14,289 research outputs found
Long and Short GRB
We report evidence from the 3B Catalogue that short (T_90 < 10 s) and long
(T_90 > 10 s) GRB represent different populations and processes: Their spectral
behavior is qualitatively different, with short bursts harder in the BATSE
range, but chiefly long bursts detected at higher photon energies; \langle
V/V_max \rangle = 0.385 \pm 0.019 for short GRB but \langle V/V_max \rangle =
0.282 \pm 0.014 for long GRB, differing by 0.103 \pm 0.024. Long GRB may be the
consequence of accretion-induced collapse, but this mechanism fails for short
GRB, for which we suggest colliding neutron stars.Comment: 5 pp., latex, no figures, revised to work around bug in latex
compile
Cosmic ray modulation in a random anisotropic magnetic field
Inhomogeneities of the interplanetary magnetic field can be divided into small scale and large scale ones as may be required by the character of the problem of cosmic ray (CR) propagation. CR propagation in stochastic magnetic fields is of diffusion character. The main contribution into the scattering of CR particles is made by their interaction with inhomogeneities of the magnetic field H which have characteristic dimensions 1 of the order of Larmor radius R=cp/eH of particle (p is the absolute value of particle momentum, e is particle charge, c is velocity of light). Scattering of particles on such inhomogeneities leads to their diffusion mostly along a magnetic field with characteristic dimensions of variation in space exceeding the mean free path
When hot water freezes before cold
I suggest that the origin of the Mpemba effect (the freezing of hot water
before cold) is freezing-point depression by solutes, either gaseous or solid,
whose solubility decreases with increasing temperature so that they are removed
when water is heated. They are concentrated ahead of the freezing front by zone
refining in water that has not been heated, reduce the temperature of the
freezing front, and thereby reduce the temperature gradient and heat flux,
slowing the progress of the front. I present a simple calculation of this
effect, and suggest experiments to test this hypothesis.Comment: 7 pages, 1 figur
Scanning optical pyrometer for measuring temperatures in hollow cathodes
Life-limiting processes in hollow cathodes are determined largely by the temperature of the electron emitter. To support cathode life assessment, a noncontact temperature measurement technique which employs a stepper motor-driven fiber optic probe was developed. The probe is driven inside the hollow cathode and collects light radiated by the hot interior surface of the emitter. Ratio pyrometry is used to determine the axial temperature profile. Thermocouples on the orifice plate provide measurements of the external temperature during cathode operation and are used to calibrate the pyrometer system in situ with a small oven enclosing the externally heated cathode. The diagnostic method and initial measurements of the temperature distribution in a hollow cathode are discussed
Polar orbit electrostatic charging of objects in shuttle wake
A survey of DMSP data has uncovered several cases where precipitating auroral electron fluxes are both sufficiently intense and energetic to charge spacecraft materials such as teflon to very large potentials in the absence of ambient ion currents. Analytical bounds are provided which show that these measured environments can cause surface potentials in excess of several hundred volts to develop on objects in the orbiter wake for particular vehicle orientations
Prioritising the care of critically ill children: a pilot study using SCREEN reduces clinic waiting times
Objective In low-resource settings, childhood mortality secondary to delays in triage and treatment remains high. This paper seeks to evaluate the impact of the novel Sick Children Require Emergency Evaluation Now (SCREEN) tool on the waiting times of critically ill children who present for care to primary healthcare clinics in Cape Town, South Africa. Methods We used a pre/postevaluation study design to calculate the median waiting times of all children who presented to four randomly chosen clinics for 5 days before, and 5 days after, the implementation of SCREEN. Findings The SCREEN programme resulted in statistical and clinically significant reductions in waiting times for children with critical illness to see a professional nurse (2 hours 45 min to 1 hour 12 min; p<0.001). There was also a statistically significant reduction in the proportion of children who left without being seen by a professional nurse (25.8% to 18.48%; p<0.001). Conclusions SCREEN is a novel programme that uses readily available laypersons, trained to make a subjective assessment of children arriving at primary healthcare centres, and provides a low cost, simple methodology to prioritise children and reduce waiting times in low-resource healthcare clinics
The Long and the Short of Gamma-Ray Bursts
We report evidence from the 3B Catalogue that long ( s) and
short ( s) gamma-ray bursts represent distinct source populations.
Their spatial distributions are significantly different, with long bursts
having but short bursts having
, differing by ,
significant at the level. Long and short bursts also differ
qualitatively in their spectral behavior, with short bursts harder in the BATSE
(50--300 KeV) band, but long bursts more likely to be detected at photon
energies > 1 MeV. This implies different spatial origin and physical processes
for long and short bursts. Long bursts may be explained by accretion-induced
collapse. Short bursts require another mechanism, for which we suggest neutron
star collisions. These are capable of producing neutrino bursts as short as a
few ms, consistent with the shortest observed time scales in GRB. We briefly
investigate the parameters of clusters in which neutron star collisons may
occur, and discuss the nuclear evolution of expelled and accelerated matter.Comment: 21 pp., AAS latex, 1 figure added as ps fil
Superpotentials from variational derivatives rather than Lagrangians in relativistic theories of gravity
The prescription of Silva to derive superpotential equations from variational
derivatives rather than from Lagrangian densities is applied to theories of
gravity derived from Lovelock Lagrangians in the Palatini representation.
Spacetimes are without torsion and isolated sources of gravity are minimally
coupled. On a closed boundary of spacetime, the metric is given and the
connection coefficients are those of Christoffel. We derive equations for the
superpotentials in these conditions. The equations are easily integrated and we
give the general expression for all superpotentials associated with Lovelock
Lagrangians. We find, in particular, that in Einstein's theory, in any number
of dimensions, the superpotential, valid at spatial and at null infinity, is
that of Katz, Bicak and Lynden-Bell, the KBL superpotential. We also give
explicitly the superpotential for Gauss-Bonnet theories of gravity. Finally, we
find a simple expression for the superpotential of Einstein-Gauss-Bonnet
theories with an anti-de Sitter background: it is minus the KBL superpotential,
confirming, as it should, the calculation of the total mass-energy of spacetime
at spatial infinity by Deser and Tekin.Comment: Scheduled to appear in Class. Quantum Grav. August 200
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