947,361 research outputs found
Unfolding spectral analysis of the Fermi-LAT data
The Large Area Telescope (LAT) onboard the Fermi satellite is observing the
gamma-ray sky in the high energy region, above 20 MeV. We have developed a
method to reconstruct the energy spectra of the gamma-rays detected by the
Fermi LAT instrument based on a Bayesian unfolding approach, that takes into
account the energy dispersion introduced by the instrument response. The method
has been successfully applied to reconstruct the energy spectra of both steady
and pulsating point sources. The analysis technique will be illustrated and the
results obtained in some significant test cases will be discussed.Comment: 6 pages, 9 figure
Comprehensive Spectral Analysis of Cyg X-1 using RXTE Data
We analyse a large number () pointed RXTE observations of Cyg X-1 and
model the spectrum of each one. A subset of the observations for which there is
simultaneous reliable measure of the hardness ratio by the All Sky Monitor,
shows that the sample covers nearly all the spectral shapes of Cyg X-1. The
relative strength, width of the Iron line and the reflection parameter are in
general correlated with the high energy photon spectral index . This is
broadly consistent with a geometry where for the hard state (low ) there is a hot inner Comptonizing region surrounded by a truncated cold
disk. The inner edge of the disk moves inwards as the source becomes softer
till finally in the soft state (high ) the disk fills the inner
region and active regions above the disk produce the Comptonized component.
However, the reflection parameter shows non-monotonic behaviour near the
transition region (), suggestive of a more complex geometry or
physical state of the reflector. Additionally, the inner disk temperature,
during the hard state, is on the average higher than in the soft one, albeit
with large scatter. These inconsistencies could be due to limitations in the
data and the empirical model used to fit them. The flux of each spectral
component is well correlated with which shows that unlike some other
black hole systems, Cyg X-1 does not show any hysteresis behaviour. In the soft
state, the flux of the Comptonized component is always similar to the disk one,
which confirms that the ultra-soft state (seen in other brighter black hole
systems) is not exhibited by Cyg X-1. The rapid variation of the Compton
Amplification factor with , naturally explains the absence of spectra
with , despite a large number having .Comment: 12 pages, 8 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
Midinfrared spectral investigations of carbonates: Analysis of remotely sensed data
Recent airborne thermal infrared observations of Mars from the Kuiper Airborne Observatory (KAO) have provided evidence for the presence of carbonates, sulfates, and hydrates. Using the optical properties of calcite and anhydrite, it was estimated that CO3's and SO4's constituted about 1 to 3 and 10 to 15 wt. percent, repectively of the materials composing the atmospheric dust. Using the derived value as an estimate of total CO3 abundance, and making an assumption that the CO3's were uniformly distributed within the Martian regolith, it was estimated that such a CO3 reservoir could contain roughly 2 to 5 bars of CO2. While the results indicate that several volatile-bearing materials are present on Mars, the observations from the KAO are inherently limited in their ability to determine the spatial distributions of these materials. However, previous spacecraft observations of Mars provide both the spectral coverage necessary to identify these materials, as well as the potential for investigating their spatial variability. This has prompted us to pursue a reinvestigation of the Mariner 6 and 7 infrared spectrometer and Mariner 9 infrared interferometer spectrometer observations. The former data have been recently made available in digital format and calibration of wavelengths and intensities are almost complete. Additionally, we are pursuing the derivation of optical constants of more appropriate carbonates and sulfates
Making Laplacians commute
In this paper, we construct multimodal spectral geometry by finding a pair of
closest commuting operators (CCO) to a given pair of Laplacians. The CCOs are
jointly diagonalizable and hence have the same eigenbasis. Our construction
naturally extends classical data analysis tools based on spectral geometry,
such as diffusion maps and spectral clustering. We provide several synthetic
and real examples of applications in dimensionality reduction, shape analysis,
and clustering, demonstrating that our method better captures the inherent
structure of multi-modal data
The spectral analysis of nonstationary categorical time series using local spectral envelope
Most classical methods for the spectral analysis are based on the assumption that the time
series is stationary. However, many time series in practical problems shows nonstationary
behaviors. The data from some fields are huge and have variance and spectrum which changes
over time. Sometimes,we are interested in the cyclic behavior of the categorical-valued time
series such as EEG sleep state data or DNA sequence, the general method is to scale the
data, that is, assign numerical values to the categories and then use the periodogram to find
the cyclic behavior. But there exists numerous possible scaling. If we arbitrarily assign the
numerical values to the categories and proceed with a spectral analysis, then the results will
depend on the particular assignment. We would like to find the all possible scaling that
bring out all of the interesting features in the data. To overcome these problems, there have
been many approaches in the spectral analysis.
Our goal is to develop a statistical methodology for analyzing nonstationary categorical
time series in the frequency domain. In this dissertation, the spectral envelope methodology
is introduced for spectral analysis of categorical time series. This provides the general
framework for the spectral analysis of the categorical time series and summarizes information
from the spectrum matrix. To apply this method to nonstationary process, I used the
TBAS(Tree-Based Adaptive Segmentation) and local spectral envelope based on the piecewise
stationary process. In this dissertation,the TBAS(Tree-Based Adpative Segmentation)
using distance function based on the Kullback-Leibler divergence was proposed to find the
best segmentation
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