arXiv subjects

D. Pugliese

Publications and source records attributed to D. Pugliese.

At least 37 records · Page 2Linked to original sources

arXiv2020

Constraining LQG Graph with Light Surfaces: Properties of BH Thermodynamics for Mini-Super-Space, Semi-Classical Polymeric BH

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.

gr-qc↗

arXiv2020

Limiting effects in clusters of misaligned toroids orbiting static SMBHs

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.

astro-ph.HE↗

arXiv2019

On the metric bundles of axially symmetric spacetimes

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.

gr-qc↗

arXiv2019

On the accreting tori sequences in Ringed Accretion Disks models. Polytropic equations of state and toroidal magnetic fields

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.

astro-ph.HE↗

arXiv2019

Multi-accretion events from corotating and counterrotating SMBHs tori

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

astro-ph.HE↗

arXiv2019

Killing horizons, throats and bottlenecks in the ergoregion of the Kerr spacetime

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.

gr-qc↗

arXiv2019

RADs energetics and constraints on emerging tori collisions around super-massive Kerr Black Holes

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.

astro-ph.HE↗

arXiv2019

Disclosing connections between black holes and naked singularities: Horizon remnants, Killing throats and bottlenecks

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.

gr-qc↗

arXiv2018

Relating Kerr SMBHs in Active Galactic Nuclei to RAD configurations

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<a<M, constrained by the RAD properties. The spins are remarkably close providing an accurate characterization of the Kerr BHs based on the RAD properties. RAD dynamics is richer in the spacetimes of high spins. One of the critical predictions states that a RAD tori couple formed by an outer accreting corotating and an inner accreting counterrotating torus is expected to be observed only around slowly spinning (a<0.46M) BHs. The analysis strongly binds the fluid and BH characteristics providing indications on the situations where to search for RADs observational evidences. Obscuring and screening tori, possibly evident as traces in Xray spectrum emission, are strongly constrained, eventually ruling out many assumptions used in the current investigations of the screening effects. We expect relevance of our classification of Kerr BHs in relation to AGNs phenomena that could be observed by the Xray satellite ATHENA.

astro-ph.HE↗

arXiv2018

Proto-jets configurations in RADs orbiting a Kerr SMBH: symmetries and limiting surfaces

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.

gr-qc↗

arXiv2018

Influence of toroidal magnetic field in multi-accreting tori

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.

astro-ph.HE↗

arXiv2018

Constraints on two accretion disks centered on the equatorial plane of a Kerr SMBH

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.

gr-qc↗

arXiv2018

Observers in Kerr spacetimes: the ergoregion on the equatorial plane

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.

gr-qc↗

arXiv2017

Tori sequences as remnants of multiple accreting periods of Kerr SMBHs

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.

astro-ph.HE↗

arXiv2017

Ringed accretion disks: evolution of double toroidal configurations

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.

astro-ph.HE↗

arXiv2017

General classification of charged test particle circular orbits in Reissner--Nordström spacetime

We investigate charged particles circular motion in the gravitational field of a charged mass distribution described by the Reissner-Nordström spacetime. We introduce a set of independent parameters completely characterizing the different spatial regions in which circular motion is allowed. We provide most complete classification of circular orbits for different sets of particle and source charge-to-mass ratios. We study both black holes and naked singularities and show that the behavior of charged particles depend drastically on the type of source. Our analysis shows in an alternative manner that the behavior of circular orbits can in principle be used to distinguish between black holes and naked singularities. From this analysis, special limiting values for the dimensionless charge of black hole and naked singularity emerge, namely, Q/M=1/2, $Q/M=\sqrt{13}/5$ and $Q/M=\sqrt{2/3}$ for the black hole case and Q/M=1, $Q/M=5/ (2 \sqrt{6})$, $Q/M=3 \sqrt{6}/7$, and finally $Q/M= \sqrt{9/8}$ for the naked singularity case. Similarly and surprisingly, analogue limits emerge for the orbiting particles charge-to-mass ratio $ε$, for positive charges $ε=1$, $ε=2$ and $ε=M/Q$. These limits play an important role in the study of the coupled electromagnetic and gravitational interactions, and the investigation of the role of the charge in the gravitational collapse of compact objects.

gr-qc↗

arXiv2017

A thermodynamic analysis of the spider silk and the importance of complexity

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".

nlin.AO↗

arXiv2016

Implications of the Co-rotation Theorem on the MRI in Axial Symmetry

We analyze the linear stability of an axially symmetric ideal plasma disk, embedded in a magnetic field and endowed with a differential rotation. This study is performed by adopting the magnetic flux function as the fundamental dynamical variable, in order to outline the role played by the co-rotation theorem on the linear mode structure. Using some specific assumptions (e.g. plasma incompressibility and propagation of the perturbations along the background magnetic field), we select the Alfvenic nature of the Magneto-Rotational Instability and, in the geometric optics limit, we determine the dispersion relation describing the linear spectrum. We show how the implementation of the co-rotation theorem (valid for the background configuration) on the linear dynamics produces the cancellation of the vertical derivative of the disk angular velocity (we check such a feature also in the standard vector formalism to facilitate comparison with previous literature, both in the axisymmetric and three-dimensional case). As a result, we clarify that the unstable modes have, for a stratified disk, the same morphology, proper of a thin disk profile, and the $z$ dependence has a simple parametric role.

astro-ph.HE↗