9 research outputs found
Scattering of Gravitational Waves by the Weak Gravitational Fields of Lens Objects
We consider the scattering of the gravitational waves by the weak
gravitational fields of lens objects. We obtain the scattered gravitational
waveform by treating the gravitational potential of the lens to first order,
i.e. using the Born approximation. We find that the effect of scattering on the
waveform is roughly given by the Schwarzschild radius of the lens divided by
the wavelength of gravitational wave for a compact lens object. If the lenses
are smoothly distributed, the effect of scattering is of the order of the
convergence field along the line of sight to the source. In the short
wavelength limit, the amplitude is magnified by , which is consistent
with the result in weak gravitational lensing.Comment: 4 pages, 2 figures, A&A Letters, in press, minor changes, references
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Quasi-geometrical Optics Approximation in Gravitational Lensing
The gravitational lensing of gravitational waves should be treated in the
wave optics instead of the geometrical optics when the wave length of
the gravitational waves is larger than the Schwarzschild radius of the lens
mass . The wave optics is based on the diffraction integral which represents
the amplification of the wave amplitude by lensing. We study the asymptotic
expansion of the diffraction integral in the powers of the wave length
. The first term, arising from the short wavelength limit , corresponds to the geometrical optics limit. The second term, being of the
order of , is the leading correction term arising from the
diffraction effect. By analyzing this correction term, we find that (1) the
lensing magnification is modified to , where is
of the order of , and (2) if the lens has cuspy (or singular)
density profile at the center (), the diffracted image is formed at the lens center with the magnification
.Comment: 9 pages, 4 figures. Revised version accepted for publication in A&
Exact Wave Propagation in a Spacetime with a Cosmic String
We present exact solutions of the massless Klein-Gordon equation in a
spacetime in which an infinite straight cosmic string resides. The first
solution represents a plane wave entering perpendicular to the string
direction. We also present and analyze a solution with a static point-like
source. In the short wavelength limit these solutions approach the results
obtained by using the geometrical optics approximation: magnification occurs if
the observer lies in front of the string within a strip of angular width , where is the string tension. We find that when the distance from
the observer to the string is less than , where is the
wave length, the magnification is significantly reduced compared with the
estimate based on the geometrical optics due to the diffraction effect. For
gravitational waves from neutron star(NS)-NS mergers the several lensing events
per year may be detected by DECIGO/BBO.Comment: 15 pages, 8 figures, reference adde
Cosmological Black Holes
In this paper we propose a model for the formation of the cosmological voids.
We show that cosmological voids can form directly after the collapse of
extremely large wavelength perturbations into low-density black holes or
cosmological black holes (CBH). Consequently the voids are formed by the
comoving expansion of the matter that surrounds the collapsed perturbation. It
follows that the universe evolves, in first approximation, according to the
Einstein-Straus cosmological model. We discuss finally the possibility to
detect the presence of these black holes through their weak and strong lensing
effects and their influence on the cosmic background radiation.Comment: 14 pages, new completely revised version, to appear on GR
On the possible sources of gravitational wave bursts detectable today
We discuss the possibility that galactic gravitational wave sources might
give burst signals at a rate of several events per year, detectable by
state-of-the-art detectors. We are stimulated by the results of the data
collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which
suggest an excess of coincidences between the two detectors, when the resonant
bars are orthogonal to the galactic plane. Signals due to the coalescence of
galactic compact binaries fulfill the energy requirements but are problematic
for lack of known candidates with the necessary merging rate. We examine the
limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW
emission, and we use them to put constraints also on the GW radiation from
exotic objects, like binaries made of primordial black holes. We discuss the
possibility that the events are due to GW bursts coming repeatedly from a
single or a few compact sources. We examine different possible realizations of
this idea, such as accreting neutron stars, strange quark stars, and the highly
magnetized neutron stars (``magnetars'') introduced to explain Soft Gamma
Repeaters. Various possibilities are excluded or appear very unlikely, while
others at present cannot be excluded.Comment: 24 pages, 20 figure