8,653 research outputs found
A Search for X-Ray Bright Distant Clusters of Galaxies
We present the results of a search for X--ray luminous distant clusters of
galaxies. We found extended X--ray emission characteristic of a cluster towards
two of our candidate clusters of galaxies. They both have a luminosity in the
ROSAT bandpass of and a redshift of ;
thus making them two of the most distant X--ray clusters ever observed.
Furthermore, we show that both clusters are optically rich and have a known
radio source associated with them. We compare our result with other recent
searches for distant X--ray luminous clusters and present a lower limit of
for the number density of such high redshift
clusters. This limit is consistent with the expected abundance of such clusters
in a standard (b=2) Cold Dark Matter Universe. Finally, our clusters provide
important high redshift targets for further study into the origin and evolution
of massive clusters of galaxies. Accepted for publication in the 10th September
1994 issue of ApJ.Comment: 20 pages Latex file + 1 postscript figure file appende
A Wavelet-Based Algorithm for the Spatial Analysis of Poisson Data
Wavelets are scaleable, oscillatory functions that deviate from zero only
within a limited spatial regime and have average value zero. In addition to
their use as source characterizers, wavelet functions are rapidly gaining
currency within the source detection field. Wavelet-based source detection
involves the correlation of scaled wavelet functions with binned,
two-dimensional image data. If the chosen wavelet function exhibits the
property of vanishing moments, significantly non-zero correlation coefficients
will be observed only where there are high-order variations in the data; e.g.,
they will be observed in the vicinity of sources.
In this paper, we describe the mission-independent, wavelet-based source
detection algorithm WAVDETECT, part of the CIAO software package. Aspects of
our algorithm include: (1) the computation of local, exposure-corrected
normalized (i.e. flat-fielded) background maps; (2) the correction for exposure
variations within the field-of-view; (3) its applicability within the
low-counts regime, as it does not require a minimum number of background counts
per pixel for the accurate computation of source detection thresholds; (4) the
generation of a source list in a manner that does not depend upon a detailed
knowledge of the point spread function (PSF) shape; and (5) error analysis.
These features make our algorithm considerably more general than previous
methods developed for the analysis of X-ray image data, especially in the low
count regime. We demonstrate the algorithm's robustness by applying it to
various images.Comment: Accepted for publication in Ap. J. Supp. (v. 138 Jan. 2002). 61
pages, 23 figures, expands to 3.8 Mb. Abstract abridged for astro-ph
submissio
Theoretical methods for the calculation of Bragg curves and 3D distributions of proton beams
The well-known Bragg-Kleeman rule RCSDA = A dot E0p has become a pioneer work
in radiation physics of charged particles and is still a useful tool to
estimate the range RCSDA of approximately monoenergetic protons with initial
energy E0 in a homogeneous medium. The rule is based on the
continuous-slowing-down-approximation (CSDA). It results from a generalized
(nonrelativistic) Langevin equation and a modification of the phenomenological
friction term. The complete integration of this equation provides information
about the residual energy E(z) and dE(z)/dz at each position z (0 <= z <=
RCSDA). A relativistic extension of the generalized Langevin equation yields
the formula RCSDA = A dot (E0 +E02/2M dot c2)p. The initial energy of
therapeutic protons satisfies E0 << 2M dot c2 (M dot c2 = 938.276 MeV), which
enables us to consider the relativistic contributions as correction terms.
Besides this phenomenological starting-point, a complete integration of the
Bethe-Bloch equation (BBE) is developed, which also provides the determination
of RCSDA, E(z) and dE(z)/dz and uses only those parameters given by the BBE
itself (i.e., without further empirical parameters like modification of
friction). The results obtained in the context of the aforementioned methods
are compared with Monte-Carlo calculations (GEANT4); this Monte-Carlo code is
also used with regard to further topics such as lateral scatter, nuclear
interactions, and buildup effects. In the framework of the CSDA, the energy
transfer from protons to environmental atomic electrons does not account for
local fluctuations.Comment: 97 pages review pape
An extension of the SHARC survey
We report on our search for distant clusters of galaxies based on optical and
X-ray follow up observations of X-ray candidates from the SHARC survey. Based
on the assumption that the absence of bright optical or radio counterparts to
possibly extended X-ray sources could be distant clusters. We have obtained
deep optical images and redshifts for several of these objects and analyzed
archive XMM-Newton or Chandra data where applicable. In our list of candidate
clusters, two are probably galaxy structures at redshifts of z0.51 and
0.28. Seven other structures are possibly galaxy clusters between z0.3
and 1. Three sources are identified with QSOs and are thus likely to be X-ray
point sources, and six more also probably fall in this category. One X-ray
source is spurious or variable. For 17 other sources, the data are too sparse
at this time to put forward any hypothesis on their nature. We also
serendipitously detected a cluster at z=0.53 and another galaxy concentration
which is probably a structure with a redshift in the [0.15-0.6] range. We
discuss these results within the context of future space missions to
demonstrate the necessity of a wide field of view telescope optimized for the
0.5-2 keV range.Comment: Accepted in A&
OSSE observations of galactic 511 keV annihilation radiation
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described
The Butcher-Oemler Effect at Moderate Redshift
We present the results of Butcher-Oemler-style analysis of three moderate-
redshift (0.1<z<0.2) clusters which have bimodal X-ray surface brightness
profiles. We find that at least two of these clusters exhibit unusually high
fractions of blue galaxies as compared to clusters at comparable redshifts
studied by Butcher and Oemler (1984). This implies that star formation is
occurring in a high fraction of the galaxies in the two clusters. Our results
are consistent with hierarchical clustering models in which subcluster-
subcluster mergers create shocks in the intracluster medium. The shocks, in
turn, induce simultaneous starbursts in a large fraction of cluster galaxies.
Our study therefore lends weight to the hypothesis that the Butcher-Oemler
effect is an environmental, as well as evolutionary, phenomenon.Comment: 22 pages, 8 figures; accepted for publication in A
Cl 1205+44, a fossil group at z = 0.59
This is a report of Chandra, XMM-Newton, HST and ARC observations of an
extended X-ray source at z = 0.59. The apparent member galaxies range from
spiral to elliptical and are all relatively red (i'-Ks about 3). We interpret
this object to be a fossil group based on the difference between the brightness
of the first and second brightest cluster members in the i'-band, and because
the rest-frame bolometric X-ray luminosity is about 9.2x10^43 h70^-2 erg s^-1.
This makes Cl 1205+44 the highest redshift fossil group yet reported. The
system also contains a central double-lobed radio galaxy which appears to be
growing via the accretion of smaller galaxies. We discuss the formation and
evolution of fossil groups in light of the high redshift of Cl 1205+44.Comment: 21 pages, 13 figures, minor corrections to match published ApJ
versio
Geochemistry in the modern soil survey program
Elemental analysis has played an important role in the characterization of soils since inception of the soil survey in the US. Recent efforts in analysis of trace and major elements (geochemistry) have provided necessary data to soil survey users in a variety of areas. The first part of this paper provides a brief overview of elemental sources, forms, mobility, and bioavailability; critical aspects important to users of soil survey geochemical data for appropriate use and interpretations. Examples are provided based on data gathered as part of the US soil survey program. The second part addresses the organization of sample collection in soil survey and how soil surveys are ideally suited as a sampling strategy for soil geochemical studies. Geochemistry is functional in characterization of soil types, determining soil processes, ecological evaluation, or issues related to soil quality and health, such as evaluating suitability of soils for urban or agricultural land use. Applications of geochemistry are on-going across the US and are documented herein. This analytical direction of soil survey complements historic efforts of the National Cooperative Soil Survey Program and addresses the increasing need of soil survey users for data that assists in understanding the influence of human activities on soil properties
Posthumanist Education
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