8,886 research outputs found
CATTLE/BEEF SUBSECTOR'S STRUCTURE AND COMPETITION UNDER FREE TRADE
Industrial Organization, International Relations/Trade,
Implications of Spontaneous Glitches in the Mass and Angular Momentum in Kerr Space-Time
The outward-pointing principal null direction of the Schwarzschild Riemann
tensor is null hypersurface-forming. If the Schwarzschild mass spontaneously
jumps across one such hypersurface then the hypersurface is the history of an
outgoing light-like shell. The outward-- pointing principal null direction of
the Kerr Riemann tensor is asymptotically (in the neighbourhood of future null
infinity) null hypersurface-forming. If the Kerr parameters of mass and angular
momentum spontaneously jump across one such asymptotic hypersurface then the
asymptotic hypersurface is shown to be the history of an outgoing light-like
shell and a wire singularity-free spherical impulsive gravitational wave.Comment: 16 pages, TeX, no figures, accepted for publication in Phys. Rev.
X-Ray Observations of Black Widow Pulsars
We describe the first X-ray observations of five short orbital period ( day), -ray emitting, binary millisecond pulsars. Four of these, PSRs
J0023+0923, J11243653, J1810+1744, and J22561024 are `black-widow'
pulsars, with degenerate companions of mass , three of which
exhibit radio eclipses. The fifth source, PSR J2215+5135, is an eclipsing
`redback' with a near Roche-lobe filling 0.2 solar mass non-degenerate
companion. Data were taken using the \textit{Chandra X-Ray Observatory} and
covered a full binary orbit for each pulsar. Two pulsars, PSRs J2215+5135 and
J22561024, show significant orbital variability while PSR J11243653 shows
marginal orbital variability. The lightcurves for these three pulsars have
X-ray flux minima coinciding with the phases of the radio eclipses. This
phenomenon is consistent with an intrabinary shock emission interpretation for
the X-rays. The other two pulsars, PSRs J0023+0923 and J1810+1744, are fainter
and do not demonstrate variability at a level we can detect in these data. All
five spectra are fit with three separate models: a power-law model, a blackbody
model, and a combined model with both power-law and blackbody components. The
preferred spectral fits yield power-law indices that range from 1.3 to 3.2 and
blackbody temperatures in the hundreds of eV. The spectrum for PSR J2215+5135
shows a significant hard X-ray component, with a large number of counts above 2
keV, which is additional evidence for the presence of intrabinary shock
emission and is similar to what has been detected in the low-mass X-ray binary
to millisecond pulsar transition object PSR J1023+0038.Comment: 8 pages, 6 figures, 2 tables, submitted to Ap
Errors in kinematic distances and our image of the Milky Way Galaxy
Errors in the kinematic distances, under the assumption of circular gas
orbits, were estimated by performing synthetic observations of a model disk
galaxy. It was found that the error is < 0.5 kpc for most of the disk when the
measured rotation curve was used, but larger if the real rotation curve is
applied. In both cases, the error is significantly larger at the positions of
the spiral arms. The error structure is such that, when kinematic distances are
used to develope a picture of the large scale density distribution, the most
significant features of the numerical model are significantly distorted or
absent, while spurious structure appears. By considering the full velocity
field in the calculation of the kinematic distances, most of the original
density structures can be recovered.Comment: Accepted for publication in A
Spectral Energy Distributions for Disk and Halo M--Dwarfs
We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk
dwarfs with spectral type M1 to M6.5. These data are compared to synthetic
spectra generated by the latest model atmospheres of Allard & Hauschildt.
Photospheric parameters metallicity, effective temperature and radius are
determined for the sample. We find good agreement between observation and
theory except for known problems due to incomplete molecular data for metal
hydrides and water. The metal-poor M subdwarfs are well matched by the models
as oxide opacity sources are less important in this case. The derived effective
temperatures for the sample range from 3600K to 2600K; at these temperatures
grain formation and extinction are not significant in the photosphere. The
derived metallicities range from solar to one-tenth solar. The radii and
effective temperatures derived agree well with recent models of low mass stars.Comment: 24 pages including 13 figures, 4 Tables; accepted by Ap
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Challenges of ultra large scale integration of biomedical computing systems
The NCRI Informatics Initiative is overseeing the implementation of an informatics
framework for the UK cancer research community. The framework advocates an integrated
multidisciplinary method of working between scientific and medical communities. Key to this
process is community adoption of high quality acquisition, storage, sharing and integration of
diverse data elements to improve knowledge of the causes, prevention and treatment of
cancer. The integration of the complex data and meta-data used by these multiple
communities is a significant challenge and there are technical, resource-based and
sociological issues to be addressed. In this paper we review progress aimed at establishing
the framework and outline key challenges in ultra large scale integration of biomedical
computing systems
Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion
This paper explores the long-term evolution of a subglacial fjord landscape in the Shackleton Range, Antarctica. We propose that prolonged ice-sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice-sheet surface, leaving adjacent mountains exposed. Geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence. Simple calculations suggest that isostatic compensation caused by the deepening of bounding ice-stream troughs lowered the ice-sheet surface relative to the mountains by ~800m. Use of multiple cosmogenic isotopes on bedrock and erratics (26Al, 10Be, 21Ne) provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the Quaternary. Perhaps this occurred in the mid-Miocene, as elsewhere in East Antarctica in the McMurdo Dry Valleys and the Lambert basin. The implication is that glacial erosion instigates feedback that can change ice-sheet thickness, extent, and direction of flow. Indeed, as the subglacial troughs evolve over millions of years, they increase topographic relief; and this changes the dynamics of the ice sheet. © 2013 Elsevier B.V
The Distance to the Soft Gamma Repeater SGR 1627-41
We report millimeter observations of the line of sight to the recently
discovered Soft Gamma Repeater, SGR 1627-41, which has been tentatively
associated with the supernova remnant SNR G337.0-0.1 Among the eight molecular
clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is
probably interacting with one of the most massive giant molecular clouds (GMC)
in the Galaxy, at a distance of 11 kpc from the sun. Based on the high
extinction to the persistent X-ray counterpart of SGR 1627-41, we present
evidence for an association of this new SGR with the SNR G337.0-0.1; they both
appear to be located on the near side of the GMC. This is the second SGR
located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a
neutron star with a high transverse velocity (~ 1,000 \kms) escaping the young
(~ 5,000 years) supernova remnant G337.0-0.1Comment: 17 pages, including 2 figures. Accepted for publication in the
Astrophysical Journal Letter
Metastable dark matter mechanisms for INTEGRAL 511 keV rays and DAMA/CoGeNT events
We explore dark matter mechanisms that can simultaneously explain the
galactic 511 keV gamma rays observed by INTEGRAL/SPI, the DAMA/LIBRA annual
modulation, and the excess of low-recoil dark matter candidates observed by
CoGeNT. It requires three nearly degenerate states of dark matter in the 4-7
GeV mass range, with splittings respectively of order an MeV and a few keV. The
top two states have the small mass gap and transitions between them, either
exothermic or endothermic, can account for direct detections. Decays from one
of the top states to the ground state produce low-energy positrons in the
galaxy whose associated 511 keV gamma rays are seen by INTEGRAL. This decay can
happen spontaneously, if the excited state is metastable (longer-lived than the
age of the universe), or it can be triggered by inelastic scattering of the
metastable states into the shorter-lived ones. We focus on a simple model where
the DM is a triplet of an SU(2) hidden sector gauge symmetry, broken at the
scale of a few GeV, giving masses of order \lsim 1 GeV to the dark gauge
bosons, which mix kinetically with the standard model hypercharge. The purely
decaying scenario can give the observed angular dependence of the 511 keV
signal with no positron diffusion, while the inelastic scattering mechanism
requires transport of the positrons over distances \sim 1 kpc before
annihilating. We note that an x-ray line of several keV in energy, due to
single-photon decays involving the top DM states, could provide an additional
component to the diffuse x-ray background. The model is testable by proposed
low-energy fixed target experiments.Comment: 27 pp, 19 figures; v2. minor clarification, added refs; v3. corrected
observed rate of positron production, added new section responding to
criticisms of arXiv:0904.1025; v4. corrected typos in eqs. (6) and (40
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