1,937 research outputs found
Simulation of radio emission from air showers in atmospheric electric fields
We study the effect of atmospheric electric fields on the radio pulse emitted
by cosmic ray air showers. Under fair weather conditions the dominant part of
the radio emission is driven by the geomagnetic field. When the shower charges
are accelerated and deflected in an electric field additional radiation is
emitted. We simulate this effect with the Monte Carlo code REAS2, using
CORSIKA-simulated showers as input. In both codes a routine has been
implemented that treats the effect of the electric field on the shower
particles. We find that the radio pulse is significantly altered in background
fields of the order of ~100 V/cm and higher. Practically, this means that air
showers passing through thunderstorms emit radio pulses that are not a reliable
measure for the shower energy. Under other weather circumstances significant
electric field effects are expected to occur rarely, but nimbostratus clouds
can harbor fields that are large enough. In general, the contribution of the
electric field to the radio pulse has polarization properties that are
different from the geomagnetic pulse. In order to filter out radio pulses that
have been affected by electric field effects, radio air shower experiments
should keep weather information and perform full polarization measurements of
the radio signal.Comment: 26 pages, 12 figures, accepted for publication in Astroparticle
Physic
Alterations in immunoglobulin levels in uninfected children born to HIV infected women
Background
Immunoglobulin levels are known to be elevated in HIV infected children. However, little is known about the effect of maternal HIV infection and the maternal altered immune system on immunoglobulin levels in uninfected children. As few data are available on immunoglobulins from young healthy children, we used data from uninfected children born to hepatitis C virus (HCV) infected women as a comparison.
Methods
Prospective data on immunoglobulin levels were available from birth to 5 years for children enrolled in the European Collaborative Study (ECS) of children born to HIV-1 infected women and from birth to 24 months for children enrolled in the European Paediatric HCV Network (EPHN). Children born to HIV/HCV co-infected women were excluded. Smoothers (running means) illustrated patterns of immunoglobulins over age by infection status. Associations between infant and maternal factors and child log10 total IgG, IgM and IgA levels were quantified in linear regression analyses allowing for repeated measures within child. Further analyses were performed using only data of HIV exposed uninfected children to investigate associations between child immunoglobulins and maternal immunological and virological factors and anti-retroviral therapy exposure.
Results
1751 HIV uninfected, 190 HIV infected children (ECS), 173 HCV uninfected and 30 HCV infected children (EPHN) were included. HIV infected children had higher levels of all immunoglobulins compared to uninfected children over all ages. HIV uninfected children had significantly higher IgG, IgM and IgA levels than HCV uninfected children upto at least 24 months, adjusting for gender, prematurity and race. Prematurity was associated with significantly lower levels of immunoglobulins upto 24 months. Children born to African women had higher IgG and IgA levels upto 24 months than those born to white women but lower IgM in the first 6 months.
Among HIV uninfected children higher IgG levels were associated with elevated maternal IgG levels, as well for measurements from 18 months to 5 years of age. No significant effect of maternal CD4 count was observed. ART exposure was associated with significantly lower IgG levels at 6-24 months. Race was not associated with immunoglobulin levels in multivariable analyses in this sub-group.
Conclusions
These findings indicate significant alterations in immunoglobulin levels in uninfected children born to HIV infected women. This suggests that exposure to an activated maternal immune system is associated with an altered humoral response in children without antigen stimulation, and warrants further research
Prospects for determining air shower characteristics through geosynchrotron emission arrival times
Using simulations of geosynchrotron radiation from extensive air showers, we
present a relation between the shape of the geosynchrotron radiation front and
the distance of the observer to the maximum of the air shower. By analyzing the
relative arrival times of radio pulses at several radio antennas in an air
shower array, this relation may be employed to estimate the depth of maximum of
an extensive air shower if its impact position is known, allowing an estimate
for the primary particle's species. Vice versa, the relation provides an
estimate for the impact position of the shower's core if an external estimate
of the depth of maximum is available. In realistic circumstances, the method
delivers reconstruction uncertainties down to 30 g/cm^2 when the distance to
the shower core does not exceed 7 km. The method requires that the arrival
direction is known with high precision.Comment: 7 pages, 9 figures. Accepted for publication in Astroparticle
Physics
Monte Carlo simulations of air showers in atmospheric electric fields
The development of cosmic ray air showers can be influenced by atmospheric
electric fields. Under fair weather conditions these fields are small, but the
strong fields inside thunderstorms can have a significant effect on the
electromagnetic component of a shower. Understanding this effect is
particularly important for radio detection of air showers, since the radio
emission is produced by the shower electrons and positrons. We perform Monte
Carlo simulations to calculate the effects of different electric field
configurations on the shower development. We find that the electric field
becomes important for values of the order of 1 kV/cm. Not only can the energy
distribution of electrons and positrons change significantly for such field
strengths, it is also possible that runaway electron breakdown occurs at high
altitudes, which is an important effect in lightning initiation.Comment: 24 pages, 19 figures, accepted for publication in Astroparticle
Physic
Spatially and Spectrally Resolved Observations of a Zebra Pattern in Solar Decimetric Radio Burst
We present the first interferometric observation of a zebra-pattern radio
burst with simultaneous high spectral (~ 1 MHz) and high time (20 ms)
resolution. The Frequency-Agile Solar Radiotelescope (FASR) Subsystem Testbed
(FST) and the Owens Valley Solar Array (OVSA) were used in parallel to observe
the X1.5 flare on 14 December 2006. By using OVSA to calibrate the FST the
source position of the zebra pattern can be located on the solar disk. With the
help of multi-wavelength observations and a nonlinear force-free field (NLFFF)
extrapolation, the zebra source is explored in relation to the magnetic field
configuration. New constraints are placed on the source size and position as a
function of frequency and time. We conclude that the zebra burst is consistent
with a double-plasma resonance (DPR) model in which the radio emission occurs
in resonance layers where the upper hybrid frequency is harmonically related to
the electron cyclotron frequency in a coronal magnetic loop.Comment: Accepted for publication in Ap
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
Does treating proximal cavities in primary molars non-restoratively affect intra-arch space and alignment of successor teeth negatively?:A 4-year longitudinal study
Background: Removing plaque with toothbrush and toothpaste from proximal cavities in primary molars without restoring them follows sound cariological principles. But does this treatment affect space for and alignment of their permanent successors negatively? Hypothesis: There is no difference in impaction and displacement of the premolars, as well as in the D+E space in quadrants with three different statuses of the proximal surface of primary molars over a 4-year period. Methods: A total of 936 quadrants (466 maxillary and 470 mandibular quadrants) in 233 children were assessed. Treatment of cavities in the proximal surfaces of the primary molars consisted of amalgam and ART restorations using high-viscosity glass-ionomer cement, and cleaning of open large- and medium-sized cavities with toothbrush and toothpaste (UCT) under supervision for 220 days per year over 3 years. Dental casts were made at baseline, and after two, three, and 4 years. The D+E spaces were measured digitally. Status of the proximal surface of the primary molars was assessed by two calibrated examiners, and quadrants were grouped into normal anatomy, defective restoration, and proximal cavity. ANCOVA, ANOVA and LSD tests were applied. Results: There was a statistically significant difference between groups (p <= 0.001) and between evaluation times (p < 0.001), for the D+E space in both the maxilla and mandible. A sex difference related to the D+E space in the maxilla was found (p = 0.007). For boys, quadrants in the maxilla of the group âproximal cavityâ showed a significant shorter D+E space when compared to quadrants of the group ânormal anatomyâ at the 3- and 4-year evaluation time. For girls the difference between the two groups was only present at the 3-year evaluation time. There was no significant difference between the D+E space in quadrants with defective restorations and those with normal anatomy in the mandible and in the maxilla. Displacement and impaction of the premolars showed no significant difference between groups. Conclusion: Primary molars with open proximal cavities that are cleaned with toothbrush and toothpaste do not result in displacement and impaction of the successor teeth, neither do primary molars with defective restorations in proximal tooth surfaces
Development of an Off-Grid Solar-Powered Autonomous Chemical Mini-Plant for Producing Fine Chemicals
Photochemistry using inexhaustible solar energy is an eco-friendly way to produce fine chemicals outside the typical laboratory or chemical plant environment. However, variations in solar irradiation conditions and the need for an external energy source to power electronic components limits the accessibility of this approach. In this work, a chemical solar-driven âmini-plantâ centred around a scaled-up luminescent solar concentrator photomicroreactor (LSC-PM) was built. To account for the variations in solar irradiance at ground level and passing clouds, a responsive control system was designed that rapidly adapts the flow rate of the reagents to the light received by the reaction channels. Supplying the plant with solar panels, integrated into the module by placing it behind the LSC to utilize the transmitted fraction of the solar irradiation, allowed this setup to be self-sufficient and fully operational off-grid. Such a system can shine in isolated environments and in a distributed manufacturing world, allowing to decentralize the production of fine chemicals
A burst from the direction of UZ Fornacis with XMM-Newton
The XMM-Newton pointing towards the magnetic cataclysmic variable UZ For
finds the source to be a factor > 10^3 fainter than previous EXOSAT and ROSAT
observations. The source was not detected for the majority of a 22 ksec
exposure with the EPIC cameras, suggesting that the accretion rate either
decreased, or stopped altogether. However a 1.1 ksec burst was detected from UZ
For during the observation. Spectral fits favour optically thin, kT = 4.4 keV
thermal emission. Detection of the burst by the on-board Optical Monitor
indicates that this was most probably an accretion event. The 0.1-10 keV
luminosity of 2.1 x 10^30 erg/s is typical for accretion shock emission from
high state polars and would result from the potential energy release of ~ 10^16
g of gas. There is no significant soft excess due to reprocessing in the white
dwarf atmosphere.Comment: 7 pages, 2 postscript figures, ApJL, in pres
Cyclotron modeling phase-resolved infrared spectroscopy of polars I: EF Eridani
We present phase-resolved low resolution infrared spectra of the polar EF
Eridani obtained over a period of 2 years with SPEX on the IRTF. The spectra,
covering the wavelength range 0.8 to 2.4 microns, are dominated by cyclotron
emission at all phases. We use a ``Constant Lambda'' prescription to attempt to
model the changing cyclotron features seen in the spectra. A single cyclotron
emission component with B = 12.6 MG, and a plasma temperature of kT = 5.0 keV,
does a reasonable job in matching the features seen in the H and K bands, but
fails to completely reproduce the morphology shortward of 1.6 microns. We find
that a two component model, where both components have similar properties, but
whose contributions differ with viewing geometry, provides an excellent fit to
the data. We discuss the implications of our models and compare them with
previously published results. In addition, we show that a cyclotron model with
similar properties to those used for modeling the infrared spectra, but with a
field strength of B = 115 MG, can explain the GALEX observations of EF Eri.Comment: 25 pages, 5 figures, to appear in Ap
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