7,968 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
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
Of NBOs and kHz QPOs: a low-frequency modulation in resonant oscillations of relativistic accretion disks
The origin of quasi periodic modulations of flux in the kilohertz range (kHz
QPOs), observed in low-mass X-ray binaries, is usually assumed to be physically
distinct from that of the ``normal branch oscillations'' (NBOs) in the
Z-sources. We show that a low-frequency modulation of the kHz QPOs is a natural
consequence of the non-linear relativistic resonance suggested previously to
explain the properties of the high-frequency twin peaks. The theoretical
results discussed here are reminiscent of the 6 Hz variations of frequency and
amplitude of the kHz QPOs reported by Yu, van der Klis and Jonker (2001).Comment: Accepted for publication in PASJ; 4 pages, 1 figur
Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities
The "radiation inner edge" of an accretion disk is defined as the inner
boundary of the region from which most of the luminosity emerges. Similarly,
the "reflection edge" is the smallest radius capable of producing a significant
X-ray reflection of the fluorescent iron line. For black hole accretion disks
with very sub-Eddington luminosities these and all other "inner edges" locate
at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner
edge of the black hole accretion disk. However, even for moderate luminosities,
there is no such unique inner edge as differently defined edges locate at
different places. Several of them are significantly closer to the black hole
than ISCO. The differences grow with the increasing luminosity. For nearly
Eddington luminosities, they are so huge that the notion of the inner edge
losses all practical significance.Comment: 12 pages, 15 figures, submitted to A&
Mass of a Black Hole Firewall
Quantum entanglement of Hawking radiation has been supposed to give rise to a
Planck density "firewall" near the event horizon of old black holes. We show
that Planck density firewalls are excluded by Einstein's equations for black
holes of mass exceeding the Planck mass. We find an upper limit of
to the surface density of a firewall in a Schwarzschild black hole of mass ,
translating for astrophysical black holes into a firewall density smaller than
Planck density by more than 30 orders of magnitude. A strict upper limit on the
firewall density is given by the Planck density times the ratio .Comment: 6 pages, version published in Phys. Rev. Let
Dynamics of thick discs around Schwarzschild-de Sitter black holes
We consider the effects of a cosmological constant on the dynamics of
constant angular momentum discs orbiting Schwarzschild-de Sitter black holes.
The motivation behind this study is to investigate whether the presence of a
radial force contrasting the black hole's gravitational attraction can
influence the occurrence of the runaway instability, a robust feature of the
dynamics of constant angular momentum tori in Schwarzschild and Kerr
spacetimes. In addition to the inner cusp near the black hole horizon through
which matter can accrete onto the black hole, in fact, a positive cosmological
constant introduces also an outer cusp through which matter can leave the torus
without accreting onto the black hole. To assess the impact of this outflow on
the development of the instability we have performed time-dependent and
axisymmetric hydrodynamical simulations of equilibrium initial configurations
in a sequence of background spacetimes of Schwarzschild-de Sitter black holes
with increasing masses. The simulations have been performed with an unrealistic
value for the cosmological constant which, however, yields sufficiently small
discs to be resolved accurately on numerical grids and thus provides a first
qualitative picture of the dynamics. The calculations, carried out for a wide
range of initial conditions, show that the mass-loss from the outer cusp can
have a considerable impact on the instability, with the latter being rapidly
suppressed if the outflow is large enough.Comment: 12 pages; A&A, in pres
Centrifugal Force and Ellipticity behaviour of a slowly rotating ultra compact object
Using the optical reference geometry approach, we have derived in the
following, a general expression for the ellipticity of a slowly rotating fluid
configuration using Newtonian force balance equation in the conformally
projected absolute 3-space, in the realm of general relativity. Further with
the help of Hartle-Thorne (H-T) metric for a slowly rotating compact object, we
have evaluated the centrifugal force acting on a fluid element and also
evaluated the ellipticity and found that the centrifugal reversal occurs at
around , and the ellipticity maximum at around . The result has been compared with that of Chandrasekhar and
Miller which was obtained in the full 4-spacetime formalism
Extracting black-hole rotational energy: The generalized Penrose process
In the case involving particles the necessary and sufficient condition for
the Penrose process to extract energy from a rotating black hole is absorption
of particles with negative energies and angular momenta. No torque at the
black-hole horizon occurs. In this article we consider the case of arbitrary
fields or matter described by an unspecified, general energy-momentum tensor
and show that the necessary and sufficient condition for
extraction of a black hole's rotational energy is analogous to that in the
mechanical Penrose process: absorption of negative energy and negative angular
momentum. We also show that a necessary condition for the Penrose process to
occur is for the Noether current (the conserved energy-momentum density vector)
to be spacelike or past directed (timelike or null) on some part of the
horizon. In the particle case, our general criterion for the occurrence of a
Penrose process reproduces the standard result. In the case of relativistic
jet-producing "magnetically arrested disks" we show that the negative energy
and angular-momentum absorption condition is obeyed when the Blandford-Znajek
mechanism is at work, and hence the high energy extraction efficiency up to
found in recent numerical simulations of such accretion flows
results from tapping the black hole's rotational energy through the Penrose
process. We show how black-hole rotational energy extraction works in this case
by describing the Penrose process in terms of the Noether current.Comment: 24 pages, 14 figures, version published in Phys. Rev.
Photon capture cones and embedding diagrams of the Ernst spacetime
The differences between the character of the Schwarzschild and Ernst
spacetimes are illustrated by comparing the photon capture cones, and the
embedding diagrams of the sections of the equatorial planes
of both the ordinary and optical reference geometry of these spacetimes. The
non-flat asymptotic character of the Ernst spacetime reflects itself in two
manifest facts: the escape photon cones correspond to purely outward radial
direction, and the embedding diagrams of both the ordinary and optical geometry
shrink to zero radius asymptotically. Using the properties of the embedding
diagrams, regions of these spacetimes which could have similar character are
estimated, and it is argued that they can exist for the Ernst spacetimes with a
sufficiently low strength of the magnetic field.Comment: 12 pages, 7 figure
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