8,281 research outputs found
The centrifugal force reversal and X-ray bursts
Heyl (2000) made an interesting suggestion that the observed shifts in QPO
frequency in type I X-ray bursts could be influenced by the same geometrical
effect of strong gravity as the one that causes centrifugal force reversal
discovered by Abramowicz and Lasota (1974). However, his main result contains a
sign error. Here we derive the correct formula and conclude that constraints on
the M(R) relation for neutron stars deduced from the rotational-modulation
model of QPO frequency shifts are of no practical interest because the correct
formula implies a weak condition R* > 1.3 Rs, where Rs is the Schwarzschild
radius. We also argue against the relevance of the rotational-modulation model
to the observed frequency modulations.Comment: 3 pages, Minor revisions, A&A Letters, in pres
Centrifugal force in Kerr geometry
We have obtained the correct expression for the centrifugal force acting on a
particle at the equatorial circumference of a rotating body in the locally
non-rotating frame of the Kerr geometry. Using this expression for the
equilibrium of an element on the surface of a slowly rotating Maclaurin
spheroid, we obtain the expression for the ellipticity (as discussed earlier by
Abramowicz and Miller) and determine the radius at which the ellipticity is
maximum.Comment: 6 pages, LateX macro
An intuitive approach to inertial forces and the centrifugal force paradox in general relativity
As the velocity of a rocket in a circular orbit near a black hole increases,
the outwardly directed rocket thrust must increase to keep the rocket in its
orbit. This feature might appear paradoxical from a Newtonian viewpoint, but we
show that it follows naturally from the equivalence principle together with
special relativity and a few general features of black holes. We also derive a
general relativistic formalism of inertial forces for reference frames with
acceleration and rotation. The resulting equation relates the real experienced
forces to the time derivative of the speed and the spatial curvature of the
particle trajectory relative to the reference frame. We show that an observer
who follows the path taken by a free (geodesic) photon will experience a force
perpendicular to the direction of motion that is independent of the observers
velocity. We apply our approach to resolve the submarine paradox, which regards
whether a submerged submarine in a balanced state of rest will sink or float
when given a horizontal velocity if we take relativistic effects into account.
We extend earlier treatments of this topic to include spherical oceans and show
that for the case of the Earth the submarine floats upward if we take the
curvature of the ocean into account.Comment: 14 pages, 21 figure
Generalizing Optical Geometry
We show that by employing the standard projected curvature as a measure of
spatial curvature, we can make a certain generalization of optical geometry
(Abramowicz and Lasota 1997, Class. Quantum Grav. 14 (1997) A23). This
generalization applies to any spacetime that admits a hypersurface orthogonal
shearfree congruence of worldlines. This is a somewhat larger class of
spacetimes than the conformally static spacetimes assumed in standard optical
geometry. In the generalized optical geometry, which in the generic case is
time dependent, photons move with unit speed along spatial geodesics and the
sideways force experienced by a particle following a spatially straight line is
independent of the velocity. Also gyroscopes moving along spatial geodesics do
not precess (relative to the forward direction). Gyroscopes that follow a
curved spatial trajectory precess according to a very simple law of
three-rotation. We also present an inertial force formalism in coordinate
representation for this generalization. Furthermore, we show that by employing
a new sense of spatial curvature (Jonsson, Class. Quantum Grav. 23 (2006) 1)
closely connected to Fermat's principle, we can make a more extensive
generalization of optical geometry that applies to arbitrary spacetimes. In
general this optical geometry will be time dependent, but still geodesic
photons move with unit speed and follow lines that are spatially straight in
the new sense. Also, the sideways experienced (comoving) force on a test
particle following a line that is straight in the new sense will be independent
of the velocity.Comment: 19 pages, 1 figure. A more general analysis is presented than in the
former version. See also the companion papers arXiv:0708.2493,
arXiv:0708.2533 and arXiv:0708.253
The upper kHz QPO: a gravitationally lensed vertical oscillation
We show that a luminous torus in the Schwarzschild metric oscillating along
its own axis gives rise to a periodically varying flux of radiation, even
though the source of radiation is steady and perfectly axisymmetric. This
implies that the simplest oscillation mode in an accretion flow, axisymmetric
up-and-down motion at the meridional epicyclic frequency, may be directly
observable when it occurs in the inner parts of accretion flow around neutron
stars and black holes. The high-frequency modulations of the X-ray flux
observed in low-mass X-ray binaries at two frequencies (twin kHz QPOs) could
then be a signature of strong gravity both because radial and meridional
oscillations have different frequencies in non-Newtonian gravity, and because
strong gravitational deflection of light rays causes the flux of radiation to
be modulated at the higher frequency.Comment: 8 p., 4 fig
Epicyclic orbital oscillations in Newton's and Einstein's dynamics
We apply Feynman's principle, ``The same equations have the same solutions'',
to Kepler's problem and show that Newton's dynamics in a properly curved 3-D
space is identical with that described by Einstein's theory in the 3-D optical
geometry of Schwarzschild's spacetime. For this reason, rather unexpectedly,
Newton's formulae for Kepler's problem, in the case of nearly circular motion
in a static, spherically spherical gravitational potential accurately describe
strong field general relativistic effects, in particular vanishing of the
radial epicyclic frequency at the marginally stable orbit.Comment: 8 page
Inertial forces and the foundations of optical geometry
Assuming a general timelike congruence of worldlines as a reference frame, we
derive a covariant general formalism of inertial forces in General Relativity.
Inspired by the works of Abramowicz et. al. (see e.g. Abramowicz and Lasota,
Class. Quantum Grav. 14 (1997) A23), we also study conformal rescalings of
spacetime and investigate how these affect the inertial force formalism. While
many ways of describing spatial curvature of a trajectory has been discussed in
papers prior to this, one particular prescription (which differs from the
standard projected curvature when the reference is shearing) appears novel. For
the particular case of a hypersurface-forming congruence, using a suitable
rescaling of spacetime, we show that a geodesic photon is always following a
line that is spatially straight with respect to the new curvature measure. This
fact is intimately connected to Fermat's principle, and allows for a certain
generalization of the optical geometry as will be further pursued in a
companion paper (Jonsson and Westman, Class. Quantum Grav. 23 (2006) 61). For
the particular case when the shear-tensor vanishes, we present the inertial
force equation in three-dimensional form (using the bold face vector notation),
and note how similar it is to its Newtonian counterpart. From the spatial
curvature measures that we introduce, we derive corresponding covariant
differentiations of a vector defined along a spacetime trajectory. This allows
us to connect the formalism of this paper to that of Jantzen et. al. (see e.g.
Bini et. al., Int. J. Mod. Phys. D 6 (1997) 143).Comment: 42 pages, 7 figure
Optical reference geometry of the Kerr-Newman spacetimes
Properties of the optical reference geometry related to Kerr-Newman
black-hole and naked-singularity spacetimes are illustrated using embedding
diagrams of their equatorial plane. Among all inertial forces defined in the
framework of the optical geometry, just the centrifugal force plays a
fundamental role in connection to the embedding diagrams because it changes
sign at the turning points of the diagrams. The limits of embeddability are
given, and it is established which of the photon circular orbits hosted the by
Kerr-Newman spacetimes appear in the embeddable regions. Some typical embedding
diagrams are constructed, and the Kerr-Newman backgrounds are classified
according to the number of embeddable regions of the optical geometry as well
as the number of their turning points. Embedding diagrams are closely related
to the notion of the radius of gyration which is useful for analyzing fluid
rotating in strong gravitational fields.Comment: 28 pages, 17 figure
On the Polish doughnut accretion disk via the effective potential approach
We revisit the Polish doughnut model of accretion disks providing a
comprehensive analytical description of the Polish doughnut structure. We
describe a perfect fluid circularly orbiting around a Schwarzschild black hole,
source of the gravitational field, by the effective potential approach for the
exact gravitational and centrifugal effects. This analysis leads to a detailed,
analytical description of the accretion disk, its toroidal surface, the
thickness, the distance from the source. We determine the variation of these
features with the effective potential and the fluid angular momentum. Many
analytical formulas are given. In particular it turns out that the distance
from the source of the inner surface of the torus increases with increasing
fluid angular momentum but decreases with increasing energy function defined as
the value of the effective potential for that momentum. The location of torus
maximum thickness moves towards the external regions of the surface with
increasing angular momentum, until it reaches a maximum an then decreases.
Assuming a polytropic equation of state we investigate some specific cases.Comment: 33 pages, 28 figures, 1 table. This is a revised version to meet the
published articl
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