6,939 research outputs found
Evolution of Massive Haloes in non-Gaussian Scenarios
We have performed high-resolution cosmological N-body simulations of a
concordance LCDM model to study the evolution of virialized, dark matter haloes
in the presence of primordial non-Gaussianity. Following a standard procedure,
departures from Gaussianity are modeled through a quadratic Gaussian term in
the primordial gravitational potential, characterized by a dimensionless
non-linearity strength parameter f_NL. We find that the halo mass function and
its redshift evolution closely follow the analytic predictions of Matarrese et
al.(2000). The existence of precise analytic predictions makes the observation
of rare, massive objects at large redshift an even more attractive test to
detect primordial non-Gaussian features in the large scale structure of the
universe.Comment: 7 pages,3 figures, submitted to MNRA
GX 339-4: back to life
We report preliminary results of a RossiXTE campaign on the 2002 outburst of
the black-hole candidate GX 339-4. We show power density spectra of five
observations during the early phase of the outburst. The first four power
spectra show a smooth transition between a Low State and a Very High State. The
fifth power spectrum resembles a High State, but a strong 6 Hz QPO appears
suddenly within 16 seconds.Comment: 3 pages, 3 figures; to appear in Proceedings of the 4th Microquasar
Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the
Center for Space Physics: Kolkat
Precise mass-dependent QED contributions to leptonic g-2 at order alpha^2 and alpha^3
Improved values for the two- and three-loop mass-dependent QED contributions
to the anomalous magnetic moments of the electron, muon, and tau lepton are
presented. The Standard Model prediction for the electron (g-2) is compared
with its most precise recent measurement, providing a value of the
fine-structure constant in agreement with a recently published determination.
For the tau lepton, differences with previously published results are found and
discussed. An updated value of the fine-structure constant is presented in
"Note added after publication."Comment: 6 pages, 1 figure. v2: New determination of alpha presented (based on
the recent electron g-2 measurement). v3: New formulae added in Sec.IIB. v4:
Updated value of alpha presente
Carbon release by selective alloying of transition metal carbides
We have performed first principles density functional theory calculations on
TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to
Zn. The theory is accompanied with experimental investigations, both as regards
materials synthesis as well as characterization. Our results show that by
dissolving a metal with a weak ability to form carbides, the stability of the
alloy is lowered and a driving force for the release of carbon from the carbide
is created. During thin film growth of a metal carbide this effect will favor
the formation of a nanocomposite with carbide grains in a carbon matrix. The
choice of alloying elements as well as their concentrations will affect the
relative amount of carbon in the carbide and in the carbon matrix. This can be
used to design the structure of nanocomposites and their physical and chemical
properties. One example of applications is as low-friction coatings. Of the
materials studied, we suggest the late 3d transition metals as the most
promising elements for this phenomenon, at least when alloying with TiC.Comment: 9 pages, 6 figure
INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4
On 2004 August 15, we observed a fast (shorter than 10 hours) state
transition in the bright black-hole transient GX 339-4 simultaneously with
RossiXTE and INTEGRAL. This transition was evident both in timing and spectral
properties. Combining the data from PCA, HEXTE and IBIS, we obtained good
quality broad-band (3-200 keV) energy spectra before and after the transition.
These spectra indicate that the hard component steepened. Also, the high-energy
cutoff that was present at ~70 keV before the transition was not detected after
the transition. This is the first time that an accurate determination of the
broad-band spectrum across such a transition has been measured on a short time
scale. It shows that, although some spectral parameters do not change abruptly
through the transition, the high-energy cutoff increases/disappears rather
fast. These results constitute a benchmark on which to test theoretical models
for the production of the hard component in these systems.Comment: Accepted for publication in MNRAS (9 pages, 6 figures
Vortex pinning by a columnar defect in planar superconductors with point disorder
We study the effect of a single columnar pin on a dimensional array
of vortex lines in planar type II superconductors in the presence of point
disorder. In large samples, the pinning is most effective right at the
temperature of the vortex glass transition. In particular, there is a
pronounced maximum in the number of vortices which are prevented from tilting
by the columnar defect in a weak transverse magnetic field. Using
renormalization group techniques we show that the columnar pin is irrelevant at
long length scales both above and below the transition, but due to very
different mechanisms. This behavior differs from the disorder-free case, where
the pin is relevant in the low temperature phase. Solutions of the
renormalization equations in the different regimes allow a discussion of the
crossover between the pure and disordered cases. We also compute density
oscillations around the columnar pin and the response of these oscillations to
a weak transverse magnetic field.Comment: 12 pages, 5 figures, minor typos corrected, a new reference adde
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