471 research outputs found
Images archivées, images d’archives : fortunes terminologiques
On ne peut manquer d’être frappé de la quasi-totale absence dans l’institution archivistique française et dans les milieux professionnels qui l’animent, du moins jusqu’à une date récente, non seulement de l’expression « image d’archives » mais encore du mot même « image ». Cette constatation constitue le point de départ d’une enquête terminologique et lexicologique : d’où vient cette expression « images d’archives » ? Quelles sont les étapes et les raisons de sa diffusion
Comment on "On the importance of the free energy for elasticity under pressure"
Marcus et al. (Marcus P, Ma H and Qiu S L 2002 J. Phys.: Condens. Matter 14
L525) claim that thermodynamic properties of materials under pressure must be
computed using the Gibbs free energy , rather than the internal energy .
Marcus et al. state that ``The minima of , but not of , give the
equilibrium structure; the second derivatives of , but not of , with
respect to strains at the equilibrium structure give the equilibrium elastic
constants.'' Both statements are incorrect.Comment: Commen
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)
We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A
galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU
J132519.9430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4
keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July.
The inferred peak isotropic luminosity of the source therefore exceeded 3
10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous
X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second
bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December.
The source is detected and varies significantly within three additional
observations but is below the detection threshold in 7 observations. The X-ray
spectrum in 1999 December is best described as a cut-off power law or a
disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W
~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST
images taken 195~days before the nearest X-ray observation. The optical
brightness of this source is consistent with a late O or early B star at the
distance of Cen A. If the optical source is the counterpart, then the X-ray and
optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray
pulsar A 0538-66.Comment: 7 pages, 8 figures. ApJ (accepted
RXTE Observations of the Anomalous Pulsar 4U 0142+61
We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional
Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE) in March
1996. The pulse frequency was measured as f = 0.11510039(3) Hz with an upper
limit of df/dt < 4 * 10^(-13) Hz/s upon the short term change in frequency over
the 4.6 day span of the observations. A compilation of all historical
measurements showed an overall spin-down trend with slope df/dt = (-3.0 +/-
0.1) * 10^(-14) Hz/s. Searches for orbital modulations in pulse arrival times
yielded an upper limit of a_x sin i < 0.26 lt-s (99% confidence) for the period
range 70 s to 2.5 days. These limits combined with previous optical limits and
evolutionary arguments suggest that 4U 0142+61 is probably not a member of a
binary system.Comment: 20 pages (LaTeX) including 7 figures. Accepted for publication in the
Astrophysical Journa
Observations of Seyferts by OSSE and parameters of their X-ray/gamma-ray sources
We present a summary of spectra of Seyfert galaxies observed by the OSSE
detector aboard Compton Gamma Ray Observatory. We obtain average spectra of
Seyferts of type 1 and 2, and find they are well fitted by thermal
Comptonization. We present detailed parameter ranges for the plasma temperature
and the Compton parameter in the case of spherical and slab geometries. We find
both the average and individual OSSE spectra of Seyfert 2s are significantly
harder than those of Seyfert 1s, which difference can be due to anisotropy of
Compton reflection and/or Thomson-thick absorption.Comment: ApJ, 10 Nov. 2000, in press, 13 page
Absence of lattice strain anomalies at the electronic topological transition in zinc at high pressure
High pressure structural distortions of the hexagonal close packed (hcp)
element zinc have been a subject of controversy. Earlier experimental results
and theory showed a large anomaly in lattice strain with compression in zinc at
about 10 GPa which was explained theoretically by a change in Fermi surface
topology. Later hydrostatic experiments showed no such anomaly, resulting in a
discrepancy between theory and experiment. We have computed the compression and
lattice strain of hcp zinc over a wide range of compressions using the
linearized augmented plane wave (LAPW) method paying special attention to
k-point convergence. We find that the behavior of the lattice strain is
strongly dependent on k-point sampling, and with large k-point sets the
previously computed anomaly in lattice parameters under compression disappears,
in agreement with recent experiments.Comment: 9 pages, 6 figures, Phys. Rev. B (in press
Bayesian echo classification for Australian single-polarization weather radar with application to assimilation of radial velocity observations
The Australian Bureau of Meteorology’s operational weather radar network comprises a heterogeneous radar collection covering diverse geography and climate. A naïve Bayes classifier has been developed to identify a range of common echo types observed with these radars. The success of the classifier has been evaluated against its training dataset and by routine monitoring. The training data indicate that more than 90% of precipitation may be identified correctly. The echo types most difficult to distinguish from rainfall are smoke, chaff, and anomalous propagation ground and sea clutter. Their impact depends on their climatological frequency. Small quantities of frequently misclassified persistent echo (like permanent ground clutter or insects) can also cause quality control issues. The Bayes classifier is demonstrated to perform better than a simple threshold method, particularly for reducing misclassification of clutter as precipitation. However, the result depends on finding a balance between excluding precipitation and including erroneous echo. Unlike many single-polarization classifiers that are only intended to extract precipitation echo, the Bayes classifier also discriminates types of nonprecipitation echo. Therefore, the classifier provides the means to utilize clear air echo for applications like data assimilation, and the class information will permit separate data handling of different echo types
The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma
Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky
(20-1000 keV) between 1991 April and 2000 May (9.1y). Using the Earth
Occultation Technique to extract flux information, a catalog of sources using
data from the BATSE large area detectors has been prepared. The first part of
the catalog consists of results from the monitoring of 58 sources, mostly
Galactic. For these sources, we have included tables of flux and spectral data,
and outburst times for transients. Light curves (or flux histories) have been
placed on the world wide web. We then performed a deep-sampling of 179 objects
(including the aforementioned 58 objects) combining data from the entire 9.1y
BATSE dataset. Source types considered were primarily accreting binaries, but a
small number of representative active galaxies, X-ray-emitting stars, and
supernova remnants were also included. The deep sample results include definite
detections of 83 objects and possible detections of 36 additional objects. The
definite detections spanned three classes of sources: accreting black hole and
neutron star binaries, active galaxies and supernova remnants. Flux data for
the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and
160-430 keV. The limiting average flux level (9.1 y) for the sample varies from
3.5 to 20 mCrab (5 sigma) between 20 and 430 keV, depending on systematic
error, which in turn is primarily dependent on the sky location. To strengthen
the credibility of detection of weaker sources (5-25 mCrab), we generated Earth
occultation images, searched for periodic behavior using FFT and epoch folding
methods, and critically evaluated the energy-dependent emission in the four
flux bands.Comment: 64 pages, 17 figures, abstract abridged, Accepted by ApJ
Newly Developed and Validated Eosinophilic Esophagitis Histology Scoring System and Evidence that it Outperforms Peak Eosinophil Count for Disease Diagnosis and Monitoring
Eosinophilic esophagitis is diagnosed by symptoms, and at least 15 intraepithelial eosinophils per high power field in an esophageal biopsy. Other pathologic features have not been emphasized. We developed a histology scoring system for esophageal biopsies that evaluates eight features: eosinophil density, basal zone hyperplasia, eosinophil abscesses, eosinophil surface layering, dilated intercellular spaces, surface epithelial alteration, dyskeratotic epithelial cells and lamina propria fibrosis. Severity (grade) and extent (stage) of abnormalities were scored using a 4 point scale (0 normal; 3 maximum change). Reliability was demonstrated by strong to moderate agreement among 3 pathologists who scored biopsies independently (p≤0.008). Several features were often abnormal in 201 biopsies (101 distal, 100 proximal) from 104 subjects (34 untreated, 167 treated). Median grade and stage scores were significantly higher in untreated compared to treated subjects (p≤0.0062). Grade scores for features independent of eosinophil counts were significantly higher in biopsies from untreated compared to treated subjects (basal zone hyperplasia p≤0.024 and dilated intercellular spaces p≤0.005), and were strongly correlated (r-square\u3e0.67). Principal components analysis identified 3 principal components that explained 78.2% of the variation in the features. In logistic regression models, 2 principal components more closely associated with treatment status than log distal peak eosinophil count (r-square 17, area under the curve 77.8 vs r-square 9, area under the curve 69.8). In summary, the eosinophilic esophagitis histology scoring system provides a method to objectively assess histologic changes in the esophagus beyond eosinophil number. Importantly, it discriminates treated from untreated patients, uses features commonly found in such biopsies, and is utilizable by pathologists after minimal training. These data provide rationales and a method to evaluate esophageal biopsies for features in addition to peak eosinophil count
Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures
Photo-ionization is the accepted mechanism for the propagation of positive
streamers in air though the parameters are not very well known; the efficiency
of this mechanism largely depends on the presence of both nitrogen and oxygen.
But experiments show that streamer propagation is amazingly robust against
changes of the gas composition; even for pure nitrogen with impurity levels
below 1 ppm streamers propagate essentially with the same velocity as in air,
but their minimal diameter is smaller, and they branch more frequently.
Additionally, they move more in a zigzag fashion and sometimes exhibit a
feathery structure. In our simulations, we test the relative importance of
photo-ionization and of the background ionization from pulsed repetitive
discharges, in air as well as in nitrogen with 1 ppm O2 . We also test
reasonable parameter changes of the photo-ionization model. We find that photo-
ionization dominates streamer propagation in air for repetition frequencies of
at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition
frequency has to be included above 1 Hz. Finally, we explain the feather-like
structures around streamer channels that are observed in experiments in
nitrogen with high purity, but not in air.Comment: 12 figure
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