2,637 research outputs found
Six degrees of freedom vibration isolation using electromagnetic suspension
Experimental data are presented for modeling an electromagnet. Control laws are considered with and without flux feedback and with position and orientation information of the suspended body. Base motion and sensor noise are the principal disturbances. Proper selection of the geometrical operating point minimizes the passive coupling above the bandwidth of the control and filtering can attenuate the high frequency content of sensor noise. Six electromagnets are arranged in a configuration which optimizes the load support and provides control over all six degrees of freedom of the suspended body. The design is based on experimental data generated with a specially designed test facility. Application for suspension of a gravity wave antenna is discussed
Polygenic risk for schizophrenia and season of birth within the UK Biobank cohort
Background:
There is strong evidence that people born in winter and in spring have a small increased risk of schizophrenia. As this ‘season of birth’ effect underpins some of the most influential hypotheses concerning potentially modifiable risk exposures, it is important to exclude other possible explanations for the phenomenon.
Methods:
Here we sought to determine whether the season of birth effect reflects gene-environment confounding rather than a pathogenic process indexing environmental exposure. We directly measured, in 136 538 participants from the UK Biobank (UKBB), the burdens of common schizophrenia risk alleles and of copy number variants known to increase the risk for the disorder, and tested whether these were correlated with a season of birth.
Results:
Neither genetic measure was associated with season or month of birth within the UKBB sample.
Conclusions:
As our study was highly powered to detect small effects, we conclude that the season of birth effect in schizophrenia reflects a true pathogenic effect of environmental exposure
Unbiased molecular analysis of T cell receptor expression using template-switch achored RT-PCR
A detailed knowledge of the principles that guide clonal selection within the memory and effector T cell pools is essential to further our understanding of the factors that influence effective T cell-mediated immunity and has direct implications for the rational design of vaccines and immunotherapies. This unit provides methods for the unbiased quantification and characterization of all expressed T cell receptor (TCR) gene products within any defined T cell population. The approach is based on a template-switch anchored reverse transcription–polymerase chain reaction (RT-PCR) and is optimized for the analysis of antigen-specific T cells isolated directly ex vivo
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
A 21-cm power spectrum at 48 MHz, using the Owens Valley Long Wavelength Array
The Large-aperture Experiment to detect the Dark Age (LEDA) was designed to
measure the 21-cm signal from neutral hydrogen at Cosmic Dawn, 15-30. Using observations made with the 200 m diameter core of the
Owens Valley Long Wavelength Array (OVRO-LWA), we present a 2-D cylindrical
spatial power spectrum for data at 43.1-53.5 MHz ()
incoherently integrated for 4 hours, and an analysis of the array sensitivity.
Power from foregrounds is localized to a "wedge" within
space. After calibration of visibilities using 5 bright compact sources
including VirA, we measure $\Delta^2(k) \approx 2 \times 10^{12}\
\mathrm{mK}^2\Delta^2(k)z = \Delta^2(k) \approx
100^2^{-1}\alpha$ and X-ray sources, as
predicted for a range of theoretical model parameters.Comment: 16 pages, 11 figures. Accepted for MNRAS; replaced with accepted
versio
On the symmetry of the vacuum in theories with spontaneous symmetry breaking
We review the usual account of the phenomena of spontaneous symmetry breaking
(SSB), pointing out the common misunderstandings surrounding the issue, in
particular within the context of quantum field theory. In fact, the common
explanations one finds in this context, indicate that under certain conditions
corresponding to the situation called SSB, the vacuum of the theory does not
share the symmetries of the Lagrangian. We explain in detail why this statement
is incorrect in general, and in what limited set of circumstances such
situation could arise. We concentrate on the case of global symmetries, for
which we found no satisfactory exposition in the existing literature, and
briefly comment on the case of gauge symmetries where, although insufficiently
publicized, accurate and complete descriptions exist. We briefly discuss the
implications for the phenomenological manifestations usually attributed to the
phenomena of spontaneous symmetry breaking, analyzing which might be affected
by our analysis and which are not. In particular we describe the mass
generation mechanism in a fully symmetric scheme (i.e., with a totally
symmetric vacuum), and briefly discuss the implications of this analysis to the
problem of formation of topological defects in the early universe
Spectral index of the Galactic foreground emission in the 50-87 MHz range
Radiometry using individual dipole antennas is a potentially effective way to
study the cosmological epoch referred to as Cosmic Dawn (z~20) through
measurement of sky brightness arising from the 21~cm transition of neutral
hydrogen, provided this can be disentangled from much stronger Galactic and
extragalactic foregrounds. In the process, measured spectra of integrated sky
brightness temperature can be used to quantify properties of the foreground
emission. In this work we analyze data from the Large-aperture Experiment to
Detect the Dark Age (LEDA) in the range 50-87 MHz to constrain the spectral
index of foreground emission in the northern sky. We focus on two
zenith-directed LEDA radiometers and study how estimates of vary with
local sidereal time (LST). We correct for the effect of gain pattern
chromaticity and compare estimated absolute temperatures with simulations.
During times with the best observing conditions, for a "reference" radiometer,
we estimate that varies from -2.55 to -2.58, consistent with previous
measurements of the southern sky and simulated sky models. Using data from the
second, experimental, radiometer, we observe a similar trend vs. LST although
with slightly smaller , in the range. We infer
good instrument stability from consistency in computed spectral indices at a
level of 1-2 for LST=9-12.5h, using data distributed between mid-2018
to mid-2019. Evidence for spectral curvature is weak owing to residual
systematic errors, other than when the Galactic Center is in the sky, at which
time we find evidence for negative curvature, ~-0.4.Comment: 13 pages, 19 figure
A Complete Catalog of Swift GRB Spectra and Durations: Demise of a Physical Origin for Pre-Swift High-Energy Correlations
We calculate durations and spectral paramaters for 218 Swift bursts detected
by the BAT instrument between and including GRBs 041220 and 070509, including
77 events with measured redshifts. Incorporating prior knowledge into the
spectral fits, we are able to measure the characteristic spectral
peak energy and the isotropic equivalent energy
(1-- keV) for all events. This complete and rather extensive catalog,
analyzed with a unified methodology, allows us to address the persistence and
origin of high-energy correlations suggested in pre-Swift observations. We find
that the - correlation is present in the Swift
sample; however, the best-fit powerlaw relation is inconsistent with the
best-fit pre-Swift relation at >5 sigma significance. Moreover, it has a factor
>~ 2 larger intrinsic scatter, after accounting for large errors on . A large fraction of the Swift events are hard and subluminous
relative to (and inconsistent with) the pre-Swift relation, in agreement with
indications from BATSE GRBs without redshift. Moreover, we determine an
experimental threshold for the BAT detector and show how the -- correlation arises artificially due to partial
correlation with the threshold. We show that pre-Swift correlations found by
Amati et al.(2002), Yonetoku et al. (2004), Firmani et al.(2006) (and
independently by others) are likely unrelated to the physical properties of
GRBs and are likely useless for tests of cosmology. Also, an explanation of
these correlations in terms of a detector threshold provides a natural and
quantitative explanation for why short-duration GRBs and events at low redshift
tend to be outliers to the correlations.Comment: 25 pages, 9 figures, 2 tables, Accepted to Ap
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