes, while the remnant of the black hole state is kept in the time-domain profile of the wormhole perturbation at the initial stage of the exponential fall off. Calculations of quasinormal modes using the WKB method with Pad\'{e} expansion and the time-domain integration are in good agreement. An analytical formula governing quasinormal modes in the eikonal regime is given., Comment: 8 pages, 4 figures, revtex, the version accepted for publication in Classical and Quantum Gravity, references added

Quasinormal modes of black holes were previously calculated in a non-linear electrodynamics and in the Gauss-Bonnet gravity theory. Here we take into consideration both of the above factors and find quasinormal modes of a (massive) scalar field in the background of a black hole in the five-dimensional Einstein-Gauss-Bonnet gravity coupled to a non-linear electrodynamics having Maxwellian weak-field limit. For the non-linear electrodynamics we considered the high frequency (eikonal) regime of oscillations analytically, while for the lower multipoles the higher order WKB analysis with the help of Pad\'{e} approximants and the time domain integration were used. We found that perturbations of a test scalar field violate the inequality between the damping rate of the least damped mode and the Hawking temperature, known as the Hod's proposal. This does not exclude the situation in which gravitational spectrum may restore the Hod's inequality, so that only the analysis of the full spectrum, including gravitational perturbations, will show if the quasinormal modes we found here for the scalar field can be a counterexample to the Hod's conjecture or not. We also revealed that in such a system, which includes the higher curvature corrections and non-linear electrodynamics, for perturbations of a massive scalar field there exists the phenomenon of the arbitrary long lived quasinormal modes - quasiresonances., Comment: 10 pages, 9 figures, revtex4-1

We consider ringed accretion disks (RADs), representing models of aggregates of corotating and counterrotating toroids orbiting a central Kerr super-massive black hole (SMBH). We comment on system of two-tori governed by the polytropic equation of state and including a toroidal magnetic field. We found the RADs leading function describing the RAD inner structure and governing the distribution of orbiting toroidal structures and the emergence of the (hydro-mechanical) instabilities in the disk. We perform this analysis first in pure hydrodynamical models by considering one-specie perfect fluid toroids and then by considering the contribution of toroidal magnetic field., Comment: 11 pages, 3 figures, RAGtime 20, 2018, Opava, Czech Republic

Ringed accretion disks (RADs) are aggregates of corotating and counterrotating toroidal accretion disks orbiting a central Kerr super-massive Black Hole (SMBH) in AGNs. The dimensionless spin of the central BH and the fluids relative rotation are proved to strongly affect the RAD dynamics. There is evidence of a strict correlation between SMBH spin, fluid rotation and magnetic fields in RADs formation and evolution. Recently, the model was extended to consider RADs constituted by several magnetized accretion tori and the effects of a toroidal magnetic field in RAD dynamics have been investigated. The analysis poses constraints on tori formation and emergence of RADs instabilities in the phases of accretion onto the central attractor and tori collision emergence. Magnetic fields and fluids rotation are proved to be strongly constrained and influence tori formation and evolution in RADs, in dependence on the toroidal magnetic fields parameters. Eventually, the RAD frame investigation constraints specific classes of tori that could be observed around some specific SMBHs identified by their dimensionless spin, Comment: 7 pages; 2 figures; CNUM: C18-07-01; (15th Marcel Grossmann Meeting :: Rome, July 1-7, 2018)

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 $R_{S}=2\cal{M}$ 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 stable stellar model, such as a positive density everywhere and decreasing monotonously from the centre, as well as a non-uniform and monotonic pressure., Comment: 20 pages, 4 figures. arXiv admin note: text overlap with arXiv:1905.10162

We employ the minimal geometric deformation approach to gravitational decoupling (MGD- decoupling) in order to build an exact anisotropic version of the Schwarzschild interior solution in a space-time with cosmological constant. Contrary to the well-known Schwarzschild interior, the matter density in the new solution is not uniform and possesses subluminal sound speed. It therefore satisfies all standard physical requirements for a candidate astrophysical object., Comment: 15 pages, 6 figures

The Schwarzschild star is an ultracompact object beyond the Buchdahl limit, which has Schwarzschild geometry outside its surface and positive pressure in the external layer which vanishes at the surface. Recently it has been shown that the Schwarzschild star is stable against spherically symmetric perturbations. Here we study arbitrary axial nonspherical perturbations, and show that the observable quasinormal modes can be as close to the Schwarzschild limit as one wishes, what makes the Schwarzschild star a very good mimicker of a black hole. The decaying time-domain profiles prove that the Schwarzschild star is stable against nonspherical perturbations as well. Another peculiar feature is the absence of echoes at the end of the ringdown. Instead we observe a nonoscillating mode which might belong to the class of algebraically special modes. At asymptotically late times, Schwarzschildian power-law tails dominate in the signal., Comment: 7 pages, 3 figures

We provide constraints on possible configurations and interactions of two coplanar tori orbiting a central Kerr black hole (BH), in dependence on its dimensionless spin. The two-tori configurations can be directly linked to the current models featuring the obscuration of galactic BH X-ray emission. The emergence of each torus instability phases is discussed and tori collision has been also investigated. The first simple evaluation of the center-of-mass energy proves that collision-energy-efficiency increases with the dimensionless BH spin. We explore the phenomenological aspects of the corotating and counterrotating tori by analyzing properties of the orbiting toroidal configurations related to the fluid enthalpy flux, the mass-flux, the mass-accretion-rates, and the cusp luminosity in the two cases of corotating and counterrotating fluids in dependence on the SMBH spin. The analysis resulted ultimately in a comparative investigation of the properties of corotating versus counterrotating tori, demonstrating that two accretion tori can orbit around the central Kerr attractor only under very specific conditions. Our results also demonstrate that the dynamics of the unstable phases of these double tori systems is significant for the high energy phenomena which could be observable in the X-ray emission and extremely energetic phenomena in active galactic nuclei and quasar., Comment: 30 pages, 16 multi-panels figures, 2 tables

Classical (quasinormal) and quantum (Hawking) radiations are investigated for test fields in the background of a four dimensional, spherically symmetric and asymptotically flat black hole in the Einstein-dilaton-Gauss-Bonnet (EdGB) theory. The geometry of the EdGB black hole deviates from the Schwarzschild geometry only slightly. Therefore, here we observe that the quasinormal spectrum also deviates from its Schwarzschild limit at most moderately, allowing for a $9\%$ decrease in the damping rate and up to a $6\%$ decrease in the real oscillation frequency. However, the intensity of Hawking radiation of an electromagnetic field turned out to be much more sensitive characteristic than its quasinormal spectrum, allowing for a $54\%$ increase of the energy emission rate. The analytical formula for the eikonal regime of quasinormal modes is derived for test fields and it is shown that the correspondence between the eikonal quasinormal modes and null geodesics is indeed fulfilled for test fields, but is not expected for the gravitational one., Comment: 10 pages, revtex, minor changes

We investigate how a spherically symmetric scalar field can modify the Schwarzschild vacuum solution when there is no exchange of energy-momentum between the scalar field and the central source of the Schwarzschild metric. This system is described by means of the gravitational decoupling by Minimal Geometric Deformation (MGD-decoupling), which allows us to show that, under the MGD paradigm, the Schwarzschild solution is modified in such a way that a naked singularity appears., Comment: 16 pages, 2 figures. arXiv admin note: text overlap with arXiv:1804.03468

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 [9, 11], 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., Comment: 5 pages, 5 figures

Here we show that the phenomenon of arbitrarily long-lived quasinormal modes (called quasiresonances) of a massive scalar field in the vicinity of a black hole is not an artifact of the test field approximation, but takes place also when the (derivative) coupling of a scalar field with the Einstein tensor is taken into consideration. We observe that at large coupling and high multipole numbers, the growing modes appear in the spectrum, which are responsible for the eikonal instability of the field. For small coupling, when the configuration is stable, there appear the purely imaginary quasinormal modes which are nonperturbative in the coupling constant. At the sufficiently small coupling the nonminimal scalar field is stable and the asymptotic late-time tails are not affected by the coupling term. The accurate calculations of quasinormal frequencies for a massive scalar field with the derivative coupling in the Reissner-Nordstr\"om black-hole background are performed with the help of Frobenius method, time-domain integration and WKB expansion., Comment: 8 pages, 4 figures

There is strong observational evidence that many active galactic nuclei (AGNs) harbour supermassive black holes (SMBHs), demonstrating multi-accretion episodes during their lifetime. In such AGNs, corotating and counterrotating tori, or strongly misaligned disks, as related to the central SMBH spin, can report traces of the AGNs evolution. Here we concentrate on aggregates of accretion disks structures, ringed accretion disks (RADs) orbiting a central Kerr SMBH, assuming that each torus of the RADs is centered in the equatorial plane of the BH, tori are coplanar and axisymmetric. Many RAD aspects are governed mostly by the Kerr geometry spin. We classify Kerr BHs due to their dimensionless spin, according to possible combinations of corotating and counterrotating equilibrium or accreting tori composing the RADs. The number of accreting tori in RADs cannot exceed n=2. We present list of 14 characteristic values of the BH spin a governing the classification in the BH range 0

We investigate how a spherically symmetric fluid modifies the Schwarzschild vacuum solution when there is no exchange of energy-momentum between the fluid and the central source of the Schwarzschild metric. This system is described by means of the gravitational decoupling realised via the minimal geometric deformation approach, which allows us to prove that the fluid must be anisotropic. Several cases are then explicitly shown, Comment: New section with a regular hairy black hole solution; references added

Ringed accretion disks (RADs) are agglomerations of perfect-fluid tori orbiting around a single central attractor that could arise during complex matter inflows in active galactic nuclei. We focus our analysis to axi-symmetric accretion tori orbiting in the equatorial plane of a supermassive Kerr black hole; equilibrium configurations, possible instabilities, and evolutionary sequences of RADs were discussed in our previous works. In the present work we discuss special instabilities related to open equipotential surfaces governing the material funnels emerging at various regions of the RADs, being located between two or more individual toroidal configurations of the agglomerate. These open structures could be associated to proto-jets. Boundary limiting surfaces are highlighted, connecting the emergency of the jet-like instabilities with the black hole dimensionless spin. These instabilities are observationally significant for active galactic nuclei, being related to outflows of matter in jets emerging from more than one torus of RADs orbiting around supermassive black holes., Comment: 13 pages, double column, 4 figures multi-panels To appear in Class. Quantum Grav. https://doi.org/10.1088/1361-6382/aab99d

The possibility that two toroidal accretion configurations may be orbiting around a~super--massive Kerr black hole has been addressed. Such tori may be formed during different stages of the Kerr attractor accretion history. We consider the relative rotation of the tori and the corotation or counterrotation of a~single torus with respect to the Kerr attractor. We give classification of the couples of accreting and non--accreting tori in dependence on the Kerr black hole dimensionless spin. We demonstrate that only in few cases a~double accretion tori system may be formed under specific conditions., Comment: 18 pages, 5 figures multi-panels Proceedings of RAGtime 17-19: Workshops on black holes and neutron stars, 17-19/23-26 Oct., 1-5 Nov. 2015/2016/2017, Opava, Czech Republic http://proceedings.physics.cz/index.php/proceedings-of-ragtime-17-19

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., Comment: 14 pages

Super-massive black holes (SMBHs) hosted in active galactic nuclei (AGNs) can be characterized by multi-accreting periods as the attractors interact with the environment during their life-time. These multi-accretion episodes should leave traces in the matter orbiting the attractor. Counterrotating and even misaligned structures orbiting around the SMBHs would be consequences of these episodes. Our task in this work is to consider situations where such accretions occur and to trace their remnants represented by several toroidal accreting fluids, corotating or counterrotating relative to the central Kerr attractor, and created in various regimes during the evolution of matter configurations around SMBHs. We focus particularly on the emergence of matter instabilities, i.e., tori collisions, accretion onto the central Kerr black hole, or creation of jet-like structures (proto-jets). Each orbiting configuration is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluid. We prove that sequences of configurations and hot points, where an instability occurs, characterize the Kerr SMBHs, depending mainly on their spin-mass ratios. The occurrence of tori accretion or collision are strongly constrained by the fluid rotation with respect to the central black hole and the relative rotation with respect to each other. Our investigation provides characteristic of attractors where traces of multi-accreting episodes can be found and observed., Comment: 43 pages, 17 multi-panels figures, 1 Table

In Phys. Rev. D 79, 064016 (2009) 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., Comment: 8 pages, revtex, refereed version, accepted for publication in Physics Letters B

We investigate ringed accretion disks constituted by two tori (rings) orbiting on the equatorial plane of a central super-massive Kerr black hole. We discuss the emergence of the instability phases of each ring of the macro-configuration (ringed disk) according to Paczynski violation of mechanical equilibrium. In the full general relativistic treatment, we consider the effects of the geometry of the Kerr spacetimes relevant in the characterization of the evolution of these configurations. The discussion of the rings stability in different spacetimes enables us to identify particular classes of central Kerr attractors in dependence of their dimensionless spin. As a result of this analysis we set constraints of the evolutionary schemes of the ringed disks related to the tori morphology and their rotation relative to the central black hole and to each other. The dynamics of the unstable phases of this system is significant for the high energy phenomena related to accretion onto super-massive black holes in active galactic nuclei (AGNs), and the extremely energetic phenomena in quasars which could be observable in their X-ray emission., Comment: 35 pages, 15 figures multipanels, 1 table. To appear in APJS

In this work we have obtained the set of new exact solutions of the Einstein equations that generalize the known Lemaitre-Tolman-Bondi solution for the certain case of nonzero pressure under zero spatial curvature. These solutions are pretending to describe the black hole immersed in the nonstatic cosmological background and give a possibility to investigate the problems concerning the effects of the cosmological expansion in gravitationally bounded systems. They may also be used as a seed models in the problem of structure formation in the universe at the epoch of matter and radiation decoupling. It was shown that each of the solutions obtained contains either the Reissner-Nordstrom or the Schwarzschild black hole in the central region of the space. It is demonstrated that the approach of the mass function use in solving the Einstein equations allows clear physical interpretation of the resulting solutions that is of much benefit to any their further application., Comment: 8 pages

We analyze the possibility that several instability points may be formed, due to the Paczy\'nski mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider recently proposed model of ringed accretion disk, made up by several tori (rings) which can be corotating or counterrotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor., Comment: 15 pages, 2 figures multipanels, 3 tables. to appear in Astrophysical Journal Supplement Series

We investigate a model of ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the General Relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can be then determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high energy phenomena demonstrated in active galactic nuclei., Comment: 64 pages, 10 figures multi-panels, 2 Tables. Accepted for publication in The Astrophysical Journal Supplement

Estimations of black hole spin in the three Galactic microquasars GRS 1915+105, GRO J1655-40, and XTE J1550-564 have been carried out based on spectral and timing X-ray measurements and various theoretical concepts. Among others, a non-linear resonance between axisymmetric epicyclic oscillation modes of an accretion disc around a Kerr black hole has been considered as a model for the observed high-frequency quasi-periodic oscillations (HF QPOs). Estimates of spin predicted by this model have been derived based on the geodesic approximation of the accreted fluid motion. Here we assume accretion flow described by the model of a pressure-supported torus and carry out related corrections to the mass-spin estimates. We find that for dimensionless black hole spin a<0.9, the resonant eigenfrequencies are very close to those calculated for the geodesic motion. Their values slightly grow with increasing torus thickness. These findings agree well with results of a previous study carried out in the pseudo-Newtonian approximation. The situation becomes different for a>0.9, in which case the resonant eigenfrequencies rapidly decrease as the torus thickness increases. We conclude that the assumed non-geodesic effects shift the lower limit of the spin, implied for the three microquasars by the epicyclic model and independently measured masses, from a~0.7 to a~0.6. Their consideration furthermore confirms compatibility of the model with the rapid spin of GRS 1915+105 and provides highly testable predictions of the QPO frequencies. Individual sources with a moderate spin (a<0.9) should exhibit a smaller spread of the measured 3:2 QPO frequencies than sources with a near-extreme spin (a~1). This should be further examined using the large amount of high-resolution data expected to become available with the next generation of X-ray instruments, such as the proposed Large Observatory for X-ray Timing (LOFT)., Comment: 6 pages, 4 figures, accepted by Astronomy & Astrophysics

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study., Comment: Proc. SPIE 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray, 91442T