31,151 research outputs found
Some Physical Consequences of Abrupt Changes in the Multipole Moments of a Gravitating Body
The Barrab\`es-Israel theory of light-like shells in General Relativity is
used to show explicitly that in general a light-like shell is accompanied by an
impulsive gravitational wave. The gravitational wave is identified by its
Petrov Type N contribution to a Dirac delta-function term in the Weyl conformal
curvature tensor (with the delta-function singular on the null hypersurface
history of the wave and shell). An example is described in which an
asymptotically flat static vacuum Weyl space-time experiences a sudden change
across a null hypersurface in the multipole moments of its isolated axially
symmetric source. A light-like shell and an impulsive gravitational wave are
identified, both having the null hypersurface as history. The stress-energy in
the shell is dominated (at large distance from the source) by the jump in the
monopole moment (the mass) of the source with the jump in the quadrupole moment
mainly responsible for the stress being anisotropic. The gravitational wave
owes its existence principally to the jump in the quadrupole moment of the
source confirming what would be expected.Comment: 26 pages, tex, no figures, to appear in Phys.Rev.
Peeling properties of lightlike signals in General Relativity
The peeling properties of a lightlike signal propagating through a general
Bondi-Sachs vacuum spacetime and leaving behind another Bondi-Sachs vacuum
space-time are studied. We demonstrate that in general the peeling behavior is
the conventional one which is associated with a radiating isolated system and
that it becomes unconventional if the asymptotically flat space-times on either
side of the history of the light-like signal tend to flatness at future null
infinity faster than the general Bondi-Sachs space-time. This latter situation
occurs if, for example, the space-times in question are static Bondi-Sachs
space- times.Comment: 14 pages, LaTeX2
The Bell-Szekeres Solution and Related Solutions of the Einstein-Maxwell Equations
A novel technique for solving some head-on collisions of plane homogeneous
light-like signals in Einstein-Maxwell theory is described. The technique is a
by-product of a re-examination of the fundamental Bell-Szekeres solution in
this field of study. Extensions of the Bell-Szekeres collision problem to
include light-like shells and gravitational waves are described and a family of
solutions having geometrical and topological properties in common with the
Bell-Szekeres solution is derived.Comment: 18 pages, Latex fil
The re-emission spectrum of digital hardware subjected to EMI
The emission spectrum of digital hardware under the influence of external electromagnetic interference is shown to contain information about the interaction of the incident energy with the digital circuits in the system. The generation mechanism of the re-emission spectrum is reviewed, describing how nonlinear effects may be a precursor to the failure of the equipment under test. Measurements on a simple circuit are used to demonstrate how the characteristics of the re-emission spectrum may be correlated with changes to the digital waveform within the circuit. The technique is also applied to a piece of complex digital hardware where Similar, though more subtle, effects can be measured. It is shown that the re-emission spectrum can be used to detect the interaction of the interference with the digital devices at a level well below that which is able to cause static failures in the circuits. The utility of the technique as a diagnostic tool for immunity testing of digital hardware, by identifying which subsystems are being affected by external interference, is also demonstrated
The replacement histone H2A.Z in a hyperacetylated form is a feature of active genes in the chicken
The replacement histone H2A.Z is variously reported
as being linked to gene expression and preventing the
spread of heterochromatin in yeast, or concentrated
at heterochromatin in mammals. To resolve this
apparent dichotomy, affinity-purified antibodies
against the N-terminal region of H2A.Z, in both a triacetylatedandnon-
acetylatedstate, areusedin native
chromatin immmuno-precipitation experiments with
mononucleosomes from three chicken cell types. The
hyperacetylated species concentrates at the 50 end of
active genes, both tissue specific and housekeeping
but is absent from inactive genes, while the
unacetylated form is absent from both active and
inactive genes. A concentration of H2A.Z is also
found at insulators under circumstances implying a
link to barrier activity but not to enhancer blocking.
Although acetylated H2A.Z is widespread throughout
the interphase genome, at mitosis its acetylation is
erased, the unmodified form remaining. Thus,
although H2A.Z may operate as an epigenetic marker
for active genes, its N-terminal acetylation does not
Shearing Interferometer for Quantifying the Coherence of Hard X-Ray Beams
We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 m. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics
The replacement histone H2A.Z in a hyperacetylated form is a feature of active genes in the chicken
The replacement histone H2A.Z is variously reported
as being linked to gene expression and preventing the
spread of heterochromatin in yeast, or concentrated
at heterochromatin in mammals. To resolve this
apparent dichotomy, affinity-purified antibodies
against the N-terminal region of H2A.Z, in both a triacetylatedandnon-
acetylatedstate, areusedin native
chromatin immmuno-precipitation experiments with
mononucleosomes from three chicken cell types. The
hyperacetylated species concentrates at the 50 end of
active genes, both tissue specific and housekeeping
but is absent from inactive genes, while the
unacetylated form is absent from both active and
inactive genes. A concentration of H2A.Z is also
found at insulators under circumstances implying a
link to barrier activity but not to enhancer blocking.
Although acetylated H2A.Z is widespread throughout
the interphase genome, at mitosis its acetylation is
erased, the unmodified form remaining. Thus,
although H2A.Z may operate as an epigenetic marker
for active genes, its N-terminal acetylation does not
Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
This paper investigates the discrepancy between recent measurements of the
density profile of the globular cluster 47 Tuc that have used HST data sets.
Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive
r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is
a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band
FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c
= 12" +/- 2". Differences in the adopted cluster centers are not the cause of
the discrepancy. Our independent analysis of the data used by De Marchi et al.
reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is
spuriously low, a result of radially-varying bias in the star counts in a
magnitude limited sample -- photometric errors and a steeply rising stellar
luminosity function cause more stars to scatter across the limiting magnitude
into the sample than out of it, especially near the cluster center where
crowding effects are most severe. (2) Changing the limiting magnitude to the
main sequence turnoff, away from the steep part of the luminosity function,
partially alleviates the problem and results in r_c = 18". (3) Combining such a
limiting magnitude with accurate photometry derived from PSF fitting, instead
of the less accurate aperture photometry employed by De Marchi et al., results
in a reliable measurement of the density profile which is well fit by r_c = 22"
+/- 2". Archival WFPC2 data are used to derive a star list with a higher degree
of completeness, greater photometric accuracy, and wider areal coverage than
the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above
conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see
http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure
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