25,011 research outputs found
ASCA Observations of the Seyfert 2 Galaxy NGC 7582: An Obscured and Scattered View of the Hidden Nucleus
ASCA observations of the Seyfert 2 galaxy NGC 7582 revealed it was highly
variable on the timescale of s in the hard X-ray (2-10 keV)
band, while the soft X-ray (0.5-2 keV) flux remained constant during the
observations.
The spectral analysis suggests that this object is seen through an obscuring
torus with the thickness of N. The
hard X-ray is an absorbed direct continuum from a hidden Seyfert 1 nucleus; the
soft X-ray is dominated by the scattered central continuum from an extended
spatial region. Thus we have an obscured/absorbed and a scattered view of this
source as expected from the unification model for Seyfert galaxies.
More interestingly, the inferred X-ray column was observed to increase by
from 1994 to 1996, suggesting a ``patchy''
torus structure, namely the torus might be composed of many individual clouds.
The observed iron line feature near 6.4 keV with the equivalent width of 170 eV
is also consistent with the picture of the transmission of nuclear X-ray
continuum through a non-uniform torus.Comment: 10 pages, 6 figures. To be appear in PASJ 50 No.5 (1998 Oct.25 issue
Multi-Estimator Full Left Ventricle Quantification through Ensemble Learning
Cardiovascular disease accounts for 1 in every 4 deaths in United States.
Accurate estimation of structural and functional cardiac parameters is crucial
for both diagnosis and disease management. In this work, we develop an ensemble
learning framework for more accurate and robust left ventricle (LV)
quantification. The framework combines two 1st-level modules: direct estimation
module and a segmentation module. The direct estimation module utilizes
Convolutional Neural Network (CNN) to achieve end-to-end quantification. The
CNN is trained by taking 2D cardiac images as input and cardiac parameters as
output. The segmentation module utilizes a U-Net architecture for obtaining
pixel-wise prediction of the epicardium and endocardium of LV from the
background. The binary U-Net output is then analyzed by a separate CNN for
estimating the cardiac parameters. We then employ linear regression between the
1st-level predictor and ground truth to learn a 2nd-level predictor that
ensembles the results from 1st-level modules for the final estimation.
Preliminary results by testing the proposed framework on the LVQuan18 dataset
show superior performance of the ensemble learning model over the two base
modules.Comment: Jiasha Liu, Xiang Li and Hui Ren contribute equally to this wor
Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322
We report results from a systematic study of the spectral energy distribution
(SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The
jets of both sources have been directly imaged at both radio and X-ray
frequencies, which makes it possible to constrain the spectrum of the radiating
electrons in the jets. We modelled the observed SEDs of the jet `blobs' with
synchrotron emission alone and with synchrotron emission plus inverse Compton
scattering. The results favor a pure synchrotron origin of the observed jet
emission. Moreover, we found evidence that the shape of the electron spectral
distribution is similar for all jet `blobs' seen. Assuming that this is the
case for the jet as a whole, we then applied the synchrotron model to the radio
spectrum of the total emission and extrapolated the results to higher
frequencies. In spite of significant degeneracy in the fits, it seems clear
that, while the synchrotron radiation from the jets can account for nearly 100%
of the measured radio fluxes, it contributes little to the observed X-ray
emission, when the source is relatively bright. In this case, the X-ray
emission is most likely dominated by emission from the accretion flows. When
the source becomes fainter, however, the jet emission becomes more important,
even dominant, at X-ray energies. We also examined the spectral properties of
the sources during outbursts and the correlation between the observed radio and
X-ray variabilities. The implication of the results is discussed.Comment: 9 pages, 11 figures, MNRAS, accepted; the paper has been much
expanded (e.g., arguments strengthened, another source H 1743-322 added) and
rewritten (e.g., title changed, abstract revised); the main conclusions
remain unchange
Leptons from Dark Matter Annihilation in Milky Way Subhalos
Numerical simulations of dark matter collapse and structure formation show
that in addition to a large halo surrounding the baryonic component of our
galaxy, there also exists a significant number of subhalos that extend hundreds
of kiloparsecs beyond the edge of the observable Milky Way. We find that for
dark matter (DM) annihilation models, galactic subhalos can significantly
modify the spectrum of electrons and positrons as measured at our galactic
position. Using data from the recent Via Lactea II simulation we include the
subhalo contribution of electrons and positrons as boundary source terms for
simulations of high energy cosmic ray propagation with a modified version of
the publicly available GALPROP code. Focusing on the DM DM -> 4e annihilation
channel, we show that including subhalos leads to a better fit to both the
Fermi and PAMELA data. The best fit gives a dark matter particle mass of 1.2
TeV, for boost factors of 90 in the main halo and 1950-3800 in the subhalos
(depending on assumptions about the background), in contrast to the 0.85 TeV
mass that gives the best fit in the main halo-only scenario. These fits suggest
that at least a third of the observed electron cosmic rays from DM annihilation
could come from subhalos, opening up the possibility of a relaxation of recent
stringent constraints from inverse Compton gamma rays originating from the
high-energy leptons.Comment: 8 pages, 13 figures; added referenc
Boundary between the thermal and statistical polarization regimes in a nuclear spin ensemble
As the number of spins in an ensemble is reduced, the statistical uctuations
in its polarization eventually exceed the mean thermal polarization. This
transition has now been surpassed in a number of recent nuclear magnetic
resonance experiments, which achieve nanometer-scale detection volumes. Here,
we measure nanometer- scale ensembles of nuclear spins in a KPF6 sample using
magnetic resonance force microscopy. In particular, we investigate the
transition between regimes dominated by thermal and statistical nuclear
polarization. The ratio between the two types of polarization provides a
measure of the number of spins in the detected ensemble
Scaling analysis of Schottky barriers at metal-embedded semiconducting carbon nanotube interfaces
We present an atomistic self-consistent tight-binding study of the electronic
and transport properties of metal-semiconducting carbon nanotube interfaces as
a function of the nanotube channel length when the end of the nanotube wire is
buried inside the electrodes. We show that the lineup of the nanotube band
structure relative to the metal Fermi-level depends strongly on the metal work
function but weakly on the details of the interface. We analyze the
length-dependent transport characteristics, which predicts a transition from
tunneling to thermally-activated transport with increasing nanotube channel
length.Comment: To appear in Phys.Rev.B Rapid Communications. Color figures available
in PRB online versio
Nonclassical photon pairs generated from a room-temperature atomic ensemble
We report experimental generation of non-classically correlated photon pairs
from collective emission in a room-temperature atomic vapor cell. The
nonclassical feature of the emission is demonstrated by observing a violation
of the Cauchy-Schwarz inequality. Each pair of correlated photons are separated
by a controllable time delay up to 2 microseconds. This experiment demonstrates
an important step towards the realization of the Duan-Lukin-Cirac-Zoller scheme
for scalable long-distance quantum communication.Comment: 4 pages, 2 figure
Quantum walk on a line for a trapped ion
We show that a multi-step quantum walk can be realized for a single trapped
ion with interpolation between quantum and random walk achieved by randomizing
the generalized Hadamard coin flip phase. The signature of the quantum walk is
manifested not only in the ion's position but also its phonon number, which
makes an ion trap implementation of the quantum walk feasible.Comment: 5 pages, 3 figure
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