16,280 research outputs found
Threshold detection in an on-off binary communications channel with atmospheric scintillation
The optimum detection threshold in an on-off binary optical communications system operating in the presence of atmospheric turbulence was investigated assuming a poisson detection process and log normal scintillation. The dependence of the probability of bit error on log amplitude variance and received signal strength was analyzed and semi-emperical relationships to predict the optimum detection threshold derived. On the basis of this analysis a piecewise linear model for an adaptive threshold detection system is presented. Bit error probabilities for non-optimum threshold detection system were also investigated
Unveiling Palomar 2: The Most Obscure Globular Cluster in the Outer Halo
We present the first color-magnitude study for Palomar 2, a distant and
heavily obscured globular cluster near the Galactic anticenter. Our (V,V-I)
color-magnitude diagram (CMD), obtained with the UH8K camera at the CFHT,
reaches V(lim) = 24 and clearly shows the principal sequences of the cluster,
though with substantial overall foreground absorption and differential
reddening. The CMD morphology shows a well populated red horizontal branch with
a sparser extension to the blue, similar to clusters such as NGC 1261, 1851, or
6229 with metallicities near [Fe/H] = -1.3, placing it about 34 kpc
from the Galactic center. We use starcounts of the bright stars to measure the
core radius, half-mass radius, and central concentration of the cluster. Its
integrated luminosity is M_V = -7.9, making it clearly brighter and more
massive than most other clusters in the outer halo.Comment: 25 pages, aastex, with 8 postscript figures; accepted for publication
in AJ, September 1997. Also available by e-mail from
[email protected]. Please consult Harris directly for (big)
postscript files of Figures 1a,b (the images of the cluster
The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability
We examine the formation and evolution of the density enhancement (density
spike) that appears downstream of strong, cosmic-ray-modified shocks. This
feature results from temporary overcompression of the flow by the combined
cosmic-ray shock precursor/gas subshock. Formation of the density spike is
expected whenever shock modification by cosmic-ray pressure increases strongly.
That occurence may be anticipated for newly generated strong shocks or for
cosmic-ray-modified shocks encountering a region of higher external density,
for example. The predicted mass density within the spike increases with the
shock Mach number and with shocks more dominated by cosmic-ray pressure. We
find this spike to be linearly unstable under a modified Rayleigh-Taylor
instability criterion at the early stage of its formation. We confirm this
instability numerically using two independent codes based on the two-fluid
model for cosmic-ray transport. These two-dimensional simulations show that the
instability grows impulsively at early stages and then slows down as the
gradients of total pressure and gas density decrease. Observational discovery
of this unstable density spike behind shocks, possibly through radio emission
enhanced by the amplified magnetic fields would provide evidence for the
existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap
Relativistic effects in Ni II and the search for variation of the fine structure constant
Theories unifying gravity and other interactions suggest the possibility of
spatial and temporal variation of physical ``constants'' in the Universe.
Detection of high redshift absorption systems intersecting the sight lines
towards distant quasars provide a powerful tool for measuring these variations.
In the present paper we demonstrate that high sensitivity to variation of the
fine structure constant alpha can be obtained by comparing cosmic and
laboratory spectra of the Ni II ion. Relativistic effects in Ni II reveal many
interesting features. The Ni II spectrum exhibits avoided level crossing
phenomenon under variation of alpha and the intervals between the levels have
strong nonlinear dependencies on relativistic corrections. The values of the
transition frequency shifts, due to the change of alpha, vary significantly
from state to state including change of the sign. This enhances the sensitivity
to the variation of alpha and reduces possible systematic errors. The
calculations of alpha-dependence of the nickel ion spectral lines that are
detectable in quasar absorption spectra have been performed using a
relativistic configuration interaction method.Comment: 13 pages, 1 figure, accepted by Phys. Rev. A, typos corrected,
acknowledgment adde
How well do antivirals shorten genital herpes pain duration?
Q: How well do antivirals shorten genital herpes pain duration? Evidence-based answer: Oral and intravenous (IV) acyclovir each shorten the duration of pain for a first primary outbreak of herpes by about 50%; topical acyclovir shortens it by about 25% (strength of recommendation [SOR]: B, small randomized controlled trials [RCTs] with some methodological flaws). Oral valacyclovir and famcyclovir are equivalent to oral acyclovir. Adding topical acyclovir to oral acyclovir doesn't produce additional benefit (SOR: B, RCTs). Patients with severe disease may require IV acyclovir (SOR: C, expert opinion). General treatment measures that may improve patient comfort include keeping lesions clean and dry, avoiding tight clothing, taking analgesics, and using ice packs or taking warm baths (SOR: C, expert opinion). Evidence for using complementary and alternative medicine to treat genital herpes is lacking or conflicting [SOR: C, narrative review of clinical trials]
GTF2IRD1 regulates transcription by binding an evolutionarily conserved DNA motif âGUCEâ
AbstractGTF2IRD1 is a member of a family of transcription factors whose defining characteristic is varying numbers of a helixâloopâhelix like motif, the I-repeat. Here, we present functional analysis of human GTF2IRD1 in regulation of three genes (HOXC8, GOOSECOID and TROPONIN ISLOW). We define a regulatory motif (GUCEâGTF2IRD1 Upstream Control Element) common to all three genes. GUCE is bound in vitro by domain I-4 of GTF2IRD1 and mediates transcriptional regulation by GTF2IRD1 in vivo. Definition of this site will assist in identification of other downstream targets of GTF2IRD1 and elucidation of its role in the human developmental disorder WilliamsâBeuren syndrome
Possible evidence for a variable fine structure constant from QSO absorption lines: systematic errors
Comparison of quasar absorption spectra with laboratory spectra allow us to
probe possible variations in the fundamental constants over cosmological
time-scales. In a companion paper we present an analysis of Keck/HIRES spectra
and report possible evidence suggesting that the fine structure constant,
alpha, may have been smaller in the past: da/a = (-0.72 +/- 0.18) * 10^{-5}
over the redshift range 0.5 < z < 3.5. In this paper we describe a
comprehensive investigation into possible systematic effects. Most of these do
not significantly influence our results. When we correct for those which do
produce a significant systematic effect in the data, the deviation of da/a from
zero becomes more significant. We are lead increasingly to the interpretation
that alpha was slightly smaller in the past.Comment: 15 pages, 8 figures, Accepted by MNRAS, small changes after referee's
comments, accepted versio
A Search for Time Variation of the Fine Structure Constant
A method offering an order of magnitude sensitivity gain is described for
using quasar spectra to investigate possible time or space variation in the
fine structure constant, alpha. Applying the technique to a sample of 30
absorption systems, spanning redshifts 0.5 < z< 1.6, obtained with the Keck I
telescope, we derive limits on variations in alpha over a wide range of epochs.
For the whole sample Delta(alpha)/alpha = -1.1 +/- 0.4 x 10^{-5}. This
deviation is dominated by measurements at z > 1, where Delta(alpha)/alpha =
-1.9 +/- 0.5 x 10^{-5}. For z < 1, Delta(alpha)/alpha = -0.2 +/- 0.4 x 10^{-5},
consistent with other known constraints. Whilst these results are consistent
with a time-varying alpha, further work is required to explore possible
systematic errors in the data, although careful searches have so far not
revealed any.Comment: 4 pages, 1 figure, accepted for publication in Physical Review
Letter
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