4,825 research outputs found
Lensing by Lyman Limit Systems: Determining the Mass to Gas Ratio
We present a new method to determine the total mass-to-neutral gas ratio in
Lyman-limits systems. The method exploits the relation between the neutral
hydrogen column density and the magnification of background sources due to the
weak gravitational lensing that these systems induce. Because weak lensing does
not provide a direct measure of mass, one must use this relation in a
statistical sense to solve for the average mass-to-gas ratio and its
distribution. We use a detailed mock catalog of quasars (sources) and
Lyman-limit systems (lenses) to demonstrate the applicability of this approach
through our ability to recover the parameter. This mock catalog also allows us
to check for systematics in the method and to sketch its limitations. For a
universal constant mass-to-gas ratio and a sample of N quasars, we obtain an
unbiased estimate of its value with 95% confidence limits (independent of its
actual value) of +/- 140 {10^5/N)^0.5.Comment: 20 pages, 11 figures submitted to Ap
On the harmonic measure of stable processes
Using three hypergeometric identities, we evaluate the harmonic measure of a
finite interval and of its complementary for a strictly stable real L{\'e}vy
process. This gives a simple and unified proof of several results in the
literature, old and recent. We also provide a full description of the
corresponding Green functions. As a by-product, we compute the hitting
probabilities of points and describe the non-negative harmonic functions for
the stable process killed outside a finite interval
Do Gamma-Ray Burst Sources Repeat?
The demonstration of repeated gamma-ray bursts from an individual source
would severely constrain burst source models. Recent reports (Quashnock and
Lamb 1993; Wang and Lingenfelter 1993) of evidence for repetition in the first
BATSE burst catalog have generated renewed interest in this issue. Here, we
analyze the angular distribution of 585 bursts of the second BATSE catalog
(Meegan et al. 1994). We search for evidence of burst recurrence using the
nearest and farthest neighbor statistic and the two-point angular correlation
function. We find the data to be consistent with the hypothesis that burst
sources do not repeat; however, a repeater fraction of up to about 20% of the
observed bursts cannot be excluded.Comment: ApJ Letters, in press, 13 pages, including three embedded figures.
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A systematic review of internet-based worksite wellness approaches for cardiovascular disease risk management: outcomes, challenges & opportunities.
ContextThe internet is gaining popularity as a means of delivering employee-based cardiovascular (CV) wellness interventions though little is known about the cardiovascular health outcomes of these programs. In this review, we examined the effectiveness of internet-based employee cardiovascular wellness and prevention programs.Evidence acquisitionWe conducted a systematic review by searching PubMed, Web of Science and Cochrane library for all published studies on internet-based programs aimed at improving CV health among employees up to November 2012. We grouped the outcomes according to the American Heart Association (AHA) indicators of cardiovascular wellbeing--weight, BP, lipids, smoking, physical activity, diet, and blood glucose.Evidence synthesisA total of 18 randomized trials and 11 follow-up studies met our inclusion/exclusion criteria. Follow-up duration ranged from 6-24 months. There were significant differences in intervention types and number of components in each intervention. Modest improvements were observed in more than half of the studies with weight related outcomes while no improvement was seen in virtually all the studies with physical activity outcome. In general, internet-based programs were more successful if the interventions also included some physical contact and environmental modification, and if they were targeted at specific disease entities such as hypertension. Only a few of the studies were conducted in persons at-risk for CVD, none in blue-collar workers or low-income earners.ConclusionInternet based programs hold promise for improving the cardiovascular wellness among employees however much work is required to fully understand its utility and long term impact especially in special/at-risk populations
Representation of Markov chains by random maps: existence and regularity conditions
We systematically investigate the problem of representing Markov chains by
families of random maps, and which regularity of these maps can be achieved
depending on the properties of the probability measures. Our key idea is to use
techniques from optimal transport to select optimal such maps. Optimal
transport theory also tells us how convexity properties of the supports of the
measures translate into regularity properties of the maps via Legendre
transforms. Thus, from this scheme, we cannot only deduce the representation by
measurable random maps, but we can also obtain conditions for the
representation by continuous random maps. Finally, we present conditions for
the representation of Markov chain by random diffeomorphisms.Comment: 22 pages, several changes from the previous version including
extended discussion of many detail
Continuous infusion of an agonist of the tumor necrosis factor receptor 2 in the spinal cord improves recovery after traumatic contusive injury.
AimThe activation of the TNFR2 receptor is beneficial in several pathologies of the central nervous system, and this study examines whether it can ameliorate the recovery process following spinal cord injury.MethodsEHD2-sc-mTNFR2 , an agonist specific for TNFR2, was used to treat neurons exposed to high levels of glutamate in vitro. In vivo, it was infused directly to the spinal cord via osmotic pumps immediately after a contusion to the cord at the T9 level. Locomotion behavior was assessed for 6 weeks, and the tissue was analyzed (lesion size, RNA and protein expression, cell death) after injury. Somatosensory evoked potentials were also measured in response to hindlimb stimulation.ResultsThe activation of TNFR2 protected neurons from glutamate-mediated excitotoxicity through the activation of phosphoinositide-3 kinase gamma in vitro and improved the locomotion of animals following spinal cord injury. The extent of the injury was not affected by infusing EHD2-sc-mTNFR2 , but higher levels of neurofilament H and 2', 3'-cyclic-nucleotide 3'-phosphodiesterase were observed 6 weeks after the injury. Finally, the activation of TNFR2 after injury increased the neural response recorded in the cortex following hindlimb stimulation.ConclusionThe activation of TNFR2 in the spinal cord following contusive injury leads to enhanced locomotion and better cortical responses to hindlimb stimulation
Single-parameter non-adiabatic quantized charge pumping
Controlled charge pumping in an AlGaAs/GaAs gated nanowire by
single-parameter modulation is studied experimentally and theoretically.
Transfer of integral multiples of the elementary charge per modulation cycle is
clearly demonstrated. A simple theoretical model shows that such a quantized
current can be generated via loading and unloading of a dynamic quasi-bound
state. It demonstrates that non-adiabatic blockade of unwanted tunnel events
can obliterate the requirement of having at least two phase-shifted periodic
signals to realize quantized pumping. The simple configuration without multiple
pumping signals might find wide application in metrological experiments and
quantum electronics.Comment: 4 pages, 4 figure
A simple analytical model for dark matter halo structure and adiabatic contraction
A simple analytical model for describing inner parts of dark matter halo is
considered. It is assumed that dark matter density is power-law. The model
deals with dark matter distribution function in phase space of adiabatic
invariants (radial action and angular momentum). Two variants are considered
for the angular part of the distribution function: narrow and broad
distribution. The model allows to describe explicitly the process of adiabatic
contraction of halo due to change of gravitational potential caused by
condensation of baryonic matter in the centre. The modification of dark matter
density in the centre is calculated, and is it shown that the standard
algorithm of adiabatic contraction calculation overestimates the compressed
halo density, especially in the case of strong radial anisotropy.Comment: 5 pages, 3 figures. v3 - major improvements, another halo model
introduced, discussion extende
Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe
We describe a calculation of electromagnetic cascading in radiation and
matter in the early universe initiated by the decay of massive particles or by
some other process. We have used a combination of Monte Carlo and numerical
techniques which enables us to use exact cross sections, where known, for all
the relevant processes. In cascades initiated after the epoch of big bang
nucleosynthesis -rays in the cascades will photodisintegrate He,
producing He and deuterium. Using the observed He and deuterium
abundances we are able to place constraints on the cascade energy deposition as
a function of cosmic time. In the case of the decay of massive primordial
particles, we place limits on the density of massive primordial particles as a
function of their mean decay time, and on the expected intensity of decay
neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with
previous work of the photon spectrum in the cascade and the limits we
calculate for the density of massive particles. The method of calculation of
photon spectra at low energies has been improved. Most figures are revised.
Our conclusions are substantially unchange
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