The Penrose process governing extraction of energy from black holes by the simple effects related to uncharged or charged particles is discussed in most of the known variants related to Kerr black holes and Kerr naked singularities, or to electrically charged static black holes. Attention is focused on the magnetic Penrose process in its extremely efficient version enabling extreme...

In this talk, I will discuss the possibilities of detecting

gravitational waves (GW) from hyperbolic encounters within the scope of

the current and future interferometric detectors. Given that the globular

clusters are most favorable to host these events, we study the scattering

events in these dense clusters, and discuss event rates for observing such

encounters. By incorporating the...

The general parametrization for spacetimes of spherically symmetric Lorentzian, traversable wormholes in an arbitrary metric theory of gravity is presented. The parametrization is similar in spirit to the post-Newtonian parametrized formalism, but with validity that extends beyond the weak field region and covers the whole space. Our method is based on a continued-fraction expansion in terms...

We investigate the Cho-Maison magnetic monopole in the presence of gravity. Unlike 't Hooft-Polyakov monopole, this hypothetical particle is a topological offspring of the ElectroWeak theory with a mass estimated around 5-7 TeV. Our goal is to study solutions within a large family of extended Electrowek models coupled minimally or non-minimally to gravity and see what kind of space-time...

The innermost part of the Nuclear Star Cluster is known as the S cluster that consists mostly of young B-type stars. The origin of these young stars at the scale of 1000 gravitational radii from Sgr A* has remained a puzzle. The brightest of those stars have been crucial for determining the mass of Sgr A* as well as for testing gravitational theories in the strong-field limit. Recently, Ali et...

Freeman Dyson made these points on Quantum Gravity:

(1.) A single graviton cannot be detected, because a measuring

two-body apparatus size should be smaller that its gravitational radius.

(2.) With no possibility of detecting a single graviton the

very concept of Quantum Gravity is practically meaningless.

(3.) There is nothing wrong with having separate theories

for the large scale...

Sagittarius A* is a super massive black hole in the center of our galaxy observed across the electromagnetic spectrum for decades. Moreover, it is the primary target of the Event Horizon Telescope (EHT) and the GRAVITY collaboration, which observe in the 230 GHz (EHT) and the NIR (GRAVITY). The compact radio emission is expected to be thermal radiation from the vicinity of the black hole. The...

We examine limits to the energy to which cosmic rays can be accelerated by the

termination shocks in the southern jet of radiogalaxy 3C 445. At high radio frequencies, the southern hotspot shows two sub structures named SE and SW.

By using the observed flux density at 22 GHz in SE and SW, we find that a

hotspot magnetic field of about 500 micro Gauss is required to explain the...

Astrophysical black holes in nuclei of galaxies are of indisputable relevance to the current research. These extreme objects are of immense interest to pure relativists as well as observational astronomers. We will present an overview of selected aspects of physical processes occurring in the inner regions of Active Galactic Nuclei (AGN). Observational evidence strongly suggests that strong...

The central super-massive black hole of the Milky Way, Sgr A*, accretes at a very low rate making it a very underluminous galactic nucleus. Despite the tens of Wolf-Rayet stars present within the inner parsec supplying $\sim10^{−3}\rm M_{\odot}\ yr^{-1}$ in stellar winds, only a negligible fraction of this material ($<10^{-4}$) ends up being accreted onto Sgr A*. The recent discovery of cold...

Recent observations by the Event Horizon Telescope of the image of the supermassive Black Hole at the center of M87 and SagA* provide a window into the strong-field regime of gravity. Several authors have studied the constraints that black hole shadow measurements cast on specific gravitational solutions or on the coefficients of more general parametrized frameworks. When multiple parameters...

We investigate the impact of the time-dependent moment of inertia on the braking index of pulsars. We assume the moment of inertia to be slowly varying in time and we explore how much must the moment of inertia change to explain the currently measured values of the braking index.

I will discuss developments with this

Recent constraints from neutron star measurements and nuclear experiments have narrowed the allowed physical regions for the compact stars equation of state (EoS) resulting in some of the well established models being less probable than before. These new constraints include the NICER mass-radius measurement of the object PSR J0740+6620, whereas laboratory experiments like Spectral Pion Ratio...

Some accreting neutron stars may produce near-Eddington luminosity. In general relativity, the atmosphere of a sufficiently luminous neutron star is disconnected from and levitates above its surface. The atmosphere is centered at a radius where the gravitational and radiation forces balance each other. On the other hand, in a number of space-time metrics in the case of naked singularities,...

We model an ultraluminous X-ray source using a global 2D axisymmetric general relativistic radiation magnetohydrodynamic simulation of super-Eddington accretion onto a neutron star with a 10 billion Gauss dipolar magnetic field. We found that the magnetic confinement of the gas into accretion columns along with the interaction of the gas with the neutron star surface produce a total luminosity...

The concept of blackholic quantum is at the base of the emission of high-energy (GeV) electromagnetic radiation and ultrahigh-energy cosmic rays from long GRBs and AGN. I assess from the quantitative viewpoint the blackholic quantum in specific sources and outline astrophysical consequences.

There is growing interest in the effects of the discs' self-gravity around black holes (BH), both from the theoretical point of view, as exact solutions of Einstein's equations, and from possible implications to black hole astrophysics. Here we apply the well known "displace, cut, and reflect" method, originally proposed by Kuzmin to construct analytical disk-galaxy models, to generate "black...

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...

We present an analytical study on a thick accretion disk model considering a charged fluid encircling a charge or non-charge black hole and endowed with an axisymmetric and stationary magnetic field. Our study is based on the Polish doughnut and provides an extension to this well-known model. Indeed, the effect of the electric and magnetic, and the gravitational fields are known on the...

Magnetic reconnection is an important phenomenon in the astrophysical plasma, e.g. in solar flares, interstellar medium, stellar evolution and supernovae, accretion discs, and jets. We study magnetic reconnection and plasmoid formation in the Orszag-Tang problem. We show magnetic energy reduction caused by magnetic reconnection. We also study the effect of resistivity and compare the numerical...

Puffy disk is a novel model of accretion disks resulting only from a complex global GRRMHD simulation. In this talk I will focus on the observational appearance of such type of disk, which is different from standart models of thin or slim disks. Complicated vertical structure of the disk can tell us a lot about the behaviour of plasma in the turbulent environment in the vicinity of compact...

The equation of state (EoS) of matter at supranuclear densities in the core of neutron stars (NSs) contains key information on the properties of the strong force and the possible existence of exotic states of matter; however it is only poorly constrained from earth-based experiments and theory. NS properties, the precise

values of M and R in particular, encode unique information on the EoS,...

We will present our progress in examining the influence of the quadrupole moment of slowly rotating neutron stars on the oscillations of non-slender accretion tori. We assume a perfect fluid, polytropic, constant speciﬁc angular momentum, non-selfgravitating torus, and analytically calculate formulas for the oscillation frequencies.

Ultraluminous X-ray sources (ULXs) provide an ideal laboratory to explore super-Eddington accretion onto stellar-mass compact objects, which may be an important episode in black hole evolution. We present the evidence of super-Eddington accretion in the case of three ULX sources on the base of multi-epoch spectral and timing analysis of X-ray data taken by Chandra, XMM-Newton, Suzaku and...

In this talk, I analyse the stability of self-gravitating spheres in the context of gravitational cracking. Besides exploring the role played by the anisotropy in the occurrence of cracking, we also study the effect of the complexity factor recently introduced in Phys. Rev. D 97, 044010 (2018). The models under study correspond to anisotropic solutions obtained in the framework of the...

Due to general relativistic effects, accretion disks surrounding black holes can host self-trapped g-modes close to their inner edges. This talk is devoted to their nonlinear evolution. I will discuss change of the trapping region and oscillation frequency of the g-modes when the amplitude of the oscillations is significant. When nonlinearities in the hydrodynamic equations are taken into...

We will explore dynamics of various nonlinear systems oscillating around black hole (charged particle, spinning particle, string loop, bumpy spacetime) and we will try to search for 3:2 resonances.

Monk (Zhang et al. 2019) is a general relativistic radiative transfer code that is capable of performing polarized radiative transfer. With Monk we systemically calculated the energy spectra and polarization of the X-ray radiation from AGNs and BHXRBs in the hard state. We also couple Monk and Dyplo (Marcel et al. 2018) to evaluate the polarization of BHXRBs in the hard state where we solve...

We will discuss the oscillations of spinning test particles and data fitting of quasi-periodic oscillations observed in the well known quasars as well as active galactic nuclei in the framework of the model of geodesic oscillations of Keplerian disks modified for the epicyclic oscillations of spinning test particles in the background of Kerr black holes.

In this presentation I will survey the projects that Pavel Bakala and I,

together with students and collaborators, carried out in recent years,

especially during Pavel's sabbatical at the Astronomical Observatory

of Rome in 2019. Strong-field GR in application to astrophysical

effects and problems was the leitmotiv of the projects, with

extreme light bending playing an especially...

We discuss some effects of breaking the axial symmetry in vacuum magnetospheres of black holes and neutron stars. In particular, we analyze the emergence of magnetic null points which may occur due to non-axisymmetry, and substantially affect the dynamics of electrically charged particles.

TBD