2,925 research outputs found
The 2000 Periastron Passage of PSR B1259-63
We report here on a sequence of 28 observations of the binary pulsar system
PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope
Compact Array around the time of the 2000 periastron passage. Observations made
on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a
maximum of rad m, some 700 times the value measured
away from periastron, and is the largest astrophysical RM measured. This value,
combined with the dispersion measure implies a magnetic field in the Be star's
wind of 6 mG. We find that the light curve of the unpulsed emission is similar
to that obtained during the 1997 periastron but that differences in detail
imply that the emission disc of the Be star is thicker and/or of higher
density. The behaviour of the light curve at late times is best modelled by the
adiabatic expansion of a synchrotron bubble formed in the pulsar/disc
interaction. The expansion rate of the bubble km s is
surprisingly low but the derived magnetic field of 1.6 G close to that
expected.Comment: 8 pages, 6 figures, 3 tables, LaTeX (mn.sty). Accepted for
publication in the Monthly Notices of the Royal Astronomical Society. Also
available at http://astronomy.swin.edu.au/staff/tconnors/publications.htm
Development of a low-maintenance measurement approach to continuously estimate methane emissions: a case study
The chemical breakdown of organic matter in landfills represents a significant source of methane gas (CH4). Current estimates suggest that landfills are responsible for between 3% and 19% of global anthropogenic emissions. The net CH4 emissions resulting from biogeochemical processes and their modulation by microbes in landfills are poorly constrained by imprecise knowledge of environmental constraints. The uncertainty in absolute CH4 emissions from landfills is therefore considerable. This study investigates a new method to estimate the temporal variability of CH4 emissions using meteorological and CH4 concentration measurements downwind of a landfill site in Suffolk, UK from July to September 2014, taking advantage of the statistics that such a measurement approach offers versus shorter-term, but more complex and instantaneously accurate, flux snapshots. Methane emissions were calculated from CH4 concentrations measured 700 m from the perimeter of the landfill with observed concentrations ranging from background to 46.4 ppm. Using an atmospheric dispersion model, we estimate a mean emission flux of 709 ÎŒg mâ2 sâ1 over this period, with a maximum value of 6.21 mg mâ2 sâ1, reflecting the wide natural variability in biogeochemical and other environmental controls on net site emission. The emissions calculated suggest that meteorological conditions have an influence on the magnitude of CH4 emissions. We also investigate the factors responsible for the large variability observed in the estimated CH4 emissions, and suggest that the largest component arises from uncertainty in the spatial distribution of CH4 emissions within the landfill area. The results determined using the low-maintenance approach discussed in this paper suggest that a network of cheaper, less precise CH4 sensors could be used to measure a continuous CH4 emission time series from a landfill site, something that is not practical using far-field approaches such as tracer release methods. Even though there are limitations to the approach described here, this easy, low-maintenance, low-cost method could be used by landfill operators to estimate time-averaged CH4 emissions and their impact downwind by simultaneously monitoring plume advection and CH4 concentrations
13 Years of Timing of PSR B1259-63
This paper summarizes the results of 13 years of timing observations of a
unique binary pulsar, PSR B125963, which has a massive B2e star companion.
The data span encompasses four complete orbits and includes the periastron
passages in 1990, 1994, 1997 and 2000. Changes in dispersion measure occurring
around the 1994, 1997 and 2000 periastrons are measured and accounted for in
the timing analysis. There is good evidence for a small glitch in the pulsar
period in 1997 August, not long after the 1997 periastron, and a significant
frequency second derivative indicating timing noise. We find that spin-orbit
coupling with secular changes in periastron longitude and projected semi-major
axis () cannot account for the observed period variations over the whole
data set. While fitting the data fairly well, changes in pulsar period
parameters at each periastron seem ruled out both by X-ray observations and by
the large apparent changes in pulsar frequency derivative. Essentially all of
the systematic period variations are accounted for by a model consisting of the
1997 August glitch and step changes in at each periastron. These changes
must be due to changes in the orbit inclination, but we can find no plausible
mechanism to account for them. It is possible that timing noise may mask the
actual changes in orbital parameters at each periastron, but the good fit to
the data of the step-change model suggests that short-term timing noise is
not significant.Comment: 9 pages, 7 figures, accepted by MNRA
A defect in myoblast fusion underlies Carey-Fineman-Ziter syndrome
Multinucleate cellular syncytial formation is a hallmark of skeletal muscle differentiation. Myomaker, encoded by Mymk (Tmem8c), is a well-conserved plasma membrane protein required for myoblast fusion to form multinucleated myotubes in mouse, chick, and zebrafish. Here, we report that autosomal recessive mutations in MYMK (OMIM 615345) cause Carey-Fineman-Ziter syndrome in humans (CFZS; OMIM 254940) by reducing but not eliminating MYMK function. We characterize MYMK-CFZS as a congenital myopathy with marked facial weakness and additional clinical and pathologic features that distinguish it from other congenital neuromuscular syndromes. We show that a heterologous cell fusion assay in vitro and allelic complementation experiments in mymk knockdown and mymkinsT/insT zebrafish in vivocan differentiate between MYMK wild type, hypomorphic and null alleles. Collectively, these data establish that MYMK activity is necessary for normal muscle development and maintenance in humans, and expand the spectrum of congenital myopathies to include cell-cell fusion deficits
A Map of the Universe
We have produced a new conformal map of the universe illustrating recent
discoveries, ranging from Kuiper belt objects in the Solar system, to the
galaxies and quasars from the Sloan Digital Sky Survey. This map projection,
based on the logarithm map of the complex plane, preserves shapes locally, and
yet is able to display the entire range of astronomical scales from the Earth's
neighborhood to the cosmic microwave background. The conformal nature of the
projection, preserving shapes locally, may be of particular use for analyzing
large scale structure. Prominent in the map is a Sloan Great Wall of galaxies
1.37 billion light years long, 80% longer than the Great Wall discovered by
Geller and Huchra and therefore the largest observed structure in the universe.Comment: Figure 8, and additional material accessible on the web at:
http://www.astro.princeton.edu/~mjuric/universe
Energetic changes caused by antigenic module insertion in a virus-like particle revealed by experiment and molecular dynamics simulations
The success of recombinant virus-like particles (VLPs) for human papillomavirus and hepatitis B demonstrates the potential of VLPs as safe and efficacious vaccines. With new modular designs emerging, the effects of antigen module insertion on the self-assembly and structural integrity of VLPs should be clarified so as to better enabling improved design. Previous work has revealed insights into the molecular energetics of a VLP subunit, capsomere, comparing energetics within various solution conditions known to drive or inhibit self-assembly. In the present study, molecular dynamics (MD) simulations coupled with the molecular mechanics-Poisson-Boltzmann surface area (MM-PBSA) method were performed to examine the molecular interactions and energetics in a modular capsomere of a murine polyomavirus (MPV) VLP designed to protect against influenza. Insertion of an influenza antigenic module is found to lower the binding energy within the capsomere, and a more active state is observed in Assembly Buffer as compared with that in Stabilization Buffer, which has been experimentally validated through measurements using differential scanning calorimetry. Further in-depth analysis based on free-energy decomposition indicates that destabilized binding can be attributed to electrostatic interaction induced by the chosen antigen module. These results provide molecular insights into the conformational stability of capsomeres and their abilities to be exploited for antigen presentation, and are expected to be beneficial for the biomolecular engineering of VLP vaccines.Lin Zhang, Ronghong Tang, Shu Bai, Natalie K. Connors, Linda H.L. Lua, Yap P. Chuan, Anton P.J. Middelberg, Yan Su
The geometry of spontaneous spiking in neuronal networks
The mathematical theory of pattern formation in electrically coupled networks
of excitable neurons forced by small noise is presented in this work. Using the
Freidlin-Wentzell large deviation theory for randomly perturbed dynamical
systems and the elements of the algebraic graph theory, we identify and analyze
the main regimes in the network dynamics in terms of the key control
parameters: excitability, coupling strength, and network topology. The analysis
reveals the geometry of spontaneous dynamics in electrically coupled network.
Specifically, we show that the location of the minima of a certain continuous
function on the surface of the unit n-cube encodes the most likely activity
patterns generated by the network. By studying how the minima of this function
evolve under the variation of the coupling strength, we describe the principal
transformations in the network dynamics. The minimization problem is also used
for the quantitative description of the main dynamical regimes and transitions
between them. In particular, for the weak and strong coupling regimes, we
present asymptotic formulas for the network activity rate as a function of the
coupling strength and the degree of the network. The variational analysis is
complemented by the stability analysis of the synchronous state in the strong
coupling regime. The stability estimates reveal the contribution of the network
connectivity and the properties of the cycle subspace associated with the graph
of the network to its synchronization properties. This work is motivated by the
experimental and modeling studies of the ensemble of neurons in the Locus
Coeruleus, a nucleus in the brainstem involved in the regulation of cognitive
performance and behavior
Low energy polarization sensitivity of the Gas Pixel Detector
An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been
proposed to be included in many upcoming space missions to fill the gap of
about 30 years from the first (and to date only) positive measurement of
polarized X-ray emission from an astrophysical source. The estimated
sensitivity of the current prototype peaks at an energy of about 3 keV, but the
lack of readily available polarized sources in this energy range has prevented
the measurement of detector polarimetric performances.
In this paper we present the measurement of the Gas Pixel Detector
polarimetric sensitivity at energies of a few keV and the new, light, compact
and transportable polarized source that was devised and built to this aim.
Polarized photons are produced, from unpolarized radiation generated with an
X-ray tube, by means of Bragg diffraction at nearly 45 degrees.
The employment of mosaic graphite and flat aluminum crystals allow the
production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from
the diffraction of unpolarized continuum or line emission. The measured
modulation factor of the Gas Pixel Detector at these energies is in good
agreement with the estimates derived from a Monte Carlo software, which was up
to now employed for driving the development of the instrument and for
estimating its low energy sensitivity. In this paper we present the excellent
polarimetric performance of the Gas Pixel Detector at energies where the peak
sensitivity is expected. These measurements not only support our previous
claims of high sensitivity but confirm the feasibility of astrophysical X-ray
photoelectric polarimetry.Comment: 15 pages, 12 figures. Accepted for publication in NIM
Characteristics of long-duration inhibitory postsynaptic potentials in rat neocortical neurons in vitro
1. The characteristics of long-duration inhibitory postsynaptic potentials (l-IPSPs) which are evoked in rat frontal neocortical neurons by local electrical stimulation were investigated with intracellular recordings from anin vitro slice preparation.
2. Stimulation with suprathreshold intensities evoked l-IPSPs with typical durations of 600â900 msec at resting membrane potential. Conductance increases of 15â60% were measured at the peak amplitude of l-IPSPs (150â250 msec poststimulus).
3. The duration of the conductance increases during l-IPSPs displayed a significant voltage dependence, decreasing as the membrance potential was depolarized and increasing with hyperpolarization.
4. The reversal potential of l-IPSPs is significantly altered by reductions in the extracellular potassium concentration. Therefore it is concluded that l-IPSPs in rat neocortical neurons are generated by the activation of a potassium conductance.
5. l-IPSPs exhibit stimulation fatigue. Stimulation with a frequency of 1 Hz produces a complete fatigue of the conductance increases during l-IPSPs after approximately 20 consecutive stimuli. Recovery from this fatigue requires minutes.
6. l-IPSPs are not blocked by bicuculline but are blocked by baclofen
Meditation and auditory attention: An ERP study of meditators and non-meditators
The findings of a study by Cahn and Polich (2009) suggests that there is an effect of a meditative state on three event-related potential (ERP) brain markers of âlow-levelâ auditory attention (i.e., acoustic representations in sensory memory) in expert meditators: the N1, the P2, and the P3a. The current study built on these findings by examining trait and state effects of meditation on the passive auditory mismatch negativity (MMN), N1, and P2 ERPs. We found that the MMN was significantly larger in meditators than non-meditators regardless of whether they were meditating or not (a trait effect), and that N1 amplitude was significantly attenuated during meditation in non-meditators but not expert meditators (an interaction between trait and state). These outcomes suggest that low-level attention is superior in long-term meditators in general. In contrast, low-level attention is reduced in non-meditators when they are asked to meditate for the first time, possibly due to auditory fatigue or cognitive overload
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