The basic adea of the Penrose process is given and its variants are introduced along with describing the role in astrophysical phenomena. These phenomena illustrate interplay of the regular and chaotic motion allowed for ionized matter. We focus on demonstration of possible occurence of ionized Keplerian disks orbiting a magnetized black hole and the situations giving rise to interesting...
Minuscule primordial black holes produced just after inflation can serve as ``time capsules" bringing back energy from the past to a later epoch when they evaporate. As these black holes behave like matter, while the rest of the Universe content behaves like radiation, the mass fraction of these black holes, that is tiny at formation, becomes significant later. If sufficiently small, these...
We argue that if in the next few years LIGO-Virgo-KAGRA interferometers would not detect a sufficient number of events involving light black holes, i.e. with masses smaller than 1.5 solar mass, this would be an unambiguous proof that the primordial black holes in the asteroid mass range are NOT present in the dark matter halo of the Galaxy.
The formation of Primordial black holes is naturally enhanced during the quark-hadron phase transition, because of the softening of the equation of state: at a scale between 1 and 3 solar masses, the threshold is reduced of about 10% with a corresponding abundance of primordial black significantly increased by more than 100 times. We show that a sub-population of primordial black black holes...
Active Galactic Nuclei (AGN) are powered by accretion onto a super-massive black hole. Despite of different mass and size-scale, the accretion processes might be similar to accretion on stellar-mass black holes in X-ray Binaries (XRBs). In this talk, I will review the observational studies aimed at comparison of accretion in AGN and XRB.
Most galaxies harbour a super-massive black hole (SMBH) in their centre but deriving its physical properties is not straightforward, especially if it is in quiescent state. If a star falls inside the tidal radius of the black hole its disruption can set on the accretion, which produces a distinctive observational signature. However, for characterising the SMBH from the observed tidal...
General relativity predicts the loss of causal structure of spacetime. This is clearly seen with the appearance of singularities in the gravitational collapse (Penrose 65), which when removed lead to the appearance of a Cauchy horizon, a null hyper-surface beyond which predictability breaks down (Poisson-Israel 89). So far no counterexample to the above is known, leading to the belief that...
A compact supermassive source Sagittarius A* located at the centre of our Galaxy has been observed at different wavelengths across the electromagnetic spectrum, being currently the best known black hole candidate. It is also the closest and largest in projection supermassive black hole candidate. At the same time, its particle acceleration capability related cosmic ray and neutrino messengers...
I will briefly discuss how the first images of the supermassive black holes M87 and Sgr A were obtained by the EHT collaboration. In particular, I will describe the theoretical aspects that have allowed us to model the dynamics of the plasma accreting onto the black hole and how such dynamics was used to generate synthetic black-hole images. I will also illustrate how the comparison between...
For the past two decades, flares (i.e. outbursts of radiation) have been observed from the centre of the Milky Way where a massive compact object of 4.3 millions solar masses resides at only 8.3 kpc. This makes this object called Sgr A* the closest supermassive black hole candidate to Earth and an unique laboratory for relativistic astrophysics. Recent observations have shown that the source...
We report on a first detection of an orbiting hot spot in ALMA polarimetric light curves of Sagittarius A*, manifesting in the loop-like patterns on the linear polarization Q-U plane following the X-ray flare detected by Chandra. The observed hot spot period corresponds to about 70 minutes, and the implied orbital radius corresponds to about 5 Schwarzschild radii. These new observations...
I will summarize our current understanding of dust-enshrouded sources that are observed in the S cluster around the supermassive black hole (SMBH). Currently, we observe a population of about 10 sources that share common characteristics: a prominent near-infrared excess and rather broad emission lines (Br-gamma, HeI). These sources are quite distinct from the majority of the surrounding B-type...
We are fitting dynamics of electrically neutral hot-spot orbiting around Sagittarius A* source in Galactic centre, represented by various modifications of the standard Kerr black hole, to the three flares observed by the GRAVITY instrument. We consider stationary, axisymmetric, and asymptotically flat spacetimes describing charged black holes in general relativity combined with nonlinear...
Large scale magnetic fields pervade the cosmic environment and the astrophysical black holes are often embedded and influenced by the mutual interaction. We will outline the appropriate mathematical framework to describe magnetized black holes within General Relativity and we will show several examples how these can be employed in the astrophysical context.
The study of charged test particle dynamic in the combined black hole gravitational field and magnetic field around it could provide important theoretical insight into astrophysical processes around such compact objects. We will examine simple but astrophysically relevant parabolic magnetic field configuration as a toy model for magnetosphere around Schwarzschild black hole. Bounded orbits and...
We study charged non-conducting fluid tori orbiting in the background given by a Schwarzschild
black hole immersed in a parabolic magnetic field introduced in the context of the Blandford–Znajek
process. In our study we focus on the off-equatorial charged tori that could be related to creation
of jets due to the Blandford-Znajek process.
I will argue that if black holes represent one the most fascinating implications of Einstein's theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the gravitational signal can provide tight constraints on the...
Weakly magnetized stars lead much more interesting and active lives than their more glamorous cousins (the magnetars and canonical TeraGauss radio and X-ray pulsars), accreting huge amounts of mass and angular momentum, emitting copious amounts of power in energetic particles and quanta, and destroying their companions. I will give a short overview of the highlights of their observed history,...
We present a simple semi-analytic model of geometrically-thin accretion flows with significant eccentricity. The structure of the flow, including three-dimensional density profile and velocity field is described by three azimuthally-dependent quantities giving a thickness of the flow in the radial and vertical direction and radial distribution of the angular momentum. These quantities are a...
Following the previous research on epicyclic oscillations of accretion disks around black holes (BHs) and neutron stars (NSs), a new model of high-frequency quasi-periodic oscillations (QPOs) considering radial precession of accretion flow, has been proposed (CT model). Within the model the configuration of accreted fluid is approximated by marginally overflowing accretion tori predicted by...
Interaction of a planetary magnetosphere with the magnetic field
of its host star defines the atmospheric outer boundary conditions on the planet and modulates the radio emission from the system. Using state-of the-art numerical simulations, we investigate magnetospheric interaction of a planet orbiting different types of stars including pulsars.
We shall discuss whether a pole-dipole-quadrupole body described by the Mathisson-Papapetrou-Dixon equations can move on geodesic trajectories in a black hole spacetime background. We shall use the Ohashi-Kyrian-Semerak spin supplementary condition to fix the centroid of the body, since under this condition the four-momentum can be set parallel to the four-velocity as holds for geodesic motion.
Accreting stellar-mass black holes in X-ray binaries (XRBs) can be found in
different states that are characterized by different spectral properties.
The two basic states are high/soft and low/hard states in which either the
thermal multi-colour black-body component originating in the accretion disc or
non-thermal power-law like Comptonisation component originating in the hot
corona...
Observing the x-ray black hole binary source GRS1915+105 has been classified as a top priority during the first year of IXPE operation. The primarily occurrence of the source in thermal state makes it the perfect candidate for black hole spin measurements. To credibly assess the source polarimetric properties, we use a multicolor black body emission model accounting for thermal radiation from...
A widely accepted picture of an accretion flow in a soft spectral state X-ray binary system is a geometrically thin disk structure much alike the classic analytic solution of Shakura and Sunyaev. Despite the fact that the analytic models are troubled by instabilities and miss important aspects of physics such as magnetic fields, they are successfully used as a framework for interpreting...
Based on the high cadence X-ray monitoring of a flare coming from the nucleus of a galaxy located cca 250 Mpc away and discovered by ASASSN in December 2020, the existence of ultra-fast outflows in the source was revealed. The outflows were quasiperiodically launched from near the SMBH with variable column density and ionization, but constant velocity, for more than three months. We interpret...
A review will be presented of the idea of black widow pulsars and of recent literature which aims to give a lower limit for the maximum neutron star mass using observational data for these objects. Comparison will be made with other related work.
Kilonovae are transient events observed in Optical-Near IR band, powered by radioactive decay of elements produced through r-process nucleosynthesis. They form in the ejecta emitted in binary neutron star mergers. The kilonova signal may accompany the gravitational wave emission and potential gamma-ray burst signal, such as in the well known event discovered as GW-GRB 170817. I will discuss...
Disc structures around black holes are of great importance in astrophysics. The usual simplification of most accreting models is neglecting the disc's self-gravitation. However, in many situations, numerous disc properties may be highly sensitive to the gravitational field. We present a full metric describing the field of a thin disc and a Schwarzschild black hole. The disc density profiles...
The purpose of this talk is discuss the possibility of detection of dark matter through multiple observations of compact stars and related phenomena. Recent scientific and technological developments have allowed for a better study of the nature of these astrophysical objects, in particular of the equation of state (EoS). As we advance on the quest for clarification of the neutron star internal...
Modern astronomical observations on the international level on the ground and space telescopes, and recent discoveries have provided convincing evidence that black holes have a significant impact on nearby objects, emitting powerful gamma-ray bursts, absorbing the next star, and stimulating the growth of newborn stars in the surrounding areas.
Study the motion of the photon around rotating...
We provide an explicit, closed and compact expression for the Debye superpotential of a circular source. This superpotential is obtained by integrating the Green function of Teukolsky Master Equation (TME). The Debye potential itself is then, for a particular configuration, calculated in the same manner as the $\phi_0$ field component is calculated from the Green function of the TME --- by...
Relativistic, broad-asymmetric Fe K emission lines are observed in many accreting systems from black holes in active galactic nuclei and X-ray binaries to neutron star low-mass X-ray binaries. The origin of the line broadening is often interpreted as due to dynamical broadening and relativistic effects of matter in accretion disks around the compact object. In the case of neutron stars, iron...
Complex modeling of the radiation spectrum of accreting neutron stars.
Strange quark stars (SQS) are a type of compact object that may be created through a second explosion after the proto-neutron star state. The magnetic field of the quark star core may reach 10^{18} G. Using the MIT bag model with the density-dependent bag constant for the equation of state, we study the structure and total energy of the SQS in the presence of the strong magnetic fields with...
Magnetars and pulsars are known for their extreme magnetic field. However, magnetism seems to be the common attribute of almost all other astrophysical objects. Can this universality of astrophysical magnetism be related to the other universal principle, that of least action? We derive (simplified) stellar structure equations from a variational principle minimizing the total energy of a star...
Linear perturbations of Robinson-Trautman radiative solutions with the
new nonlinear electrodynamics model are investigated. The results show
that unlike the solutions of Maxwell electrodynamics in the
Robinson-Trautman class, our new solutions are well-posed.
We demonstrate how the inclusion of a R^2 term and the use of the Palatini formalism allows models of inflation that have been excluded by the Planck data in their simplest realizations to become viable once again. We also discuss how the inclusion of a non-minimal derivative coupling term in the action allows the predictions of Palatini-Higgs inflation to be in contact with observations in...
Motivated by processes happening around accreting black holes,a series of papers entitled "Free motion around black holes with discs or rings: Between integrability and chaos" were published in order to classify and evaluate the chaotic behaviour that may occur in geodesic motion around static black holes if these are perturbed by gravity of some additional matter. However the static and...
In magnetized astrophysical plasma magnetic reconnection is the accepted mechanism that heats and accelerates plasmoids and affects the dynamics of the plasma. This is thought to be the mechanism for the dissipation of magnetic energy in the still poorly understood phenomena of solar flares, magnetic substorms in the Earth magnetosphere, jets, and relativistic ejections from the accretion...
We derive and analyze particularly simple phantom scalar field solution within the Weyl geometry comparing it with the Curzon--Chazy spacetime. We show that this solution contains a wormhole throat which connects to a region that behaves like a one-directional time machine. We investigate its conformal structure and analyze a non-scalar singularity appearing on the boundary of the spacetime...
This paper is devoted to investigate the possible ways of distinguishing regular and singular black holes (BHs) in modified gravity (MOG) called regular MOG (RMOG) and Schwarzschild MOG (SMOG) BHs through observational data from twin peak quasiperiodic oscillations (QPOs) which are generated by test particles in stable orbits around the BHs. The presence of MOG field causes to sufficiently the...
Astonishing precision of gravitational wave detector LIGO
requires careful analysis of the signal with new data analysis
techniques. Recurrence quantification analysis is nonlinear method for
data analysis based on number and duration of recurrences in observed
data. It is especially useful for detection of deterministic behaviour
in apparently randomly distributed data generated by...
We study shadows in the naked singularity spacetime, focusing on analytical studies, including the null naked singularity and naked singularity spacetime with quadrupole. We show that the Angular size of shadows in a different scenarios for the Schwarzschild black hole, null naked singularity and for the q-metric. The shadows of q-metric depends on the value of the quadrupole parameter.
Both spectral energy distribution (SED) of the continuum and the spectral lines offer a way to measure and model parameters of the inner and outer region of the black hole accretion disc. We introduce a perturbative term located in the orbital plane of the accretion disc and parametrize the spectral changes to reveal the decrease of the emissivity of the expected signal. We assume three...
We present 2D global Radiation GRMHD simulation of accretion disks
rotating around a 10 solar mass non-rotating black hole. The gas
initially is set up following the Penna et al. (2013) equilibrium torus
solutions threaded by a poloidal magnetic field. We examined the effect
of resolution in various quantitative values such as the accretion
rate, magnetization, various fluxes and disk...
Disk reflection and disk wind are two seemingly disconnected phenomena in the X-ray spectra of Black Hole X-ray binaries, providing independent probes to constrain important physical parameters associated with the accretion disks (e.g., inclination and density). A synergy between reflection and absorption spectroscopy, therefore, can provide an unprecedented insight into the physical...