The Schwarzschild star is an ultracompact object beyond the Buchdahl
limit, which has Schwarzschild geometry outside its surface and positive
pressure in the external layer which vanishes at the surface. Recently
it has been shown that the Schwarzschild star is stable against
spherically-symmetric perturbations. Here we study arbitrary axial non-
spherical perturbations, and show that the observable quasinormal modes
can be as close to the Schwarzschild limit as one wishes, what makes the
Schwarzschild star a very good mimicker of a black hole. The decaying
time-domain profiles prove that the Schwarzschild star is stable against
non-spherical perturbations as well. Another peculiar feature is the
absence of echoes at the end of the ringdown. Instead we observe a non-
oscillating mode which might belong to the class of algebraically
special modes. At asymptotically late times, Schwarzschildian power-law
tails dominate in the signal.
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We employ the minimal geometric deformation approach to gravitational
decoupling (MGD-decoupling) in order to generate an exact anisotropic
and non-uniform version of the ultracompact Schwarzschild star, or
'gravastar', proposed by Mazur and Mottola. This new system represents
an ultracompact configuration of radius $R_{S}=2cal{M}$ whose interior
metric can be matched smoothly to a conformally deformed Schwarzschild
exterior. Remarkably, the model satisfies some of the basic requirements
to describe a stable stellar model, such as a positive density
everywhere and decreasing monotonously from the centre, as well as a
non-uniform and monotonic pressure.
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We study the optical properties of the Kehagias-Sfetsos (KS) compact
objects, characterized by the "Hov{r}ava" parameter $omega_{_{KS}}$,
in the presence of plasma, considering its homogeneous or power-law
density distribution. The strong effects of both "Hov{r}ava" parameter
$omega_{_{KS}}$ and plasma on the shadow cast by the KS compact objects
are demonstrated. Using the weak field approximation, we investigate the
gravitational lensing effect. Strong dependence of the deflection angle
of the light on both the "Hov{r}ava" and plasma parameter is explicitly
shown. The magnification of image source due to the weak gravitational
lensing is given for both the homogeneous and inhomogeneous plasma.
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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.
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We study the appearance of Keplerian accretion disks in order to demonstrate the influence of the nonlinear electrodynamics (NED) on the gravitational lensing and frequency shifting of the images of the Keplerian disks. We focus our attention on the Bardeen black hole backgrounds with magnetic charges that could be considered to be acceptable solutions for the Einstein gravitational equations combined with those representing an NED. Photons governing the appearance of the Keplerian disks follow null geodesics of the effective geometry related to the Bardeen spacetime. We compare the appearance governed by the effective geometry to those governed by the spacetime geometry itself, and to the appearance of Keplerian disks orbiting a related Reissner-Nordstrom black hole spacetime. We demonstrate a clear and very strong difference between the disk images determined by the effective geometry and the others, both in the shape and (especially) in the frequency mapping where it exhibits a difference of three orders.
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Three well-known exact regular solutions of general relativity coupled to nonlinear electrodynamics (NED), namely the Maxwellian, Bardeen, and Hayward regular spacetimes, which can describe either a regular black hole or a geometry without horizons, have been considered. Relaxation times for the scalar, electromagnetic (EM) and gravitational perturbations of black holes and no-horizon spacetimes have been estimated in comparison with the ones of the Schwarzschild and Reissner-Nordström spacetimes. It has been shown that the considered geometries in general relativity coupled to the NED have never-vanishing circular photon orbits, and on account of this fact, these spacetimes always oscillate the EM perturbations with quasinormal frequencies. Moreover, we have shown that the EM perturbations in the eikonal regime can be a powerful tool to confirm (i) that the light rays do not follow null geodesics in the NED by the relaxation rates and (ii) if the underlying solution has a correct weak field limit to the Maxwell electrodynamics by the angular velocity of the circular photon orbit.
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A recent study of the X-ray source XMMUJ134736.6+173403 has revealed a strong quasi-periodic modulation in the X-ray flux. The observation of two quasiperiodic oscillations (QPOs) that occur on a daily timescale and exhibit a 3:1 frequency ratio strongly supports the evidence for the presence of an active galactic nucleus black hole (AGN BH). Assuming an orbital origin of QPOs, we calculated the upper and lower limit on AGN BH mass M and found M ≈ 107 - 109 M⊙. When we compare this to mass estimates of other sources, XMMUJ134736.6+173403 appears to be the most massive source with commensurable QPO frequencies, and its mass represents the current observational upper limit on AGN BH mass based on QPO observations. We note that it will be crucial for the falsification of particular resonance models of QPOs whether only a single QPO with a frequency that completes the harmonic sequence 3 : 2 : 1 is found in this source, or if a new different pair of QPOs with frequencies in the 3 : 2 ratio is found. The former case would agree with the prediction of the 3 : 2 epicyclic resonance model and BH mass M ≈ (5a2 + 8a + 8)×107 M⊙, where a is a dimensionless BH spin.
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In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced "spectral-timing-polarimetry" techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process. X-spinmeasurements
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We study in the weak field limit the gravitational lensing by spherically symmetric compact object immersed in an asymptotically uniform magnetic field in the presence of plasma and our approach is based on the medium modified Hamiltonian one. We show that the magnetized plasma in the environment of compact object may lead to split of the Einstein cross, creating additional lensed components. Finally we calculate magnification and time delay related to the individual images.
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We discuss the effects of electric charging on the equilibrium
configurations of magnetized, rotating fluid tori around black holes of
different mass. In the context of gaseous/dusty tori in galactic nuclei,
the central black hole dominates the gravitational field and it remains
electrically neutral, while the surrounding material acquires some
electric charge and exhibits non-negligible self-gravitational effect on
the torus structure. The structure of the torus is influenced by the
balance between the gravitational and electromagnetic forces. A cusp may
develop even in Newtonian tori due to the charge distribution.
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