408,013 research outputs found
Spectral Cross-calibration of the Konus-Wind, the Suzaku/WAM, and the Swift/BAT Data using Gamma-Ray Bursts
We report on the spectral cross-calibration results of the Konus-Wind, the
Suzaku/WAM, and the Swift/BAT instruments using simultaneously observed
gamma-ray bursts (GRBs). This is the first attempt to use simultaneously
observed GRBs as a spectral calibration source to understand systematic
problems among the instruments. Based on these joint spectral fits, we find
that 1) although a constant factor (a normalization factor) agrees within 20%
among the instruments, the BAT constant factor shows a systematically smaller
value by 10-20% compared to that of Konus-Wind, 2) there is a systematic trend
that the low-energy photon index becomes steeper by 0.1-0.2 and Epeak becomes
systematically higher by 10-20% when including the BAT data in the joint fits,
and 3) the high-energy photon index agrees within 0.2 among the instruments.
Our results show that cross-calibration based on joint spectral analysis is an
important step to understanding the instrumental effects which could be
affecting the scientific results from the GRB prompt emission data.Comment: 82 pages, 88 figures, accepted for publication in PAS
Using RIXS to uncover elementary charge and spin excitations in correlated materials
Despite significant progress in resonant inelastic x-ray scattering (RIXS)
experiments on cuprates at the Cu L-edge, a theoretical understanding of the
cross-section remains incomplete in terms of elementary excitations and the
connection to both charge and spin structure factors. Here we use
state-of-the-art, unbiased numerical calculations to study the low energy
excitations probed by RIXS in undoped and doped Hubbard model relevant to the
cuprates. The results highlight the importance of scattering geometry, in
particular both the incident and scattered x-ray photon polarization, and
demonstrate that on a qualitative level the RIXS spectral shape in the
cross-polarized channel approximates that of the spin dynamical structure
factor. However, in the parallel-polarized channel the complexity of the RIXS
process beyond a simple two-particle response complicates the analysis, and
demonstrates that approximations and expansions which attempt to relate RIXS to
less complex correlation functions can not reproduce the full diversity of RIXS
spectral features
Cross-spectral purity of the Stokes parameters in random nonstationary electromagnetic beams
We consider cross-spectral purity in random nonstationary electromagnetic
beams in terms of the Stokes parameters representing the spectral density and
the spectral polarization state. We show that a Stokes parameter being
cross-spectrally pure is consistent with the property that the corresponding
normalized time-integrated coherence (two-point) Stokes parameter satisfies a
certain reduction formula. The current analysis differs from the previous works
on cross-spectral purity of nonstationary light beams such that the purity
condition is in line with Mandel's original definition. In addition, in
contrast to earlier works concerning the cross-spectral purity of the
polarization-state Stokes parameters, intensity-normalized coherence Stokes
parameters are applied. It is consequently found that in addition to separate
spatial and temporal coherence factors the reduction formula contains a third
factor that depends exclusively on the polarization properties. We further show
that cross-spectral purity implies a specific structure for the electromagnetic
spectral spatial correlations. The results of this work constitute foundational
advances in the interference of random nonstationary vectorial light.Comment: 5 pages, 1 figur
Final-state interactions in semi-inclusive deep inelastic scattering off the Deuteron
Semi-inclusive deep inelastic scattering off the Deuteron with production of
a slow nucleon in recoil kinematics is studied in the virtual nucleon
approximation, in which the final state interaction (FSI) is calculated within
general eikonal approximation. The cross section is derived in a factorized
approach, with a factor describing the virtual photon interaction with the
off-shell nucleon and a distorted spectral function accounting for the
final-state interactions. One of the main goals of the study is to understand
how much the general features of the diffractive high energy soft rescattering
accounts for the observed features of FSI in deep inelastic scattering(DIS).
Comparison with the Jefferson Lab data shows good agreement in the covered
range of kinematics. Most importantly, our calculation correctly reproduces the
rise of the FSI in the forward direction of the slow nucleon production angle.
By fitting our calculation to the data we extracted the and
dependences of the total cross section and slope factor of the interaction of
DIS products, , off the spectator nucleon. This analysis shows the
scattering cross section rising with and decreasing with an increase of
. Finally, our analysis points at a largely suppressed off-shell part of
the rescattering amplitude.Comment: 27 pages, 8 figures. Corrected typos, section II.E has been expanded
a bit. Figures have been updated to conform to the publication guidelines.
Results and conclusions haven't changed. Accepted for publication in PR
Fitting dynamic factor models to non-stationary time series
Factor modelling of a large time series panel has widely proven useful to reduce its cross-sectional dimensionality. This is done by explaining common co-movements in the panel through the existence of a small number of common components, up to some idiosyncratic behaviour of each individual series. To capture serial correlation in the common components, a dynamic structure is used as in traditional (uni- or multivariate) time series analysis of second order structure, i.e. allowing for infinite-length filtering of the factors via dynamic loadings. In this paper, motivated from economic data observed over long time periods which show smooth transitions over time in their covariance structure, we allow the dynamic structure of the factor model to be non-stationary over time, by proposing a deterministic time variation of its loadings. In this respect we generalise existing recent work on static factor models with time-varying loadings as well as the classical, i.e. stationary, dynamic approximate factor model. Motivated from the stationary case, we estimate the common components of our dynamic factor model by the eigenvectors of a consistent estimator of the now time-varying spectral density matrix of the underlying data-generating process. This can be seen as time-varying principal components approach in the frequency domain. We derive consistency of this estimator in a "double-asymptotic" framework of both cross-section and time dimension tending to infinity. A simulation study illustrates the performance of our estimators.econometrics;
Stability Analysis of Picard Iteration for Coupled Neutronics/Thermal-Hydraulics Simulations
In this paper, we present a formal Fourier analysis (FA) of Picard iteration
for the coupled neutronics/thermal hydraulics (N/TH) problem and derive
theoretical predictions for the spectral radius of Picard iteration for such
coupled calculations as a function of the temperature difference between the
fuel and coolant, temperature coefficients of cross sections (i.e., Doppler
feedback), scattering ratio, and core height. An optimal underrelaxation factor
is also derived based on the Fourier analysis
Extended Cross-Referenced Analysis Using Data from the Landsat 8 And 9 Underfly Event
The Landsat 8 and 9 Underfly Event occurred in November 2021, where Landsat 9 flew beneath Landsat 8 in the final stages before settling in its final orbiting path. An analysis was performed on the images taken during this event, which resulted in a cross-referenced with uncertainties estimated to be less than 0.5%. This level of precision was due in part to the near-identical sensors aboard each instrument as well as the underfly event itself, which allowed the sensors to take nearly the exact same image at nearly the exact same time. This initial calibration was applied before the end of the on-orbit initial verification (OIV) period, which meant the analysis was performed in less than a month. While it was an effective and efficient first look at the data, a longer-term analysis was deemed prudent to have the most accurate cross-referenced with the smallest uncertainties. The three forms of uncertainty established in the initial analysis, dubbed “Phase 1”, were geometric, spectral, and angular. This paper covers Phase 2 of the underfly analysis, and several modifications were made to the Phase 1 process to improve the cross-referenced results, including a spectral correction in the form of a spectral band adjustment factor (SBAF) and a more robust filtering system that used the statistics of the reflectance data to better include important data compared to the more aggressive filters used in Phase 1. A proper uncertainty analysis was performed to more accurately quantify the uncertainty associated with the underfly cross-referenced. The final results of Phase 2 showed that the Phase 1 analysis was within its 0.5% uncertainty estimation, and the cross-referenced gain values in this paper were used by USGS EROS to update the Landsat 9 calibration at the end of 2022
Prospects for Annihilating Dark Matter in the inner Galactic halo by the Cherenkov Telescope Array
We compute the sensitivity to dark matter annihilations for the forthcoming
large Cherenkov Telescope Array (CTA) in several primary channels and over a
range of dark matter masses from 30 GeV up to 80 TeV. For all channels, we
include inverse Compton scattering of e by dark matter annihilations on
the ambient photon background, which yields substantial contributions to the
overall gamma-ray flux. We improve the analysis over previous work by: i)
implementing a spectral and morphological analysis of the gamma-ray emission;
ii) taking into account the most up-to-date cosmic ray background obtained from
a full CTA Monte Carlo simulation and a description of the diffuse
astrophysical emission; and iii) including the systematic uncertainties in the
rich observational CTA datasets. We find that our spectral and morphological
analysis improves the CTA sensitivity by roughly a factor 2. For the hadronic
channels, CTA will be able to probe thermal dark matter candidates over a broad
range of masses if the systematic uncertainties in the datasets will be
controlled better than the percent level. For the leptonic modes, the CTA
sensitivity will be well below the thermal value of the annihilation
cross-section. In this case, even with larger systematics, thermal dark matter
candidates up to masses of a few TeV will be easily studied.Comment: 15 pages, 4 figures, v2: Jfactors for two different DM profiles in
Tab.1 added; two new plots added; some clarifications and some references
added; results unchanged; matches version published on Phys. Rev.
A determination of the Spectra of Galactic components observed by WMAP
WMAP data when combined with ancillary data on free-free, synchrotron and
dust allow an improved understanding of the spectrum of emission from each of
these components. Here we examine the sky variation at intermediate latitudes
using a cross-correlation technique. In particular, we compare the observed
emission in 15 selected sky regions to three ``standard'' templates.
The free-free emission of the diffuse ionised gas is fitted by a well-known
spectrum at K and Ka band, but the derived emissivity corresponds to a mean
electron temperature of ~4000-5000K. This is inconsistent with estimates from
galactic HII regions. The origin of the discrepancy is unclear.
The anomalous emission associated with dust is clearly detected in most of
the 15 fields studied; it correlates well with the Finkbeiner et al. model 8
predictions (FDS8) at 94 GHz, with an effective spectral index between 20 and
60GHz of -2.85. Furthermore, the emissivity varies by a factor of ~2 from cloud
to cloud. A modestly improved fit to the anomalous dust at K-band is provided
by modulating the template by an estimate of the dust colour temperature,
specifically FDS8*T^n. We find a preferred value n~1.6.
The synchrotron emission steepens between GHz frequencies and the WMAP bands.
There are indications of spectral index variations across the sky but the
current data are not precise enough to accurately quantify this from region to
region. Our analysis of the WMAP data indicates strongly that the
dust-correlated emission at the low WMAP frequencies has a spectrum which is
compatible with spinning dust; we find no evidence for a synchrotron component
correlated with dust (abridged).Comment: 18 pages, 6 figures, revised version uses cross-correlation method
rather than T-T method. Paper re-organised and sent back to refere
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