1,218 research outputs found
Development of a 10.6-micron laser modulator
Electro-optic coefficient of gallium arsenide measured at wavelengths from 2 to 12 microns by 10.6 micron laser modulato
Critical Exponents and Stability at the Black Hole Threshold for a Complex Scalar Field
This paper continues a study on Choptuik scaling in gravitational collapse of
a complex scalar field at the threshold for black hole formation. We perform a
linear perturbation analysis of the previously derived complex critical
solution, and calculate the critical exponent for black hole mass, . We also show that this critical solution is unstable via a
growing oscillatory mode.Comment: 15 pages of latex/revtex; added details of numerics, in press in Phys
Rev D; 1 figure included, or available by anonymous ftp to
ftp://ftp.itp.ucsb.edu/figures/nsf-itp-95-58.ep
High-pressure Partial Melting of Mafic Lithologies in the Mantle
We review experimental phase equilibria associated with partial melting of mafic lithologies (pyroxenites) at high pressures to reveal systematic relationships between bulk compositions of pyroxenite and their melting relations. An important aspect of pyroxenite phase equilibria is the existence of the garnet-pyroxene thermal divide, defined by the enstatite-Ca-Tschermaks pyroxene-diopside plane in CaO-MgO-Al2O3-SiO2 projections. This divide appears at pressures above âŒ2âGPa in the natural system where garnet and pyroxenes are the principal residual phases in pyroxenites. Bulk compositions that reside on either side of the divide have distinct phase assemblages from subsolidus to liquidus and produce distinct types of partial melt ranging from strongly nepheline-normative to quartz-normative compositions. Solidus and liquidus locations are little affected by the location of natural pyroxenite compositions relative to the thermal divide and are instead controlled chiefly by bulk alkali contents and Mg-numbers. Changes in phase volumes of residual minerals also influence partial melt compositions. If olivine is absent during partial melting, expansion of the phase volume of garnet relative to clinopyroxene with increasing pressure produces liquids with high Ca/Al and low MgO compared with garnet peridotite-derived partial melt
Criticality and Bifurcation in the Gravitational Collapse of a Self-Coupled Scalar Field
We examine the gravitational collapse of a non-linear sigma model in
spherical symmetry. There exists a family of continuously self-similar
solutions parameterized by the coupling constant of the theory. These solutions
are calculated together with the critical exponents for black hole formation of
these collapse models. We also find that the sequence of solutions exhibits a
Hopf-type bifurcation as the continuously self-similar solutions become
unstable to perturbations away from self-similarity.Comment: 18 pages; one figure, uuencoded postscript; figure is also available
at http://www.physics.ucsb.edu/people/eric_hirschman
Perturbations and Critical Behavior in the Self-Similar Gravitational Collapse of a Massless Scalar Field
This paper studies the perturbations of the continuously self-similar
critical solution of the gravitational collapse of a massless scalar field
(Roberts solution). The perturbation equations are derived and solved exactly.
The perturbation spectrum is found to be not discrete, but occupying continuous
region of the complex plane. The renormalization group calculation gives the
value of the mass-scaling exponent equal to 1.Comment: 12 pages, RevTeX 3.1, 1 figur
Boosting jet power in black hole spacetimes
The extraction of rotational energy from a spinning black hole via the
Blandford-Znajek mechanism has long been understood as an important component
in models to explain energetic jets from compact astrophysical sources. Here we
show more generally that the kinetic energy of the black hole, both rotational
and translational, can be tapped, thereby producing even more luminous jets
powered by the interaction of the black hole with its surrounding plasma. We
study the resulting Poynting jet that arises from single boosted black holes
and binary black hole systems. In the latter case, we find that increasing the
orbital angular momenta of the system and/or the spins of the individual black
holes results in an enhanced Poynting flux.Comment: 7 pages, 5 figure
Rabaptin5 is recruited to endosomes by Rab4 and Rabex5 to regulate endosome maturation
Rab GTPases control membrane identity, fusion, and transport by interaction with effector proteins. Effectors that influence the activation/inactivation cycle of their own or other Rabs contribute to the timely conversion of Rab identities. Rab5 and its effector Rabaptin5 are generally considered the prime example for a positive feedback loop in which Rab5·GTP recruits Rabaptin5 complexed to Rabex5, the GDP/GTP exchange factor of Rab5, to early endosomes, thus maintaining the membrane's Rab5 identity. By deletion analysis, we found membrane recruitment of Rabaptin5 to require binding to Rab4 and Rabex5, but not Rab5. Deletion of either one of two Rab5 binding domains or silencing of Rab5 expression did not affect Rabaptin5 recruitment, but produced giant endosomes with early and late endosomal characteristics. The results contradict feedback activation of Rab5 and instead indicate that Rabaptin5 is recruited by Rabex5 recognizing ubiquitinated cargo and by Rab4 to activate Rab5 in a feed-forward manner
Standardized volumetric 3D-analysis of SPECT/CT imaging in orthopaedics: overcoming the limitations of qualitative 2D analysis
<p>Abstract</p> <p>Background</p> <p>SPECT/CT combines high resolution anatomical 3D computerized tomography (CT) and single photon emission computerized tomography (SPECT) as functional imaging, which provides 3D information about biological processes into a single imaging modality. The clinical utility of SPECT/CT imaging has been recognized in a variety of medical fields and most recently in orthopaedics; however, clinical adoption has been limited due to shortcomings of analytical tools available. Specifically, SPECT analyses are mainly qualitative due to variation in overall metabolic uptake among patients. Furthermore, most analyses are done in 2D, although rich 3D data are available. Consequently, it is difficult to quantitatively compare the position, size, and intensity of SPECT uptake regions among patients, and therefore difficult to draw meaningful clinical conclusions.</p> <p>Methods</p> <p>We propose a method for normalizing orthopaedic SPECT/CT data that enables standardised 3D volumetric quantitative measurements and comparison among patients. Our method is based on 3D localisation using clinically relevant anatomical landmarks and frames of reference, along with intensity value normalisation using clinically relevant reference regions. Using the normalised data, we describe a thresholding technique to distinguish clinically relevant hot spots from background activity.</p> <p>Results</p> <p>Using an exemplar comparison of two patients, we demonstrate how the normalised, 3D-rendered data can provide a richer source of clinical information and allow quantitative comparison of SPECT/CT measurements across patients. Specifically, we demonstrate how non-normalized SPECT/CT analysis can lead to different clinical conclusions than the normalized SPECT/CT analysis, and that normalized quantitative analysis can be a more accurate indicator of pathology.</p> <p>Conclusions</p> <p>Conventional orthopaedic frames of reference, 3D volumetric data analysis and thresholding are used to distinguish clinically relevant hot spots from background activity. Our goal is to facilitate a standardised approach to quantitative data collection and comparison of clinical studies using SPECT/CT, enabling more widespread clinical use of this powerful imaging tool.</p
Rotating magnetic solution in three dimensional Einstein gravity
We obtain the magnetic counterpart of the BTZ solution, i.e., the rotating
spacetime of a point source generating a magnetic field in three dimensional
Einstein gravity with a negative cosmological constant. The static
(non-rotating) magnetic solution was found by Clement, by Hirschmann and Welch
and by Cataldo and Salgado. This paper is an extension of their work in order
to include (i) angular momentum, (ii) the definition of conserved quantities
(this is possible since spacetime is asymptotically anti-de Sitter), (iii)
upper bounds for the conserved quantities themselves, and (iv) a new
interpretation for the magnetic field source. We show that both the static and
rotating magnetic solutions have negative mass and that there is an upper bound
for the intensity of the magnetic field source and for the value of the angular
momentum. The magnetic field source can be interpreted not as a vortex but as
being composed by a system of two symmetric and superposed electric charges,
one of the electric charges is at rest and the other is spinning. The rotating
magnetic solution reduces to the rotating uncharged BTZ solution when the
magnetic field source vanishes.Comment: Latex (uses JHEP3.cls), 12 pages. Published versio
On critical behaviour in gravitational collapse
We give an approach to studying the critical behaviour that has been observed
in numerical studies of gravitational collapse. These studies suggest, among
other things, that black holes initially form with infinitesimal mass. We show
generally how a black hole mass formula can be extracted from a transcendental
equation.
Using our approach, we give an explicit one parameter set of metrics that are
asymptotically flat and describe the collapse of apriori unspecified but
physical matter fields. The black hole mass formula obtained from this metric
exhibits a mass gap - that is, at the onset of black hole formation, the mass
is finite and non-zero.Comment: 11 pages, RevTex, 2 figures (available from VH
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