16,461 research outputs found
VHE gamma ray absorption by galactic interstellar radiation field
Adopting a recent calculation of the Galactic interstellar radiation field,
we calculate the attenuation of the very high energy gamma rays from the
Galactic sources. The infra-red radiation background near the Galactic Center
is very intense due to the new calculation and our result shows that a cutoff
of high energy gamma ray spectrum begins at about 20 TeV and reaches about 10%
for 50 TeV gamma rays.Comment: 6 pages, 1 figure, figure is changed, conclusion not change
Synthesis of Positron Emission Tomography (PET) Images via Multi-channel Generative Adversarial Networks (GANs)
Positron emission tomography (PET) image synthesis plays an important role,
which can be used to boost the training data for computer aided diagnosis
systems. However, existing image synthesis methods have problems in
synthesizing the low resolution PET images. To address these limitations, we
propose multi-channel generative adversarial networks (M-GAN) based PET image
synthesis method. Different to the existing methods which rely on using
low-level features, the proposed M-GAN is capable to represent the features in
a high-level of semantic based on the adversarial learning concept. In
addition, M-GAN enables to take the input from the annotation (label) to
synthesize the high uptake regions e.g., tumors and from the computed
tomography (CT) images to constrain the appearance consistency and output the
synthetic PET images directly. Our results on 50 lung cancer PET-CT studies
indicate that our method was much closer to the real PET images when compared
with the existing methods.Comment: 9 pages, 2 figure
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
Portal diversion in glycogen storage disease
Two children with glycogen storage disease were treated with portacaval transposition. The first is alive and in good health more than 5 years later. She underwent a rapid increase in growth after the operation, while the liver remained the same size. The second patient died within 2 days after the transposition, apparently because the portal system of the swollen liver was unable to transmit the vena caval inflow. © 1969
Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOx barriers
Perpendicular magnetic tunnel junctions with GdOX tunneling barriers have
shown a unique voltage controllable interlayer magnetic coupling effect. Here
we investigate the quality of the GdOX barrier and the coupling mechanism in
these junctions by examining the temperature dependence of the tunneling
magnetoresistance and the interlayer coupling from room temperature down to 11
K. The barrier is shown to be of good quality with the spin independent
conductance only contributing a small portion, 14%, to the total room
temperature conductance, similar to AlOX and MgO barriers. The interlayer
coupling, however, shows an anomalously strong temperature dependence including
sign changes below 80 K. This non-trivial temperature dependence is not
described by previous models of interlayer coupling and may be due to the large
induced magnetic moment of the Gd ions in the barrier.Comment: 14 pages, 4 figure
Ultraviolet Line Emission from Metals in the Low-Redshift Intergalactic Medium
We use a high-resolution cosmological simulation that includes hydrodynamics,
multiphase star formation, and galactic winds to predict the distribution of
metal line emission at z~0 from the intergalactic medium (IGM). We focus on two
ultraviolet doublet transitions, OVI 1032,1038 and CIV 1548,1551. Emission from
filaments with moderate overdensities is orders of magnitude smaller than the
background, but isolated emission from enriched, dense regions with
T~10^5-10^5.5 K and characteristic sizes of 50-100 kpc can be detected above
the background. We show that the emission from these regions is substantially
greater when we use the metallicities predicted by the simulation (which
includes enrichment through galactic winds) than when we assume a uniform IGM
metallicity. Luminous regions correspond to volumes that have recently been
influenced by galactic winds. We also show that the line emission is clustered
on scales ~1 h^-1 Mpc. We argue that although these transitions are not
effective tracers of the warm-hot intergalactic medium, they do provide a route
to study the chemical enrichment of the IGM and the physics of galactic winds.Comment: replaced by version to appear in ApJ (conclusions unchanged, one new
figure), 16 pages (emulateapj), 11 figures, version with higher resolution
figures available at
http://www.tapir.caltech.edu/~sfurlane/metals/coverpage.htm
Tailoring Cobalt-Free La0.5Sr0.5FeO3-δ Cathode with a Non-Metal Cation Doping Strategy for High Performance Proton-Conducting Solid Oxide Fuel Cells
A nonmetal doping strategy was exploited for the conventional La0.5Sr0.5FeO3-δ (LSF) cathode, allowing high performance for proton-conducting solid oxide fuel cells (H-SOFCs). Unlike previous studies focusing on the utilization of metal oxides as dopants, phosphorus, which is a nonmetal element, was used as the cation dopant for LSF by partially replacing Fe ions to form the new La0.5Sr0.5Fe0.9P0.1O3-δ (LSFP) compound. The H-SOFC using the LSFP cathode showed a two-fold peak power density as compared to that using the LSF cathode. Both experimental studies and first-principle calculations were used to unveil the mechanisms for the high performance of the LSFP cells
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Intermittent Features of the QSO Ly Transmitted Flux: Results from Hydrodynamic Cosmological Simulations
It has been recently found that the local fluctuations of the QSO's
Ly absorption spectrum transmitted flux show spiky structures. This
implies that the mass fields of the intergalactic medium (IGM) is intermittent.
This feature cannot be explained by the clustering evolution of cosmic mass
field in the linear regimes and is also difficult to incorporate into the
hierarchical clustering scenario. We calculate the structure functions and
intermittent exponent of the IGM and HI for full hydrodynamical simulation
samples. The result shows the intermittent features of the Ly
transmitted flux fluctuations as well as the mass field of the IGM. We find
that within the error bars of current data, all the intermittent behavior of
the simulation samples are consistent with the observation. This result is
different from our earlier result (Pando et al 2002), which shows that the
intermittent behavior of samples generated by pseudo-hydro simulation cannot be
fitted with observed data. One difference between the pseudo-hydro and full
hydro simulations is in treating the dynamical relation between the IGM (or HI)
and dark matter fields. The former assumes that the IGM density distribution
traces the underlying dark matter point-by-point on scales larger than the
Jeans length in either the linear or nonlinear regimes. However, hydrodynamic
studies have found that a statistical discrepancy between the IGM field and
underlying dark matter in nonlinear regime is possible. We find that the
point-by-point correlation between the IGM density perturbations and dark
matter become weaker on comoving scales less than 2 h Mpc (in LCDM
model), which is larger than the IGM Jeans length.Comment: AAS Latex file, 38 pages,17 figures included, accepted for
publication in Ap
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