Related papers: Weak nonlinear coupling between epicyclic modes in…
In a recent publication, we have demonstrated that differentially rotating stars admit new channels of black hole formation via fragmentation instabilities. Since a higher order instability of this kind could potentially transform a…
We explore effects of the Shakura-Sunyaev alpha-viscosity on the dynamics and oscillations of slender tori. We start with a slow secular evolution of the torus. We show that the angular-momentum profile approaches the Keplerian one on the…
We study scalar perturbations of four dimensional topological nonlinear charged Lifshitz black holes with spherical and plane transverse sections, and we find numerically the quasinormal modes for scalar fields. Then, we study the stability…
We analyze the relativistic dynamical properties of Keplerian and non-Keplerian circular orbits in a general axisymmetric and stationary gravitational field, and discuss the implications for the stability of co- and counter-rotating…
An axisymmetric collapse of non-rotating gravitational waves is numerically investigated in the subcritical regime where no black holes form but where curvature attains a maximum and decreases, following the dispersion of the initial wave…
Asteroseismic studies of red giants generally assume that the oscillation modes can be treated as linear perturbations to the background star. However, observations by the Kepler mission show that the oscillation amplitudes increase…
The paper studies the uniqueness and the monotonicity of the mass-spin relation of black holes the X-ray power density spectra of which contain twin high-frequency quasi-periodic oscillations in 3:2 ratio. It is found that for geodesic…
The detection of fast quasi-periodic variability from accreting black holes and neutron stars has been used to constrain their masses, radii, and spins. If the observed oscillations are linear modes in the accretion disks, then bounds can…
The possibility that two toroidal accretion configurations may be orbiting around a~super--massive Kerr black hole has been addressed. Such tori may be formed during different stages of the Kerr attractor accretion history. We consider the…
Using a metric perturbation method, we study gravitational waves from a test particle scattered by a spherically symmetric relativistic star. We calculate the energy spectrum and the waveform of gravitational waves for axial modes. Since…
We present fully general-relativistic numerical evolutions of self-gravitating tori around spinning black holes with dimensionless spin $a/M = 0.7$ parallel or anti-parallel to the disk angular momentum. The initial disks are unstable to…
We investigate the dynamical behavior of a scalar field non-minimally coupled to Einstein's tensor and Ricci scalar in geometries of asymptotically de Sitter spacetimes. We show that the quasinormal modes remain unaffected if the scalar…
Astrophysical disks that are sufficiently cold and dense are linearly unstable to the formation of axisymmetric rings as a result of the disk's gravity. In practice, spiral structures are formed, which may in turn produce bound fragments.…
To date, axisymmetric internal wave fields, which have relevance to atmospheric internal wave fields generated by storm cells and oceanic near-inertial wave fields generated by surface storms, have been experimentally realized using an…
We model the spacetime of low-mass X-ray binaries with the Tomimatsu-Sato {\delta} = 2 (TS2) metric and study the properties of the orbital and the epicyclic frequencies. The numerical analysis shows that the properties of the…
Understanding oscillations and waves in astrophysical fluid bodies helps to elucidate their observed variability and the underlying physical mechanisms. Indeed, global oscillations and bending modes of accretion discs or tori may be…
In this work we explore the idea that the high frequency QPOs observed in LMXBs may be explained as a resonant coupling between the neutron star spin and epicyclic modes of accretion disk oscillations. We propose a new model for these QPOs…
This work investigates the echoes in axial gravitational perturbations in compact objects. To this end, we propose an alternative scheme of the finite difference method implemented in two coordinate systems, where the initial conditions are…
The fundamental resonances of rapidly rotating Kerr black holes in the eikonal limit are derived analytically. We show that there exists a critical value, $\mu_c=\sqrt{{{15-\sqrt{193}}\over{2}}}$, for the dimensionless ratio $\mu\equiv m/l$…
We calculate the quasinormal modes of static spherically symmetric dilatonic Reissner-Nordstr\"om black holes for general values of the electric charge and of the dilaton coupling constant. The spectrum of quasinormal modes is composed of…