227 research outputs found
Critical escape velocity of black holes from branes
In recent work we have shown that a black hole stacked on a brane escapes
once it acquires a recoil velocity. This result was obtained in the {\it
probe-brane} approximation, {\it i.e.}, when the tension of the brane is
negligibly small. Therefore, it is not clear whether the effect of the brane
tension may prevent the black hole from escaping for small recoil velocities.
The question is whether a critical escape velocity exists. Here, we analyze
this problem by studying the interaction between a Dirac-Nambu-Goto brane and a
black hole assuming adiabatic (quasi-static) evolution. By describing the brane
in a fixed black hole spacetime, which restricts our conclusions to lowest
order effects in the tension, we find that the critical escape velocity does
not exist for co-dimension one branes, while it does for higher co-dimension
branes.Comment: 10 pages, revte
Gregory-Laflamme instability of a slowly rotating black string
We study the Gregory-Laflamme instability of a 5-dimensional slowly rotating
black string in which the 4-dimensional section is described by the Kerr black
hole. We treat the rotation in a perturbative way introducing a small parameter
for the rotation. It is found that rotation makes the Gregory-Laflamme
instability stronger. Both the critical wavelength at the onset of instability
and the growth time-scale are found to decrease as the rotation increases.Comment: 26 pages, 1 figur
Dynamics of domain walls intersecting black holes
Previous studies concerning the interaction of branes and black holes
suggested that a small black hole intersecting a brane may escape via a
mechanism of reconnection. Here we consider this problem by studying the
interaction of a small black hole and a domain wall composed of a scalar field
and simulate the evolution of this system when the black hole acquires an
initial recoil velocity. We test and confirm previous results, however, unlike
the cases previously studied, in the more general set-up considered here, we
are able to follow the evolution of the system also during the separation, and
completely illustrate how the escape of the black hole takes place.Comment: 4 pages, 6 figure
Zeta Functions in Brane World Cosmology
We present a calculation of the zeta function and of the functional
determinant for a Laplace-type differential operator, corresponding to a scalar
field in a higher dimensional de Sitter brane background, which consists of a
higher dimensional anti-de Sitter bulk spacetime bounded by a de Sitter
section, representing a brane. Contrary to the existing examples, which all
make use of conformal transformations, we evaluate the zeta function working
directly with the higher dimensional wave operator. We also consider a generic
mass term and coupling to curvature, generalizing previous results. The
massless, conformally coupled case is obtained as a limit of the general result
and compared with known calculations. In the limit of large anti-de Sitter
radius, the zeta determinant for the ball is recovered in perfect agreement
with known expressions, providing an interesting check of our result and an
alternative way of obtaining the ball determinant.Comment: 14 pages, 1 figur
Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra
From the analysis of a ~100 ks Chandra observation of the dwarf starburst
galaxy NGC 1569, we have found that the X-ray point sources, CXOU
043048.1+645050 and CXOU 043048.6+645058, showed significant time variability.
During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10
times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted
with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity
is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass
black-hole. Since the spectrum in its faint phase was also fitted with a disk
blackbody model, the time variability can be explained by a change of the
accretion rate onto the black hole. The other variable source, CXOU
043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source
may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1.
In addition, three other weak sources showed possible time variability. Taking
all of the variability into account may suggest an abundant population of
compact X-ray sources in NGC 1569.Comment: 15 pages including 4 Postscript figures; accepted for publication in
ApJ
A New delta N Formalism for Multi-Component Inflation
The delta N formula that relates the final curvature perturbation on comoving
slices to the inflaton perturbation on flat slices after horizon crossing is a
powerful and intuitive tool to compute the curvature perturbation spectrum from
inflation. However, it is customarily assumed further that the conventional
slow-roll condition is satisfied, and satisfied by all components, during
horizon crossing. In this paper, we develop a new delta N formalism for
multi-component inflation that can be applied in the most general situations.
This allows us to generalize the idea of general slow-roll inflation to the
multi-component case, in particular only applying the general slow-roll
condition to the relevant component. We compute the power spectrum of the
curvature perturbation in multi-component general slow-roll inflation, and find
that under quite general conditions it is invertible.Comment: 24 pages, no figur
Innermost stable circular orbits around relativistic rotating stars
We investigate the innermost stable circular orbit (ISCO) of a test particle
moving on the equatorial plane around rotating relativistic stars such as
neutron stars. First, we derive approximate analytic formulas for the angular
velocity and circumferential radius at the ISCO making use of an approximate
relativistic solution which is characterized by arbitrary mass, spin, mass
quadrupole, current octapole and mass -pole moments. Then, we show that
the analytic formulas are accurate enough by comparing them with numerical
results, which are obtained by analyzing the vacuum exterior around numerically
computed geometries for rotating stars of polytropic equation of state. We
demonstrate that contribution of mass quadrupole moment for determining the
angular velocity and, in particular, the circumferential radius at the ISCO
around a rapidly rotating star is as important as that of spin.Comment: 12 pages, 2 figures, accepted for publication in Phys. Rev.
General Solutions for Tunneling of Scalar Fields with Quartic Potentials in de Sitter Space
The tunneling rates for scalar fields with quartic potentials in de Sitter
space in the limit of no gravitational back reaction are calculated numerically
and the results are fitted by analytic formulae.Comment: (Contours in Figure 1 corrected, two-dimensional fitting coefficient
corrected, references added.), 16 pages, KUNS 124
Kerr-Schild ansatz in Einstein-Gauss-Bonnet gravity: An exact vacuum solution in five dimensions
As is well-known, Kerr-Schild metrics linearize the Einstein tensor. We shall
see here that they also simplify the Gauss-Bonnet tensor, which turns out to be
only quadratic in the arbitrary Kerr-Schild function f when the seed metric is
maximally symmetric. This property allows us to give a simple analytical
expression for its trace, when the seed metric is a five dimensional maximally
symmetric spacetime in spheroidal coordinates with arbitrary parameters a and
b. We also write in a (fairly) simple form the full Einstein-Gauss-Bonnet
tensor (with a cosmological term) when the seed metric is flat and the
oblateness parameters are equal, a=b. Armed with these results we give in a
compact form the solution of the trace of the Einstein-Gauss-Bonnet field
equations with a cosmological term and a different than b. We then examine
whether this solution for the trace does solve the remaining field equations.
We find that it does not in general, unless the Gauss-Bonnet coupling is such
that the field equations have a unique maximally symmetric solution.Comment: 10 pages, no figures, references added. Last version for CQ
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