2,395 research outputs found
Stability of an Ultra-Relativistic Blast Wave in an External Medium with a Steep Power-Law Density Profile
We examine the stability of self-similar solutions for an accelerating
relativistic blast wave which is generated by a point explosion in an external
medium with a steep radial density profile of a power-law index > 4.134. These
accelerating solutions apply, for example, to the breakout of a gamma-ray burst
outflow from the boundary of a massive star, as assumed in the popular
collapsar model. We show that short wavelength perturbations may grow but only
by a modest factor <~ 10.Comment: 12 pages, 3 figures, submitted to Physical Review
Absorption of an organic film at the platinum-seawater interface
Optical polarization analysis with an ellipsometer and microelectrophoretic and contact angle measurements all indicate that large changes in the properties of the platinum metal surface occurred on contact with Chesapeake Bay water from which particles and microorganisms had been removed. These changes were not observed with the same water from which organics had been removed by a photo-oxidation procedure...
Expected Number and Flux Distribution of Gamma-Ray-Burst Afterglows with High Redshifts
If Gamma-Ray-Bursts (GRBs) occur at high redshifts, then their bright
afterglow emission can be used to probe the ionization and metal enrichment
histories of the intervening intergalactic medium during the epoch of
reionization. In contrast to other sources, such as galaxies or quasars, which
fade rapidly with increasing redshift, the observed infrared flux from a GRB
afterglow at a fixed observed age is only a weak function of its redshift. This
results from a combination of the spectral slope of GRB afterglows and the
time-stretching of their evolution in the observer's frame. Assuming that the
GRB rate is proportional to the star formation rate and that the characteristic
energy output of GRBs is ~10^{52} ergs, we predict that there are always ~15
GRBs from redshifts z>5 across the sky which are brighter than ~100 nJy at an
observed wavelength of ~2 \mu m. The infrared spectrum of these sources could
be taken with the future Next Generation Space Telescope, as a follow-up on
their early X-ray localization with the Swift satellite.Comment: 29 pages, 14 figures; submitted to Ap
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
[Accepted Manuscript] Field testing a draft version of the UNICEF/Washington Group Module on child functioning and disability. Background, methodology and preliminary findings from Cameroon and India
Background
Global child disability data are generally non-comparable, comprising different tools, methodologies and disability definitions. UNICEF and The Washington Group on Disability Statistics (WG) have developed a new tool on child functioning and disability to address this need.
Aims
The aim of this paper is to describe the development of the new module, and to present an independent field test of the draft module in two contrasting settings.
Methods
UNICEF and the WG developed a parent-reported survey module to identify children aged 2�17 years with functional difficulties in population-based surveys through: review of existing documentation, consultation with experts and cognitive testing. A field test of the draft module was undertaken in Cameroon and India within a population-based survey. Functional limitation in each of 14 domains was scored on a scale comprising �no difficulty�, �some difficulty�, �a lot of difficulty� and �cannot do�.
Results
In all, 1713 children in Cameroon and 1101 children in India were assessed. Sixty-four percent of children in Cameroon and 35% of children in India were reported to have at least some difficulty in one or more domain. The proportion reported to have either �a lot of difficulty� or �cannot do� was 9% in Cameroon and 4% in India. There were no significant differences in reported functional difficulties by sex but children aged 2�4 were reported to have fewer functional difficulties of any kind compared with older children in both countries.
Conclusion
Comparable estimates were generated between the two countries, providing an initial overview of the tool's outputs. The continued development of this standardised questionnaire for the collection of robust and reliable data on child disability is essential
Effects of Dust on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of an optical QSO by a spiral galaxy is often
counteracted by dust obscuration, since the line-of-sight to the QSO passes
close to the center of the galactic disk. The dust in the lens is likely to be
correlated with neutral hydrogen, which in turn should leave a Lyman-alpha
absorption signature on the QSO spectrum. We use the estimated dust-to-gas
ratio of the Milky-Way galaxy as a mean and allow a spread in its values to
calculate the effects of dust on lensing by low redshift spiral galaxies. Using
a no-evolution model for spirals at z<1 we find (in Lambda=0 cosmologies) that
the magnification bias due to lensing is stronger than dust obscuration for QSO
samples with a magnitude limit B<16. The density parameter of neutral hydrogen,
Omega_HI, is overestimated in such samples and is underestimated for fainter
QSOs.Comment: 18 pages, 4 figures, ApJ, in pres
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
On Tarski’s Foundations of the Geometry of Solids
The paper [Tarski: Les fondements de la géométrie des corps, Annales de la Société Polonaise de Mathématiques, pp. 29-34, 1929] is in many ways remarkable. We address three historico- philosophical issues that force themselves upon the reader. First we argue that in this paper Tarski did not live up to his own methodological ideals, but displayed instead a much more pragmatic approach. Second we show that Leśniewski's philosophy and systems do not play the significant role that one may be tempted to assign to them at first glance. Especially the role of background logic must be at least partially allocated to Russell's systems of Principia mathematica. This analysis leads us, third, to a threefold distinction of the technical ways in which the domain of discourse comes to be embodied in a theory. Having all of this in place, we discuss why we have to reject the argument in [Gruszczyński and Pietruszczak: Full development of Tarski's Geometry of Solids, The Bulletin of Symbolic Logic, vol. 4 (2008), no. 4, pp. 481-540] according to which Tarski has made a certain mistake. © 2012 Association for Symbolic Logic
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