8,113 research outputs found
Quasi-bound states of massive scalar fields in the Kerr black-hole spacetime: Beyond the hydrogenic approximation
Rotating black holes can support quasi-stationary (unstable) bound-state
resonances of massive scalar fields in their exterior regions. These spatially
regular scalar configurations are characterized by instability timescales which
are much longer than the timescale set by the geometric size (mass) of the
central black hole. It is well-known that, in the small-mass limit
(here is the mass of the scalar field), these
quasi-stationary scalar resonances are characterized by the familiar hydrogenic
oscillation spectrum: , where
the integer is the principal quantum number of
the bound-state resonance (here the integers and
are the spheroidal harmonic index and the resonance parameter of the field
mode, respectively). As it depends only on the principal resonance parameter
, this small-mass () hydrogenic spectrum is obviously
degenerate. In this paper we go beyond the small-mass approximation and analyze
the quasi-stationary bound-state resonances of massive scalar fields in
rapidly-spinning Kerr black-hole spacetimes in the regime . In
particular, we derive the non-hydrogenic (and, in general, non-degenerate)
resonance oscillation spectrum
, where is the generalized
principal quantum number of the quasi-stationary resonances. This analytically
derived formula for the characteristic oscillation frequencies of the composed
black-hole-massive-scalar-field system is shown to agree with direct numerical
computations of the quasi-stationary bound-state resonances.Comment: 7 page
Marginally stable resonant modes of the polytropic hydrodynamic vortex
The polytropic hydrodynamic vortex describes an effective -dimensional
acoustic spacetime with an inner reflecting boundary at . This
physical system, like the spinning Kerr black hole, possesses an ergoregion of
radius and an inner non-pointlike curvature singularity of
radius . Interestingly, the fundamental ratio
which characterizes the effective geometry is
determined solely by the dimensionless polytropic index of the
circulating fluid. It has recently been proved that, in the
case, the effective acoustic spacetime is characterized by an {\it infinite}
countable set of reflecting surface radii,
, that can support static
(marginally-stable) sound modes. In the present paper we use {\it analytical}
techniques in order to explore the physical properties of the polytropic
hydrodynamic vortex in the regime. In particular, we prove
that in this physical regime, the effective acoustic spacetime is characterized
by a {\it finite} discrete set of reflecting surface radii,
, that can support
the marginally-stable static sound modes (here is the azimuthal harmonic
index of the acoustic perturbation field). Interestingly, it is proved
analytically that the dimensionless outermost supporting radius
, which marks the onset of superradiant
instabilities in the polytropic hydrodynamic vortex, increases monotonically
with increasing values of the integer harmonic index and decreasing values
of the dimensionless polytropic index .Comment: 13 page
Eigenvalue spectrum of the spheroidal harmonics: A uniform asymptotic analysis
The spheroidal harmonics have attracted the attention of
both physicists and mathematicians over the years. These special functions play
a central role in the mathematical description of diverse physical phenomena,
including black-hole perturbation theory and wave scattering by nonspherical
objects. The asymptotic eigenvalues of these functions have
been determined by many authors. However, it should be emphasized that all
previous asymptotic analyzes were restricted either to the regime
with a fixed value of , or to the complementary regime with a
fixed value of . A fuller understanding of the asymptotic behavior of the
eigenvalue spectrum requires an analysis which is asymptotically uniform in
both and . In this paper we analyze the asymptotic eigenvalue spectrum
of these important functions in the double limit and
with a fixed ratio.Comment: 5 page
Self-gravitating ring of matter in orbit around a black hole: The innermost stable circular orbit
We study analytically a black-hole-ring system which is composed of a
stationary axisymmetric ring of particles in orbit around a perturbed Kerr
black hole of mass . In particular, we calculate the shift in the orbital
frequency of the innermost stable circular orbit (ISCO) due to the finite mass
of the orbiting ring. It is shown that for thin rings of half-thickness
, the dominant finite-mass correction to the characteristic ISCO
frequency stems from the self-gravitational potential energy of the ring (a
term in the energy budget of the system which is quadratic in the mass of
the ring). This dominant correction to the ISCO frequency is of order
, where is the dimensionless mass of the ring.
We show that the ISCO frequency increases (as compared to the ISCO frequency of
an orbiting test-ring) due to the finite-mass effects of the self-gravitating
ring.Comment: 11 page
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