MG12 - Talk detail
 

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Abstract

Orbital resonance models of QPOs in braneworld Kerr black holes

Rotating black holes in the brany universe of the Randall-Sundrum type with infinite additional dimension are described by the Kerr geometry with a tidal charge $b$ representing the interaction of the brany black hole and the bulk spacetime. For $b<0$ rotating black holes with dimensionless spin $a>1$ are allowed. We investigate the role of the tidal charge in the orbital resonance model of quasiperiodic oscillations (QPOs) in black hole systems. The orbital Keplerian frequency and the radial and vertical epicyclic frequencies of the equatorial, quasicircular geodetical motion are given and their radial profiles are discussed. We show how the tidal charge could influence matching of the observational data indicating the 3:2 frequency ratio observed in GRS 1915+105 and GRO J1655-40 microquasars with prediction of the orbital resonance model; limits on allowed range of the black hole parameters $a$ and $b$ are established. The "magic" dimensionless black hole spin enabling presence of strong resonant phenomena at the radius where $\nu_K:\nu_\theta:\nu_r = 3:2:1$ is determined in dependence on the tidal charge. Such strong resonances could be relevant even in sources with highly scattered resonant frequencies, as those expected in Sgr A*. We can conclude that analysing the microquasars high-frequency QPOs in the framework of orbital resonance models, we can put relevant limits on the tidal charge of brany Kerr black holes.

Quasiperiodic oscillations in braneworld neutron star spacetimes

The strong gravitational field of neutron stars in the brany universe could be described by spherically symmetric solutions with a metric in the exterior to the brany stars being of the Reissner-Nordström type containing a braneworld tidal charge representing the tidal effect of the bulk spacetime onto the star structure. We investigate the role of the tidal charge in orbital resonance models of high-frequency quasiperiodic oscillations (QPOs) observed in neutron star binary systems. We give the radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic oscillations. We show how the standard relativistic precession model modified by the tidal charge fits the observational data, giving estimates of the allowed values of the tidal charge and the brane tension based on the processes going in the vicinity of neutron stars. We compare the strong field regime restrictions with those given in the weak-field limit of solar system experiments. We make some estimates for selected atoll sources.

Multi-resonance models of QPOs and black holes admitting strong resonanant phenomena

Using known frequencies of the accretion disc 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. Higher precision of the black hole 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. We give the epicyclic frequencies with influence of $\Lambda>0$ taking into account. In the multi-resonant model, the twin peak resonances are combined properly to give the observed frequency set. The multi-resonant model is proposed in three distinct versions. In the first one, related probably to the neutron star binary systems, more instances of one resonance occur at more specific radii. In the second case, more resonances are sharing one specific radius, allowing for „cooperative“ strong resonant phenomena in the field of black holes with a specific („magic“) value of spin. In the third („ugly“) case, more resonances occur at more specific radii; we restrict our attention to the case of two such resonant radii. For special values of the spin, only triple-set of frequencies is observed because of coincidence of some frequencies, allowing determination of the spin from the triple frequency ratio set. The spin is determined precisely, but not uniquely as the same frequency set could be relevant for more than one concrete spin and combination of resonant oscillations.

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