22,291 research outputs found
The Galactic Central Diffuse X-ray Enhancement: A Differential Absorption/Emission Analysis
The soft X-ray background shows a general enhancement toward the inner region
of the Galaxy. But whether this enhancement is a local feature (e.g., a
superbubble within a distance of <= 200 pc) and/or a phenomenon related to
energetic outflows from the Galactic center/bulge remains unclear. Here we
report a comparative X-ray emission and absorption study of diffuse hot gas
along the sight lines toward 3C 273 and Mrk 421, on and off the enhancement,
but at similar Galactic latitudes. The diffuse 3/4-keV emission intensity, as
estimated from the ROSAT All Sky Survey, is about three times higher toward 3C
273 than toward Mrk 421. Based on archival \chandra grating observations of
these two AGNs, we detect X-ray absorption lines (e.g., OVII Kalpha, Kbeta, and
OVIII Kalpha transitions at z~0) and find that the mean hot gas thermal and
kinematic properties along the two sight lines are significantly different. By
subtracting the foreground and background contribution, as determined along the
Mrk 421 sight line, we isolate the net X-ray absorption and emission produced
by the hot gas associated with the enhancement in the direction of 3C 273. From
a joint analysis of these differential data sets, we obtain the temperature,
dispersion velocity, and hydrogen column density as 2.0(1.6, 2.3)E6 K,
216(104,480) km/s, and 2.2(1.4, 4.1)E19 cm^{-2}, respectively (90% confidence
intervals), assuming that the gas is approximately isothermal, solar in metal
abundances, and equilibrium in collisional ionization. We also constrain the
effective line-of-sight extent of the gas to be 3.4(1.0, 10.1) kpc, strongly
suggesting that the enhancement most likely represents a Galactic central
phenomenon.Comment: 5 pages, 4 figs, and 2 tables, accepted for publication in ApJ;
references updated; match to the version of proof
X-ray Absorption Spectroscopy of the Multi-phase Interstellar Medium: Oxygen and Neon Abundances
X-ray absorption spectroscopy provides a potentially powerful tool in
determining the metal abundances in various phases of the interstellar medium
(ISM). We present a case study of the sight line toward 4U 1820-303 (Galactic
coordinates l, b=2.79, -7.91 and distance = 7.6 kpc), based on Chandra Grating
observations. The detection of OI, OII, OIII, OVII, OVIII, and NeIX Kalpha
absorption lines allows us to measure the atomic column densities of the
neutral, warm ionized, and hot phases of the ISM through much of the Galactic
disk. By comparing these measurements with the 21 cm hydrogen emission and with
the pulsar dispersion measure along the same sight line, we estimate the mean
oxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and
2.2(1.1, 3.5) in units of Anders & Grevesse (1989) solar value. This
significant oxygen abundance difference is apparently a result of molecule/dust
grain destruction and recent metal enrichment in the warm ionized and hot
phases. We also measure the column density of neon from its absorption edge and
obtain the Ne/O ratio of the neutral plus warm ionized gas as 2.1(1.3, 3.5)
solar. Accounting for the expected oxygen contained in molecules and dust
grains would reduce the Ne/O ratio by a factor of ~1.5. From a joint-analysis
of the OVII, OVIII, and NeIX lines, we obtain the Ne/O abundance ratio of the
hot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exact
temperature distribution assumed in the absorption line modeling. These
comparable ISM Ne/O ratios for the hot and cooler gas are thus considerably
less than the value (2.85+-0.07; 1sigma) recently inferred from corona emission
of solar-like stars (Drake & Testa 2005). (abridged)Comment: 9 pages, 4 figures and 3 tables, accepted for publication in ApJ 200
Nonparametric Estimation and Symmetry Tests for Conditional Density Functions.
We suggest two new methods for conditional density estimation. The first is based on locally fitting a log-linear model, and is in the spirit of recent work on locally parametric techniques in density estimation. The second method is a constrained local polynomial estimator. Both methods always produce non-negative estimators. We propose an algorithm suitable for selecting the two bandwidths for either estimator. We also develop a new bootstrap test for the symmetry of conditional density functions. The proposed methods are illustrated by both simulation and application to a real data set.TESTING ; STATISTICAL ANALYSIS ; ESTIMATION OF PARAMETERS
Study on QoS support in 802.11e-based multi-hop vehicular wireless ad hoc networks
Multimedia communications over vehicular ad hoc networks (VANET) will play an important role in the future intelligent transport system (ITS). QoS support for VANET therefore becomes an essential problem. In this paper, we first study the QoS performance in multi-hop VANET by using the standard IEEE 802.11e EDCA MAC and our proposed triple-constraint QoS routing protocol, Delay-Reliability-Hop (DeReHQ). In particular, we evaluate the DeReHQ protocol together with EDCA in highway and urban areas. Simulation results show that end-to-end delay performance can sometimes be achieved when both 802.11e EDCA and DeReHQ extended AODV are used. However, further studies on cross-layer optimization for QoS support in multi-hop environment are required
Distributed Clustering in Cognitive Radio Ad Hoc Networks Using Soft-Constraint Affinity Propagation
Absence of network infrastructure and heterogeneous spectrum availability in cognitive radio ad hoc networks (CRAHNs) necessitate the self-organization of cognitive radio users (CRs) for efficient spectrum coordination. The cluster-based structure is known to be effective in both guaranteeing system performance and reducing communication overhead in variable network environment. In this paper, we propose a distributed clustering algorithm based on soft-constraint affinity propagation message passing model (DCSCAP). Without dependence on predefined common control channel (CCC), DCSCAP relies on the distributed message passing among CRs through their available channels, making the algorithm applicable for large scale networks. Different from original soft-constraint affinity propagation algorithm, the maximal iterations of message passing is controlled to a relatively small number to accommodate to the dynamic environment of CRAHNs. Based on the accumulated evidence for clustering from the message passing process, clusters are formed with the objective of grouping the CRs with similar spectrum availability into smaller number of clusters while guaranteeing at least one CCC in each cluster. Extensive simulation results demonstrate the preference of DCSCAP compared with existing algorithms in both efficiency and robustness of the clusters
Analyses of pion-nucleon elastic scattering amplitudes up to in extended-on-mass-shell subtraction scheme
We extend the analysis of elastic pion-nucleon scattering up to
level using extended-on-mass-shell subtraction scheme within the framework of
covariant baryon chiral perturbation theory. Numerical fits to partial wave
phase shift data up to GeV are performed to pin down the free
low energy constants. A good description to the existing phase shift data is
achieved. We find a good convergence for the chiral series at ,
considerably improved with respect to the -level analyses found in
previous literature. Also, the leading order contribution from explicit
resonance and partially-included loop
contribution are included to describe phase shift data up to
GeV. As phenomenological applications, we investigate chiral correction to the
Goldberger-Treiman relation % and find that it converges rapidly,
and the correction is found to be very small: . We also
get a reasonable prediction of pion-nucleon sigma term up to
by performing fits including both the pion-nucleon partial wave phase
shift data and the lattice QCD data. We report that MeV
from the fit without , and MeV from the
fit with explicit .Comment: The final version published in Phys.Rev. D 87, 054019 (2013
Chiral geometry of higher excited bands in triaxial nuclei with particle-hole configuration
The lowest six rotational bands have been studied in the particle-rotor model
with the particle-hole configuration
and different triaxiality parameter . Both constant and spin-dependent
variable moments of inertial (CMI and VMI) are introduced. The energy spectra,
electromagnetic transition probabilities, angular momentum components and
-distribution have been examined. It is shown that, besides the band 1 and
band 2, the predicted band 3 and band 4 in the calculations of both CMI and VMI
for atomic nuclei with could be interpreted as chiral doublet
bands.Comment: 4 pages, 4 figure
Orbital-transverse density-wave instabilities in iron-based superconductors
Besides the conventional spin-density-wave (SDW) state, a new kind of
orbital-transverse density-wave (OTDW) state is shown to exist generally in
multi-orbital systems. We demonstrate that the orbital character of Fermi
surface nesting plays an important role in density responses. The relationship
between antiferromagnetism and structural phase transition in LaFeAsO (1111)
and BaFeAs (122) compounds of iron-based superconductors may be
understood in terms of the interplay between the SDW and OTDW with a
five-orbital Hamiltonian. We propose that the essential difference between 1111
and 122 compounds is crucially determined by the presence of the
two-dimensional -like Fermi surface around (0,0) being only in 1111
parent compounds.Comment: several parts were rewritten for clarity. 6 pages, 3 figures, 1 tabl
10 to 50 nm Long Quasi Ballistic Carbon Nanotube Devices Obtained Without Complex Lithography
A simple method combining photolithography and shadow (or angle) evaporation
is developed to fabricate single-walled carbon nanotube (SWCNT) devices with
tube lengths L~10-50 nm between metal contacts. Large numbers of such short
devices are obtained without the need of complex tools such as electron beam
lithography. Metallic SWCNTs with lengths ~ 10 nm, near the mean free path
(mfp) of lop~15 nm for optical phonon scattering, exhibit near-ballistic
transport at high biases and can carry unprecedented 100 mA currents per tube.
Semiconducting SWCNT field-effect transistors (FETs) with ~ 50 nm channel
lengths are routinely produced to achieve quasi-ballistic operations for
molecular transistors. The results demonstrate highly length-scaled and
high-performance interconnects and transistors realized with SWCNTs.Comment: PNAS, in pres
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