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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

Recently, new exploratory channels have opened up for the physics of highly compact objects, such as gravitational waves and black hole shadows. Moreover, more precise analysis and observations are now possible in the physics of accretion around compact objects. These advancements provide in particular an unprecedented insight into the physics near the horizons of a black hole. In this work we focus on the shadow boundary of a Kerr black hole, introducing observables related to special null orbits, called horizons replicas, solutions of the shadow edge equations which are related to particular photon orbits, defined by constraints on their impact parameter, carrying information about the angular momentum of the central spinning object. These orbits are related to particular regions on the shadow boundary and might be used to determine the spin of the black hole. The results provide the conditions by which horizon replicas are imprinted in the black hole shadow profile, in dependence on the black hole dimensionless spin and observational angle, providing eventually new templates for the future observations., Comment: 24 pages, 12 figures. To appear in NPB

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 discuss the fundamentals of classical black hole (BH) thermodynamics in a new framework determined by light surfaces and their frequencies. This new approach allows us to study BH transitions inside the Kerr geometry. In the case of BHs, we introduce a new parametrization of the metric in terms of the maximum extractable rotational energy or, correspondingly, the irreducible mass, which is an alternative to the spin parametrization. It turns out that BH spacetimes with spins $a/M= \sqrt {8/9}$ and $a/M=1/\sqrt{2}$ show anomalies in the rotational energy extraction and surface gravity whereas the case $a/M=\sqrt{3}/2$ is of particular relevance to study the variations of the horizon area. We find the general conditions under which BH transitions can occur and express them in terms of the masses of the initial and final states. This shows that BH transitions in the Kerr geometry are not arbitrary but depend on the relationship between the mass and spin of the initial and final states. From an observational point of view, we argue that near the BH poles it is possible to detect photon orbits with frequencies that characterize the light surfaces analyzed in this work., Comment: 23 double column pages, 12 multipanels figures, 1Table, To appear in EPJC

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 present some features of Kerr black hole horizons that are replicated on orbits accessible to outside observers. We use the concepts of horizon confinement and replicas to show that outside the outer horizon there exist photon orbits whose frequencies contain information about the inner horizon and that can, in principle, be detected through the emission spectra of black holes. It is shown that such photon orbits exist close to the rotation axis of the Kerr geometry. We argue that these results could be used to recognize and further investigate black holes and their horizons, Comment: 7 pages; 2 multipanels figures

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 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

We present a graph model for a background independent, relational approach to spacetime emergence. The general idea and the graph main features, detailed in [1], are discussed. This is a combinatorial (dynamical) metric graph, colored on vertexes, endowed with a classical distribution of colors probability on the graph vertexes. The graph coloring determines the graph structure in clusters of graph vertices (events) that can be monochromatic (homogeneous loops) or polychromatic (inhomogeneous loops). The probability is conserved after the graph conformal expansion from an initial seed graph state to higher (conformally expanded) graph states. The emerging structure has self-similar characteristics on different scales (states). From the coloring, different levels of vertices and thus graph levels arise as new aggregates of colored vertices. In this second (derived) graphs level, the derived graph vertices correspond to the polychromatic edges (with differently colored vertices) of the initial graph. Vertex aggregates are related, as some levels (graph states) to plexors and twistors (involving Clifford statistics). Two metric levels are defined on the colored graph, the first level is a natural metric defined on the graph, the second level emerges from the first and related, due to symmetries. Metric structure reflects the graph colored structure under conformal transformations evolving with its states under conformal expansion. In some special cases vertices/events chains could be related to strings generalizations., Comment: 25 pages, 3 figure multipanels

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 provide a complete characterization of the metric Killing bundles (or metric bundles) of the Kerr geometry. Metric bundles, first introduced in [21] can be generally defined for axially symmetric spacetimes with Killing horizons and, for the case of Kerr geometries, are sets of black holes (BHs) or black holes and naked singularities (NSs) geometries. Each metric of a bundle has an equal limiting photon (orbital) frequency, which defines the bundle and coincides with the frequency of a Killing horizon in the extended plane. In this plane each bundle is represented as a curve tangent to the curve that represents the horizons, which thus emerge as the envelope surfaces of the metric bundles. We show that the horizons frequency can be used to establish a connection between BHs and NSs, providing an alternative representation of such spacetimes in the extended plane and an alternative definition of the BH horizons. We introduce the concept of inner horizon confinement and horizons replicas and study the possibility of detecting their frequencies. We study the bundle characteristic frequencies constraining the inner horizon confinement in the outer region of the plane i.e. the possibility of detect frequency related to the inner horizon, and the horizons replicas, structures which may be detectable, for example, from the emission spectra of BHs spacetimes. It is shown that such observations can be performed close to the rotation axis of the Kerr geometry, depending on the BH spin. We argue that these results could be used to further investigate black holes and their thermodynamic properties., Comment: 44 pages; 29 figures; 1 table

This work focuses on some key aspects of the General Relativistic (GR)-magnetohydrodynamic (MHD) applications in High-Energy Astrophysics. We discuss the relevance of the GR-HD counterparts formulation exploring the geometrically thick disk models and constraints of the GR-MHD shaping the physics of accreting configurations. Models of clusters of tori orbiting a central super-massive black hole (SMBH) are described. These orbiting tori aggregates form sets of geometrically thick, pressure supported, perfect fluid tori, associated to complex instability processes including tori collision emergence and empowering a wide range of activities related expectantly to the embedding matter environment of Active Galaxy Nuclei. Some notes are included on aggregates combined with proto-jets, represented by open cusped solutions associated to the geometrically thick tori. This exploration of some key concepts of the GR-MHD formulation in its applications to High-Energy Astrophysics starts with the discussion of the initial data problem for a most general Einstein-Euler-Maxwell system addressing the problem with a relativistic geometric background. The system is then set in quasi linear hyperbolic form, and the reduction procedure is argumented. Then, considerations follow on the analysis of the stability problem for self-gravitating systems with determined symmetries considering the perturbations also of the geometry part on the quasi linear hyperbolic onset. Thus we focus on the ideal GR-MHD and self-gravitating plasma ball. We conclude with the models of geometrically thick GR-HD disks gravitating around a Kerr SMBH in their GR-HD formulation and including in the force balance equation of the disks the influence of a toroidal magnetic field, determining its impact in tori topology and stability, Comment: 42 pages; 2 figures, 1 table

This work discusses observational evidences of quantum effects on geometry in a black hole (BH) astrophysical context. We study properties of a family of loop quantum corrected regular BH solutions and their horizons, focusing on the geometry symmetries. We explore a recent model where the geometry is determined by a metric quantum modification outside the horizon: a regular static spherical solution of minisuperspace BH metric with Loop Quantum Gravity (LQG) corrections. The solutions are characterized by some polymeric functions and the emergence of a singularity in the limiting Schwarzschild geometry. We discuss particular metric solutions for similar properties of structures, the metric Killing bundles (metric bundles MBs), related to the BH horizons properties. A comparison with the Reissner-Nordstrom geometry and the Kerr geometry, similar for their respective MBs properties is done. The analysis provides a way to recognize these geometries and detect phenomenological evidence of LQG origin by the detection of stationary/static observers and the properties of lightlike orbits with the analysis of the conformal invariant MBs related to the (local) causal structure. This approach could be applied in other quantum corrected BH solutions constraining the characteristics of the underlining LQG-graph, as the minimal loop area, through photons detection. Light surfaces associated with a diversified range of BH phenomenology and grounding MB definition provide a research channel of possible astrophysical evidence. The BHs thermodynamic characteristics are studied, luminosity, surface gravity, and temperature. Ultimately the application of this method to this spherically symmetric approximate solution provides a way to clarify some formal aspects of MBs in the presence of static spherical symmetric spacetimes., Comment: 34 pages; 16 figure multipanels; 2 tables

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 the definition of metric bundles in axially symmetric geometries and give explicit examples for solutions of Einstein equations. These structures have been introduced in Pugliese and Quevedo (2019) to explain some properties of black holes (BHs) and naked singularities (NSs), investigated through the analysis of the limiting frequencies of stationary observers, which are at the base of a Killing horizon definition for these black hole spacetimes. In Pugliese and Quevedo (2019), we introduced the concept of NS Killing throats and bottlenecks associated to, and explained by, the metric bundles. In particular, we proved that the horizon frequency can point out a connection between BHs and NSs. We detail this definition in general and review some essential properties of metric bundles as seen in different frames and exact solutions., Comment: 10 pages; 1 figure; RAGtime 20, 2018

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)

The properties of Kerr black holes (BHs)and naked singularities (NSs) are investigated by using stationary observers and their limiting frequencies. We introduce the concept of NS Killing throats and bottlenecks for slowly spinning NSs to describe the frequency of stationary observers. In particular, we show the frequency on the horizon can be used to point out a connection between BHs and NSs and to interpret the horizon in terms of frequencies. The analysis is performed on the equatorial plane of the ergoregion., Comment: 15 pages; 8 figures. (15th Marcel Grossmann Meeting :: Rome, July 1-7, 2018)

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

We study the properties of black holes and naked singularities by considering stationary observers and light surfaces in Kerr spacetimes. We reconsider the notion of Killing horizons from a special perspective by exploring the entire family of Kerr metrics. To this end, we introduce the concepts of extended plane, Killing throats and bottlenecks for weak (slowly spinning) naked singularities. Killing bottlenecks (or horizon remnants in analogy with the corresponding definition of throats in black holes) are restrictions of the Killing throats appearing in special classes of slowly spinning naked singularities. Killing bottlenecks appear in association with the concept of pre-horizon regime introduced in [1, 2]. In the extended plane of the Kerr spacetime, we introduce particular sets, metric bundles, of metric tensors which allow us to reinterpret the concept of horizon and to find connections between black holes and naked singularities throughout the horizons. To evaluate the effects of frame-dragging on the formation and structure of Killing bottlenecks and horizons in the extended plane, we consider also the Kerr-Newman and the Reissner-Norstrom spacetimes. We argue that these results might be significant for the comprehension of processes that lead to the formation and eventually destruction of Killing horizons., Comment: 33 pages, 32 multi-panels figures, 3 Tables

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

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

We analyzed the effects of a toroidal magnetic field in the formation of several magnetized accretion tori, dubbed as ringed accretion disks (RADs), orbiting around one central Kerr supermassive Black Hole (SMBH) in AGNs, where both corotating and counterotating disks are considered. Constraints on tori formation and emergence of RADs instabilities, accretion onto the central attractor and tori collision emergence, are investigated. The results of this analysis show that the role of the central BH spin-mass ratio, the magnetic field and the relative fluid rotation and tori rotation with respect the central BH, are crucial elements in determining the accretion tori features, providing ultimately evidence of a strict correlation between SMBH spin, fluid rotation and magnetic fields in RADs formation and evolution. More specifically we proved that magnetic field and disks rotation are in fact strongly constrained, as tori formation and evolution in RADs depend on the toroidal magnetic fields parameters. Eventually this analysis identifies specific classes of tori, for restrict ranges of magnetic field parameter, that can be observed around some specific SMBHs identified by their dimensionless spin., Comment: 15 pages, 7 multi-panels figures. To appear on MNRAS

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 perform a detailed analysis of the properties of stationary observers located on the equatorial plane of the ergosphere in a Kerr spacetime, including light-surfaces. This study highlights crucial differences between black hole and the super-spinner sources. In the case of Kerr naked singularities, the results allow us to distinguish between "weak" and "strong" singularities, corresponding to spin values close to or distant from the limiting case of extreme black holes, respectively. We derive important limiting angular frequencies for naked singularities. We especially study very weak singularities as resulting from the spin variation of black holes. We also explore the main properties of zero angular momentum observers for different classes of black hole and naked singularity spacetimes., Comment: 20 pages, 13 multi-panels figures, 2 tables

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

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

The spider silk is one of the most interesting bio-materials investigated in the last years. One of the main reasons that brought scientists to study this organized system is its high level of resistance if compared to other artificial materials characterized by higher density. Subsequently, researchers discovered that the spider silk is a complex system formed by different kinds of proteins, organized (or disorganized) to guarantee the required resistance, which is function of the final application and of the environmental conditions. Some spider species are able to make different silks, up to twelve, having a composition that seems to be function of the final use (i.e. dragline web, capture web, etc). The aim of this paper is to analyze the properties of the spider silk by means of a thermodynamic approach, taking advantage of the well-known theories applied to polymers, and to try to underline and develop some intriguing considerations. Moreover, this study can be taken as an example to introduce and discuss the importance of the concept of optionality and of the anti-fragile systems proposed by N. N. Thaleb in his book "Antifragile: Things that gain from disorder"., Comment: 9 pages, 4 figures

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