76,421 research outputs found
Signatures of Emerging Subsurface Structures in Acoustic Power Maps
We show that under certain conditions, subsurface structures in the solar
interior can alter the average acoustic power observed at the photosphere above
them. By using numerical simulations of wave propagation, we show that this
effect is large enough for it to be potentially used for detecting emerging
active regions before they appear on the surface. In our simulations,
simplified subsurface structures are modeled as regions with enhanced or
reduced acoustic wave speed. We investigate the dependence of the acoustic
power above a subsurface region on the sign, depth, and strength of the wave
speed perturbation. Observations from the Solar and Heliospheric
Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the emergence
of NOAA active region 10488 are used to test the use of acoustic power as a
potential precursor of magnetic flux emergence.Comment: 7 pages, 5 figures, accepted for publication in Solar Physics on 21
March 201
Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3
Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.Comment: 8 pages, 6 figure
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
Gravitational Lens Statistics for Generalized NFW Profiles: Parameter Degeneracy and Implications for Self-Interacting Cold Dark Matter
Strong lensing is a powerful probe of the distribution of matter in the cores
of clusters of galaxies. Recent studies suggest that the cold dark matter model
predicts cores that are denser than those observed in galaxies, groups and
clusters. One possible resolution of the discrepancy is that the dark matter
has strong interactions (SIDM), which leads to lower central densities. A
generalized form of the Navarro, Frenk and White profile (Zhao profile) may be
used to describe these halos. In this paper we examine gravitational lensing
statistics for this class of model. The optical depth to multiple imaging is a
very sensitive function of the profile parameters in the range of interest for
SIDM halos around clusters of galaxies. Less concentrated profiles, which
result from larger self-interaction cross-sections, can produce many fewer
lensed pairs. Lensing statistics provide a powerful test for SIDM. More
realistic and observationally oriented calculations remain to be done, however
larger self-interaction cross-sections may well be ruled out by the very
existence of strong lenses on galaxy cluster scales. The inclusion of centrally
dominant cluster galaxies should boost the cross-section to multiple imaging.
However our preliminary calculations suggest that the additional multiple
imaging rate is small with respect to the differences in multiple imaging rate
for different halo profiles. In future statistical studies, it will be
important to properly account for the scatter among halo profiles since the
optical depth to multiple imaging is dominated by the most concentrated members
of a cluster population.Comment: 58 pages, 14 figures. To be published in ApJ. Revised version
includes discussion of magnification bias and the effect of a centrally
dominant galax
Simple algebras of Weyl type
Over a field of any characteristic, for a commutative associative algebra
with an identity element and for the polynomial algebra of a
commutative derivation subalgebra of , the associative and the Lie
algebras of Weyl type on the same vector space are
defined. It is proved that , as a Lie algebra (modular its center) or as
an associative algebra, is simple if and only if is -simple and
acts faithfully on . Thus a lot of simple algebras are obtained.Comment: 9 pages, Late
- …