Using known frequencies of the twin peak quasiperiodic oscillations (QPOs) and the known mass of the central black hole, the black hole dimensionless spin a can be determined, assuming a concrete version of the orbital resonance model. However, because of large range of observationally limited values of the black hole mass, its spin can be estimated with a low precision only. Higher precision of the black hole dimensionless spin measurement is possible in the framework of multi-resonance model of QPOs inspired by complex high-frequency QPO patterns observed in some black hole and neutron star systems.
Read More
Humpy profile of the LNRF-related orbital velocity was found for accretion discs orbiting rapidly rotating Kerr black holes with a spin a > 0.9953 (Keplerian discs) and a > 0.99979 (marginally stable thick discs). Maximal positive rate of change of the orbital velocity in terms of the proper radial distance is used to define a local frequency characterising possible physical processes in the disc connected with the velocity hump. Comparing the "humpy frequency" related to distant observers with epicyclic frequencies of perturbed orbital motion, it was shown that in Keplerian discs orbiting near-extreme Kerr black holes (a > 0.998) the the ratio of radial epicyclic frequency and humpy frequency (both evaluated at the same radius) is in terms of small integers asymptotically going to the ratio 3:2 for a → 1. The Extended Orbital Resonance Model with non-linear hump-induced oscillations was applied to two X-ray variable sources GRS 1915+105 and XTE J1650-500. In the case of GRS 1915+105, the model is able to address the whole set of reported QPOs, giving the mass and spin of the central black hole: a = 0.9998; M = 14:8M⊙. For XTE J1650-500, similar ideas give values a = 0.9982; M = 5:1M⊙.
Read More
We summarize influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The shape of an equatorial thin accretion disk and profiled spectral lines of thin keplerian rings around the black holes are given and classified in terms of the black hole rotational and tidal parameters. It is shown that rising of negatively-valued tidal parameter, with rotational parameter fixed, generally strengthens the relativistic effects and suppresses the rotation induced asymmetries in the optical phenomena.
Read More
The appearance of corotating Keplerian discs orbiting Kerr superspinars has been shown to be strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile, demonstrating clear distinctions from the corotating discs around black holes. However, it has to be modified in the innermost parts of the disc by the self-illumination effect caused by the photons trapped in the strong gravitational field near the superspinars with spin a < 9. Here we demonstrate that self-illumination is irrelevant for counter-rotating Keplerian discs orbiting in the field of all Kerr superspinars since such discs are located at the regions where none of the radiated photons can be trapped. Therefore, the results obtained by considering the escaping photons only are quite relevant for the appearance of the counter-rotating Keplerian discs orbiting Kerr superspinars. The appearance and spectral continuum of such discs differ from those of counter-rotating discs in the field of Kerr black holes. The differences are of quantitative character except the existence of the disc images located inside the image of the inner edge of the disc that are created by photons reaching regions close to the surface of Kerr superspinars. Their observational relevance is realistic in the near future for accretion phenomena in the vicinity of Sgr A*.
Read More
Extremely Compact Stars (ECS) contain trapped null geodesics. When such
objects enter the evolution period admitting geodetical motion of
neutrinos, certain part of neutrinos produced in their interior will be
trapped influencing their neutrino luminosity and thermal evolution. We
study neutrino trapping in the braneworld ECS, assuming uniform
distribution of neutrino emissivity and massless neutrinos. We give the
efficiency of the neutrino trapping effects in the framework of the
simple model of the internal spacetime with uniform distribution of
energy density, and external spacetime described by the
Reissner-Nordström geometry characterized by the braneworld "tidal"
parameter b. For b < 0 the external spacetime is of the black-hole
type, while for b > 0 the external spacetime can be of both
black-hole and naked-singularity type. Then the ECS surface radius R can
be located also above the unstable (outer) photon circular orbit. Such
basically new types of the spacetimes strongly alter the trapping
phenomena as compared to the standard case of b = 0. It is shown that
the neutrino trapping effects are slightly lowered by the presence of
physically more plausible case of b < 0, as compared to the standard
internal Schwarzschild spacetime, while they can be magnified by
positive tidal charges if b < 1 and lowered for b > 1. However,
potential astrophysical relevance of the trapping phenomena is strongly
enhanced for negative tidal charges enabling a significant enlargement
of the ECS surface radius to values coherent with recent observations.
Read More
Extremely Compact Stars (ECS) contain trapped null geodesics. When such objects enter the evolution period admitting geodetical motion of neutrinos, certain part of neutrinos produced in their interior will be trapped influencing their neutrino luminosity and thermal evolution. We study neutrino trapping in the braneworld ECS, assuming uniform distribution of neutrino emissivity and massless neutrinos. We give the efficiency of the neutrino trapping effects in the framework of the simple model of the internal spacetime with uniform distribution of energy density, and external spacetime described by the Reissner-Nordström geometry characterized by the braneworld "tidal" parameter b. For b < 0 the external spacetime is of the black-hole type, while for b > 0 the external spacetime can be of both black-hole and naked-singularity type. Then the ECS surface radius R can be located also above the unstable (outer) photon circular orbit. Such basically new types of the spacetimes strongly alter the trapping phenomena as compared to the standard case of b = 0. It is shown that the neutrino trapping effects are slightly lowered by the presence of physically more plausible case of b < 0, as compared to the standard internal Schwarzschild spacetime, while they can be magnified by positive tidal charges if b < 1 and lowered for b > 1. However, potential astrophysical relevance of the trapping phenomena is strongly enhanced for negative tidal charges enabling a significant enlargement of the ECS surface radius to values coherent with recent observations.
Read More
Astrophysical fluids may acquire nonzero electrical charge because of
strong irradiation or charge separation in a magnetic field. In this
case, electromagnetic and gravitational forces may act together and
produce new equilibrium configurations, which are different from the
uncharged ones. Following our previous studies of charged test particles
and uncharged perfect fluid tori encircling compact objects, we
introduce here a simple test model of a charged perfect fluid torus in
strong gravitational and electromagnetic fields. In contrast to ideal
magnetohydrodynamic models, we consider here the opposite limit of
negligible conductivity, where the charges are tied completely to the
moving matter. This is an extreme limiting case which can provide a
useful reference against which to compare subsequent more complicated
astrophysically motivated calculations. To clearly demonstrate the
features of our model, we construct three-dimensional axisymmetric
charged toroidal configurations around Reissner-Nordström black
holes and compare them with equivalent configurations of electrically
neutral tori.
Read More
We used the equations of state (EoS) of dense nuclear matter to
construct the macroscopic properties of neutron stars and test them
using available observational results. The Dirac-Brueckner-Hartree-Fock
mean field calculations approximated by their parameterized form are the
basis of our calculations for the star interior. We calculated the
central pressure, density, radius and mass both without and with
allowance for hyperons, and compared these results with astronomical
observations.
Read More
Astrophysical fluids may acquire nonzero electrical charge because of strong irradiation or charge separation in a magnetic field. In this case, electromagnetic and gravitational forces may act together and produce new equilibrium configurations, which are different from the uncharged ones. Following our previous studies of charged test particles and uncharged perfect fluid tori encircling compact objects, we introduce here a simple test model of a charged perfect fluid torus in strong gravitational and electromagnetic fields. In contrast to ideal magnetohydrodynamic models, we consider here the opposite limit of negligible conductivity, where the charges are tied completely to the moving matter. This is an extreme limiting case which can provide a useful reference against which to compare subsequent more complicated astrophysically motivated calculations. To clearly demonstrate the features of our model, we construct three-dimensional axisymmetric charged toroidal configurations around Reissner-Nordström black holes and compare them with equivalent configurations of electrically neutral tori.
Read More
We used the equations of state (EoS) of dense nuclear matter to construct the macroscopic properties of neutron stars and test them using available observational results. The Dirac-Brueckner-Hartree-Fock mean field calculations approximated by their parameterized form are the basis of our calculations for the star interior. We calculated the central pressure, density, radius and mass both without and with allowance for hyperons, and compared these results with astronomical observations.
Read More
