10,361 research outputs found
Linear Estimating Equations for Exponential Families with Application to Gaussian Linear Concentration Models
In many families of distributions, maximum likelihood estimation is
intractable because the normalization constant for the density which enters
into the likelihood function is not easily available. The score matching
estimator of Hyv\"arinen (2005) provides an alternative where this
normalization constant is not required. The corresponding estimating equations
become linear for an exponential family. The score matching estimator is shown
to be consistent and asymptotically normally distributed for such models,
although not necessarily efficient. Gaussian linear concentration models are
examples of such families. For linear concentration models that are also linear
in the covariance we show that the score matching estimator is identical to the
maximum likelihood estimator, hence in such cases it is also efficient.
Gaussian graphical models and graphical models with symmetries form
particularly interesting subclasses of linear concentration models and we
investigate the potential use of the score matching estimator for this case
PEPSI deep spectra. I. The Sun-as-a-star
As part of the first Potsdam Echelle Polarimetric and Spectroscopic
Instrument (PEPSI) key-science project, we aim to provide well-exposed (viz.
deep) high-resolution spectra of representative stellar targets. These spectra
will be made available in form of (electronic) atlases. The first star in this
series of papers is our Sun. It also acts as a system-performance cornerstone.
The deep spectra in this paper are the results of combining up to 100
consecutive exposures per wavelength setting and are compared with other solar
flux atlases. Our software for the optimal data extraction and reduction of
PEPSI spectra is described and verified with the solar data. Three deep solar
flux spectra with a spectral resolution of up to 270,000, a continuous
wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio
(S/N) of between 2,000-8,000:1 depending on wavelength are presented.
Additionally, a time-series of 996 high-cadence spectra in one cross disperser
is used to search for intrinsic solar modulations. The wavelength calibration
based on Th-Ar exposures and simultaneous Fabry-Perot combs enables an absolute
wavelength solution within 10 m/s (rms) with respect to the HARPS laser-comb
solar atlas and a relative rms of 1.2 m/s for one day. For science
demonstration, we redetermined the disk-average solar Li abundance to
1.09+/-0.04 dex on the basis of 3D NLTE model atmospheres. We detected
disk-averaged p-mode RV oscillations with a full amplitude of 47 cm/s at 5.5
min. Comparisons with two solar FTS atlases, as well as with the HARPS solar
atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are
being taken with a sampling of one deep spectrum per day, and are supposed to
continue a full magnetic cycle of the Sun.Comment: in press, 13 pages, 8 figures, data available from pepsi.aip.d
3D Simulation of Convection and Spectral Line Formation in A-type Stars
We present first realistic numerical simulations of 3D radiative convection
in the surface layers of main sequence A-type stars with Teff = 8000 K and 8500
K, log g = 4.4 and 4.0, recently performed with the CO5BOLD radiation
hydrodynamics code. The resulting models are used to investigate the structure
of the H+HeI and the HeII convection zones in comparison with the predictions
of local and non-local convection theories, and to determine the amount of
"overshoot" into the stable layers below the HeII convection zone. The
simulations also predict how the topology of the photospheric granulation
pattern changes from solar to A-type star convection. The influence of the
photospheric temperature fluctuations and velocity fields on the shape of
spectral lines is demonstrated by computing synthetic line profiles and line
bisectors for some representative examples, allowing us to confront the 3D
model results with observations.Comment: 5 pages, 6 figures (17 figure files), 1 Tabl
Kinetic induced phase transition
An Ising model with local Glauber dynamics is studied under the influence of
additional kinetic restrictions for the spin-flip rates depending on the
orientation of neighboring spins. Even when the static interaction between the
spins is completely eliminated and only an external field is taken into account
the system offers a phase transition at a finite value of the applied field.
The transition is realized due to a competition between the activation
processes driven by the field and the dynamical rules for the spin-flips. The
result is based on a master equation approach in a quantum formulation.Comment: 13 page
Fingerprint Analysis with Marked Point Processes
We present a framework for fingerprint matching based on marked point process
models. An efficient Monte Carlo algorithm is developed to calculate the
marginal likelihood ratio for the hypothesis that two observed prints originate
from the same finger against the hypothesis that they originate from different
fingers. Our model achieves good performance on an NIST-FBI fingerprint
database of 258 matched fingerprint pairs
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