We explore the influence of nongeodesic pressure forces present in an accretion disc on the frequencies of its axisymmetric and nonaxisymmetric epicyclic oscillation modes. We discuss its implications for models of high-frequency quasi-periodic oscillations (QPOs), which have been observed in the X-ray flux of accreting black holes (BHs) in the three Galactic microquasars, GRS 1915+105, GRO J1655-40, and XTE J1550-564. We focus on previously considered QPO models that deal with low-azimuthal-number epicyclic modes, |m| ≤ 2, and outline the consequences for the estimations of BH spin, a ∈ [0, 1]. For four out of six examined models, we find only small, rather insignificant changes compared to the geodesic case. For the other two models, on the other hand, there is a significant increase of the estimated upper limit on the spin. Regarding the falsifiability of the QPO models, we find that one particular model from the examined set is incompatible with the data. If the spectral spin estimates for the microquasars that point to a > 0.65 were fully confirmed, two more QPO models would be ruled out. Moreover, if two very different values of the spin, such as a ≈ 0.65 in GRO J1655-40 and a ≈ 1 in GRS 1915+105, were confirmed, all the models except one would remain unsupported by our results. Finally, we discuss the implications for a model that was recently proposed in the context of neutron star (NS) QPOs as a disc-oscillation-based modification of the relativistic precession model. This model provides overall better fits of the NS data and predicts more realistic values of the NS mass compared to the relativistic precession model. We conclude that it also implies a significantly higher upper limit on the microquasar's BH spin (a ∼ 0.75 vs. a ∼ 0.55).
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Extremely compact objects containing a region of trapped null geodesics could be of astrophysical relevance due to trapping of neutrinos with consequent impact on cooling processes or trapping of gravitational waves. These objects have previously been studied under the assumption of spherical symmetry. In the present paper, we consider a simple generalization by studying trapping of null geodesics in the framework of the Hartle-Thorne slow-rotation approximation taken to first order in the angular velocity, and considering a uniform-density object with uniform emissivity for the null geodesics. We calculate effective potentials and escape cones for the null geodesics and how they depend on the parameters of the spacetimes, and also calculate the "local" and "global" coefficients of efficiency for the trapping. We demonstrate that due to the rotation the trapping efficiency is different for co-rotating and retrograde null geodesics, and that trapping can occur even for R >3 G M /c2 , contrary to what happens in the absence of rotation.
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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.
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Here we have developed the general parametrization for spherically symmetric and asymptotically flat black-hole spacetimes in an arbitrary metric theory of gravity. The parametrization is similar in spirit to the parametrized post-Newtonian approximation, but valid in the whole space outside the event horizon, including the near horizon region. This generalizes the continued-fraction expansion method in terms of a compact radial coordinate suggested by Rezzolla and Zhidenko [Phys. Rev. D 90, 084009 (2014), 10.1103/PhysRevD.90.084009] for the four-dimensional case. As the first application of our higher-dimensional parametrization we have approximated black-hole solutions of the Einstein-Lovelock theory in various dimensions. This allows one to write down the black-hole solution which depends on many parameters (coupling constants in front of higher curvature terms) in a very compact analytic form, which depends only upon a few parameters of the parametrization. The approximate metric deviates from the exact (but extremely cumbersome) expressions by fractions of one percent even at the first order of the continued-fraction expansion, which is confirmed here by computation of observable quantities, such as quasinormal modes of the black hole.
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In this paper, we explore the test particle motion around black hole in Einstein-Maxwell-scalar (EMS) theory using three different black hole solutions within this theory. We have first analyzed the spacetime curvature structure of these solutions and shown the existence of two singularities and the first one is at the center r =0 . In black hole spacetime, there are two regions divided by the critical value of the cosmological parameter λ0. The photon sphere around black hole in EMS theory has also been studied and found that it does not depend on cosmological parameter λ . We have analyzed the innermost stable circular orbits (ISCO) around black hole and shown that for all solutions ISCO radius for neutral particle decreases with the increase of black hole charge. We have also studied the charged particle motion around the black hole where charged particle motion is considered in the presence of gravitational field and the Coulomb potential. It is shown that ISCO radius for charged particles increases depending on the selected value of the coupling parameter which is in contradiction with observations of the inner edge of the accretion disks of the astrophysical black holes and can be used as powerful tool to rule out the EMS theory from consideration for the gravitational field theory. It also studied the fundamental frequencies governed by test particle orbiting around black hole in EMS theory. Finally, as test of black hole solution in EMS theory ISCO radii is compared with that in Kerr black hole and found that the spin parameter of Kerr can be mimic up to a /M ≃0.936 .
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We consider quasinormal modes and Hawking radiation of four-dimensional asymptotically flat black holes in the most general up to-cubic-order-in-curvature dimension-independent Einsteinian theory of gravity that shares its graviton spectrum with the Einstein theory on constant curvature backgrounds. We show that damping rate and real oscillation frequencies of quasinormal modes for scalar, electromagnetic and Dirac fields are suppressed once the coupling with the cubic term is on. The intensity of Hawking radiation is suppressed as well, leading to, roughly, one order longer lifetime at a sufficiently large coupling constant.
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Quasinormal modes of black holes were previously calculated in a non-linear electrodynamics and in the Gauss-Bonnet gravity theory. Here we take into consideration both of the above factors and find quasinormal modes of a (massive) scalar field in the background of a black hole in the five-dimensional Einstein-Gauss-Bonnet gravity coupled to a non-linear electrodynamics having Maxwellian weak-field limit. For the non-linear electrodynamics we considered the high frequency (eikonal) regime of oscillations analytically, while for the lower multipoles the higher order WKB analysis with the help of Padé approximants and the time domain integration were used. We found that perturbations of a test scalar field violate the inequality between the damping rate of the least damped mode and the Hawking temperature, known as the Hod's proposal. This does not exclude the situation in which gravitational spectrum may restore the Hod's inequality, so that only the analysis of the full spectrum, including gravitational perturbations, will show if the quasinormal modes we found here for the scalar field can be a counterexample to the Hod's conjecture or not. We also revealed that in such a system, which includes the higher curvature corrections and non-linear electrodynamics, for perturbations of a massive scalar field there exists the phenomenon of the arbitrary long lived quasinormal modes - quasiresonances.
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