72 research outputs found
Study of the factors affecting the karst volume assessment in the Dead Sea sinkhole problem using microgravity field analysis and 3-D modeling
Thousands of sinkholes have appeared in the Dead Sea (DS) coastal area in Israel and Jordan during two last decades. The sinkhole development is recently associated with the buried evaporation karst at the depth of 25â50 m from earth's surface caused by the drop of the DS level at the rate of 0.8â1.0 m/yr. Drop in the Dead Sea level has changed hydrogeological conditions in the subsurface and caused surface to collapse. The pre-existing cavern was detected using microgravity mapping in the Nahal Hever South site where seven sinkholes of 1â2 m diameter had been opened. About 5000 gravity stations were observed in the area of 200&times;200 m<sup>2</sup> by the use of Scintrex CG-3M AutoGrav gravimeter. Besides the conventional set of corrections applied in microgravity investigations, a correction for a strong gravity horizontal gradient (DS Transform Zone negative gravity anomaly influence) was inserted. As a result, residual gravity anomaly of â(0.08Ă·0.14) mGal was revealed. The gravity field analysis was supported by resistivity measurements. We applied the Emigma 7.8 gravity software to create the 3-D physical-geological models of the sinkholes development area. The modeling was confirmed by application of the <i>GSFC</i> program developed especially for 3-D combined gravity-magnetic modeling in complicated environments. Computed numerous gravity models verified an effective applicability of the microgravity technology for detection of karst cavities and estimation of their physical-geological parameters. A volume of the karst was approximately estimated as 35 000 m<sup>3</sup>. The visual analysis of large sinkhole clusters have been forming at the microgravity anomaly site, confirmed the results of microgravity mapping and 3-D modeling
Quasi-long range order in the random anisotropy Heisenberg model
The large distance behaviors of the random field and random anisotropy
Heisenberg models are studied with the functional renormalization group in
dimensions. The random anisotropy model is found to have a phase
with the infinite correlation radius at low temperatures and weak disorder. The
correlation function of the magnetization obeys a power law . The
magnetic susceptibility diverges at low fields as . In the random field model the correlation radius is found
to be finite at the arbitrarily weak disorder.Comment: 4 pages, REVTe
Nonthermal Emission from Star-Forming Galaxies
The detections of high-energy gamma-ray emission from the nearby starburst
galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of
star-driven nonthermal processes and phenomena in non-AGN star-forming
galaxies. We review basic aspects of the related processes and their modeling
in starburst galaxies. Since these processes involve both energetic electrons
and protons accelerated by SN shocks, their respective radiative yields can be
used to explore the SN-particle-radiation connection. Specifically, the
relation between SN activity, energetic particles, and their radiative yields,
is assessed through respective measures of the particle energy density in
several star-forming galaxies. The deduced energy densities range from O(0.1)
eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high
star-formation rates.Comment: 17 pages, 5 figures, to be published in Astrophysics and Space
Science Proceeding
Detection of diffuse TeV gamma-ray emission from the nearby starburst galaxy NGC 253
We report the TeV gamma-ray observations of the nearby normal spiral galaxy
NGC 253. At a distance of 2.5 Mpc, NGC 253 is one of the nearest
starburst galaxies. This relative closeness, coupled with the high star
formation rate in the galaxy, make it a good candidate TeV gamma-ray source.
Observations were carried out in 2000 and 2001 with the CANGAROO-II 10 m
imaging atmospheric Cerenkov telescope. TeV gamma-ray emission is detected at
the level with a flux of at energies 0.5 TeV. The data indicate that the
emission region is broader than the point spread function of our telescope.Comment: 4 pages, double colomn, 3 figures, aa.cl
Evidence of TeV gamma-ray emission from the nearby starburst galaxy NGC 253
TeV gamma-rays were recently detected from the nearby normal spiral galaxy
NGC 253 (Itoh et al., 2002). Observations to detect the Cherenkov light images
initiated by gamma-rays from the direction of NGC 253 were carried out in 2000
and 2001 over a total period of 150 hours. The orientation of images in
gamma-ray--like events is not consistent with emission from a point source, and
the emission region corresponds to a size greater than 10 kpc in radius. Here,
detailed descriptions of the analysis procedures and techniques are given.Comment: 16 pages, 27 figures, aa.cl
Weak reaction freeze-out constraints on primordial magnetic fields
We explore constraints on the strength of the primordial magnetic field based
upon the weak reaction freeze-out in the early universe. We find that limits on
the strength of the magnetic field found in other works are recovered simply by
examining the temperature at which the rate of weak reactions drops below the
rate of universal expansion ( H). The temperature for which the
ratio at freeze-out leads to acceptable helium production implies limits
on the magnetic field. This simplifies the application of magnetic fields to
other cosmological variants of the standard big-bang. As an illustration we
also consider effects of neutrino degeneracy on the allowed limits to the
primordial magnetic field.Comment: Submitted to Phys. Rev. D., 6 pages, 2 figure
Nonthermal radiation mechanisms
In this paper we review the possible radiation mechanisms for the observed
non-thermal emission in clusters of galaxies, with a primary focus on the radio
and hard X-ray emission. We show that the difficulty with the non-thermal,
non-relativistic Bremsstrahlung model for the hard X-ray emission, first
pointed out by Petrosian (2001) using a cold target approximation, is somewhat
alleviated when one treats the problem more exactly by including the fact that
the background plasma particle energies are on average a factor of 10 below the
energy of the non-thermal particles. This increases the lifetime of the
non-thermal particles, and as a result decreases the extreme energy
requirement, but at most by a factor of three. We then review the synchrotron
and so-called inverse Compton emission by relativistic electrons, which when
compared with observations can constrain the value of the magnetic field and
energy of relativistic electrons. This model requires a low value of the
magnetic field which is far from the equipartition value. We briefly review the
possibilities of gamma-ray emission and prospects for GLAST observations. We
also present a toy model of the non-thermal electron spectra that are produced
by the acceleration mechanisms discussed in an accompanying paper.Comment: 17 pages, 6 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 10; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations
We have performed accurate ab initio total energy calculations using the
full-potential linearized augmented plane wave (FP-LAPW) method with the
generalized gradient approximation (GGA) for the exchange-correlation potential
to systematically investigate elastic properties of 18 stable, meta-stable and
hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg,
Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline
MB2 the optimized lattice parameters, independent elastic constants (Cij), bulk
modules (B), shear modules (G) are obtained and analyzed in comparison with the
available theoretical and experimental data. For the first time numerical
estimates of a set of elastic parameters of the polycrystalline MB2 ceramics
(in the framework of the Voigt-Reuss-Hill approximation), namely bulk and shear
modules, compressibility, Young's modules, Poisson's ratio, Lame's coefficients
are performed.Comment: 24 pages, 3 figure
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