2,265 research outputs found
Test of Information Theory on the Boltzmann Equation
We examine information theory using the steady-state Boltzmann equation. In a
nonequilibrium steady-state system under steady heat conduction, the
thermodynamic quantities from information theory are calculated and compared
with those from the steady-state Boltzmann equation. We have found that
information theory is inconsistent with the steady-state Boltzmann equation.Comment: 12 page
Thermal Expansion and Magnetostriction Studies of a Kondo Lattice Compound: Ceagsb2
We have investigated a single crystal of CeAgSb2 using low field
ac-susceptibility, thermal expansion and magnetostriction measurements in the
temperature range 1.5K to 90K. The ac-susceptibility exhibits a sharp peak at
9.7K for both B//c and B perp c due to the magnetic ordering of the Ce moment.
The thermal expansion coefficient alpha, exhibits highly anisotropic behaviour
between 3K and 80K : alpha is positive for dL/L perp c, but negative for dL/L
// c. Furthermore, alpha (for dL/L) perp c (i.e. in ab-plane) exhibits a sharp
peak at TN followed by a broad maximum at 20K, while a sharp negative peak at
TN followed by a minimum at 20K has been observed for (dL/L //) the c
direction. The observed maximum and minimum in alpha(T) at 20K have been
attributed to the crystalline field effect on the J=5/2 state of the Ce3+ ion.
The magnetostriction also exhibits anisotropic behaviour with a large
magnetostriction along the c-axis. The ab-plane magnetostriction exhibits a
peak at B=3.3T at 3K, which is consistent with the observed peak in the
magnetoresistance measurements.Comment: 4 Pages (B5), 3 figures, submitted to SCES200
Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?
A weak-lensing analysis of wide-field - and -band images centered on
the cluster Abell 1942 has uncovered a mass concentration arcminutes
South of the cluster center. A statistical analysis shows that the detections
are highly significant. No strong concentration of bright galaxies is seen at
the position of the mass concentration, though a slight galaxy number
overdensity and a weak extended X-ray source are present about 1' away from its
center.
From the spatial dependence of the tangential alignment around the center of
the mass concentration, we inferred a lower bound on the mass inside a sphere
of radius \ts Mpc of , much higher
than crude mass estimates based on X-ray data. No firm conclusion can be
inferred about the nature of the clump. If it were a high-redshift cluster, the
weak X-ray flux would indicate that it had an untypically low X-ray luminosity
for its mass; if the X-ray emission were physically unrelated to the mass
concentration, this conclusion would be even stronger.
The search for massive halos by weak lensing enables us for the first time to
select halos based on their mass properties only and to detect new types of
objects, e.g., dark halos. The mass concentration in the field of A1942 may be
the first example of such a halo.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar
file. Figures with images not included, but available on ftp.iap.fr
/pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb
Oblique Convergence in the Himalayas of Western Nepal Deduced from Preliminary Results of GPS Measurements
A GPS network consisting of 29 sites was installed in central and western Nepal, with measurements taken in 1995 and partial remeasurements in 1997. Data suggest 15 +/â5 mm/yr of N180° convergence between the Higher Himalayas and India, a result that is consistent with NâS shortening across the arcuate shape of the Nepalese Himalayas and an oblique underthrusting of the Indian crust below the High Himalayas of western Nepal. A 4 +/â3 mm/year EâW extension and deviation of the principal shortening axes are inferred east of 83°E, where Quaternary faults (DarmaâBari Gad fault system and Thakkhola graben) delineate a crustal wedge. This wedge is located on the SE projection of the Karakorum fault and may segment the Himalayan thrust belt. The convergence between the outer belt of western Nepal and India is less than 3 mm/yr, an attenuation consistent with creep on a dislocation locked beneath the Lesser Himalayas. A preliminary model suggests that this N 120°E striking dislocation is affected by a 19 mm/yr thrust component and a 7 mm/yr right lateral component
A Bose-Einstein condensate in a random potential
An optical speckle potential is used to investigate the static and dynamic
properties of a Bose-Einstein condensate in the presence of disorder. For
strong disorder the condensate is localized in the deep wells of the potential.
With smaller levels of disorder, stripes are observed in the expanded density
profile and strong damping of dipole and quadrupole oscillations is seen.
Uncorrelated frequency shifts of the two modes are measured for a weak disorder
and are explained using a sum-rules approach and by the numerical solution of
the Gross-Pitaevskii equation
Probing the Slope of Cluster Mass Profile with Gravitational Einstein Rings: Application to Abell 1689
The strong lensing modelling of gravitational ``rings'' formed around massive
galaxies is sensitive to the amplitude of the external shear and convergence
produced by nearby mass condensations. In current wide field surveys, it is now
possible to find out a large number of rings, typically 10 gravitational rings
per square degree. We propose here, to systematically study gravitational rings
around galaxy clusters to probe the cluster mass profile beyond the cluster
strong lensing regions. For cluster of galaxies with multiple arc systems, we
show that rings found at various distances from the cluster centre can improve
the modelling by constraining the slope of the cluster mass profile. We outline
the principle of the method with simple numerical simulations and we apply it
to 3 rings discovered recently in Abell~1689. In particular, the lens modelling
of the 3 rings confirms that the cluster is bimodal, and favours a slope of the
mass profile steeper than isothermal at a cluster radius \sim 300 \kpc. These
results are compared with previous lens modelling of Abell~1689 including weak
lensing analysis. Because of the difficulty arising from the complex mass
distribution in Abell~1689, we argue that the ring method will be better
implemented on simpler and relaxed clusters.Comment: Accepted for publication in MNRAS. Substantial modification after
referee's repor
Detection of correlated galaxy ellipticities on CFHT data: first evidence for gravitational lensing by large-scale structures
We report the detection of a significant (5.5 sigma) excess of correlations
between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes.
This detection of a gravitational lensing signal by large-scale structure was
made using a composite high quality imaging survey of 6300 arcmin^2 obtained at
the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic
CCD cameras. The amplitude of the excess correlation is 2.2\pm 0.2 % at 1
arcmin scale, in agreement with theoretical predictions of the lensing effect
induced by large-scale structure.We provide a quantitative analysis of
systematics which could contribute to the signal and show that the net effect
is small and can be corrected for. We show that the measured ellipticity
correlations behave as expected for a gravitational shear signal. The
relatively small size of our survey precludes tight constraints on cosmological
models. However the data are in favor of cluster normalized cosmological
models, and marginally reject Cold Dark Matter models with (Omega=0.3,
sigma_8<0.6) or (Omega=1, sigma_8=1). The detection of cosmic shear
demonstrates the technical feasibility of using weak lensing surveys to measure
dark matter clustering and the potential for cosmological parameter
measurements, in particular with upcoming wide field CCD cameras.Comment: 19 pages. 19 Figures. Revised version accepted in A&
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