440 research outputs found
Suppression of Density Fluctuations in a Quantum Degenerate Fermi Gas
We study density profiles of an ideal Fermi gas and observe Pauli suppression
of density fluctuations (atom shot noise) for cold clouds deep in the quantum
degenerate regime. Strong suppression is observed for probe volumes containing
more than 10,000 atoms. Measuring the level of suppression provides sensitive
thermometry at low temperatures. After this method of sensitive noise
measurements has been validated with an ideal Fermi gas, it can now be applied
to characterize phase transitions in strongly correlated many-body systems.Comment: minor edit: fixed technical problem with arxiv's processing of .eps
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The influence of lexical selection disruptions on articulation
Interactive models of language production predict that it should be possible to observe long-distance interactions; effects that arise at one level of processing influence multiple subsequent stages of representation and processing. We examine the hypothesis that disruptions arising in nonform-based levels of planning—specifically, lexical selection—should modulate articulatory processing. A novel automatic phonetic analysis method was used to examine productions in a paradigm yielding both general disruptions to formulation processes and, more specifically, overt errors during lexical selection. This analysis method allowed us to examine articulatory disruptions at multiple levels of analysis, from whole words to individual segments. Baseline performance by young adults was contrasted with young speakers’ performance under time pressure (which previous work has argued increases interaction between planning and articulation) and performance by older adults (who may have difficulties inhibiting nontarget representations, leading to heightened interactive effects). The results revealed the presence of interactive effects. Our new analysis techniques revealed these effects were strongest in initial portions of responses, suggesting that speech is initiated as soon as the first segment has been planned. Interactive effects did not increase under response pressure, suggesting interaction between planning and articulation is relatively fixed. Unexpectedly, lexical selection disruptions appeared to yield some degree of facilitation in articulatory processing (possibly reflecting semantic facilitation of target retrieval) and older adults showed weaker, not stronger interactive effects (possibly reflecting weakened connections between lexical and form-level representations)
Deep Chandra observation and numerical studies of the nearest cluster cold front in the sky
We present the results of a very deep (500 ks) Chandra observation, along with tailored numerical simulations, of the nearest, best resolved cluster cold front in the sky, which lies 90 kpc (19 arcmin) to the north-west of M 87. The northern part of the front appears the sharpest, with a width smaller than 2.5 kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along the front, the temperature discontinuity is narrower than 4–8 kpc and the metallicity gradient is narrower than 6 kpc, indicating that diffusion, conduction and mixing are suppressed across the interface. Such transport processes can be naturally suppressed by magnetic fields aligned with the cold front. Interestingly, comparison to magnetohydrodynamic simulations indicates that in order to maintain the observed sharp density and temperature discontinuities, conduction must also be suppressed along the magnetic field lines. However, the northwestern part of the cold front is observed to have a non-zero width. While other explanations are possible, the broadening is consistent with the presence of Kelvin–Helmholtz instabilities (KHI) on length-scales of a few kpc. Based on comparison with simulations, the presence of KHI would imply that the effective viscosity of the intracluster medium is suppressed by more than an order of magnitude with respect to the isotropic Spitzer-like temperature dependent viscosity. Underneath the cold front, we observe quasi-linear features that are ∼10 per cent brighter than the surrounding gas and are separated by ∼15 kpc from each other in projection. Comparison to tailored numerical simulations suggests that the observed phenomena may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front, where the magnetic pressure reaches ∼5–10 per cent of the thermal pressure, reducing the gas density between the bright features
Large-scale Motions in the Perseus Galaxy Cluster
By combining large-scale mosaics of ROSAT PSPC, XMM-Newton, and Suzaku X-ray
observations, we present evidence for large-scale motions in the intracluster
medium of the nearby, X-ray bright Perseus Cluster. These motions are suggested
by several alternating and interleaved X-ray bright, low-temperature,
low-entropy arcs located along the east-west axis, at radii ranging from ~10
kpc to over a Mpc. Thermodynamic features qualitatively similar to these have
previously been observed in the centers of cool core clusters, and were
successfully modeled as a consequence of the gas sloshing/swirling motions
induced by minor mergers. Our observations indicate that such sloshing/swirling
can extend out to larger radii than previously thought, on scales approaching
the virial radius.Comment: 6 pages, 6 figures, accepted for publication in Ap
Relativistic Electron Shock Drift Acceleration in Low Mach Number Galaxy Cluster Shocks
An extreme case of electron shock drift acceleration in low Mach number
collisionless shocks is investigated as a plausible mechanism of initial
acceleration of relativistic electrons in large-scale shocks in galaxy clusters
where upstream plasma temperature is of the order of 10 keV and a degree of
magnetization is not too small. One-dimensional electromagnetic full particle
simulations reveal that, even though a shock is rather moderate, a part of
thermal incoming electrons are accelerated and reflected through relativistic
shock drift acceleration and form a local nonthermal population just upstream
of the shock. The accelerated electrons can self-generate local coherent waves
and further be back-scattered toward the shock by those waves. This may be a
scenario for the first stage of the electron shock acceleration occurring at
the large-scale shocks in galaxy clusters such as CIZA J2242.8+5301 which has
well defined radio relics.Comment: 26 pages, 10 figures, accepted for publication in Ap
Galaxy Clusters as Reservoirs of Heavy Dark Matter and High-Energy Cosmic Rays: Constraints from Neutrino Observations
Galaxy Clusters (GCs) are the largest reservoirs of both dark matter and
cosmic rays (CRs). Dark matter self-annihilation can lead to a high luminosity
in gamma rays and neutrinos, enhanced by a strong degree of clustering in dark
matter substructures. Hadronic CR interactions can also lead to a high
luminosity in gamma rays and neutrinos, enhanced by the confinement of CRs from
cluster accretion/merger shocks and active galactic nuclei. We show that
IceCube/KM3Net observations of high-energy neutrinos can probe the nature of
GCs and the separate dark matter and CR emission processes, taking into account
how the results depend on the still-substantial uncertainties. Neutrino
observations are relevant at high energies, especially at >10 TeV. Our results
should be useful for improving experimental searches for high-energy neutrino
emission. Neutrino telescopes are sensitive to extended sources formed by dark
matter substructures and CRs distributed over large scales. Recent observations
by Fermi and imaging atmospheric Cherenkov telescopes have placed interesting
constraints on the gamma-ray emission from GCs. We also provide calculations of
the gamma-ray fluxes, taking into account electromagnetic cascades inside GCs,
which can be important for injections at sufficiently high energies. This also
allows us to extend previous gamma-ray constraints to very high dark matter
masses and significant CR injections at very high energies. Using both
neutrinos and gamma rays, which can lead to comparable constraints, will allow
more complete understandings of GCs. Neutrinos are essential for some dark
matter annihilation channels, and for hadronic instead of electronic CRs. Our
results suggest that the multi-messenger observations of GCs will be able to
give useful constraints on specific models of dark matter and CRs. [Abstract
abridged.]Comment: 31 pages, 20 figures, 1 table, accepted for publication in JCAP,
references and discussions adde
Circum-Galactic Gas and the Isotropic Gamma Ray Background
Interactions of cosmic rays with the interstellar gas and radiation fields of
the Milky Way provide the majority of the gamma rays observed by the Fermi
Gamma Ray Space Telescope. In addition to the gas which is densely concentrated
along the Galactic Disk, hydrodynamical simulations and observational evidence
favor the presence of a halo of hot (T~10^6 K) ionized hydrogen (H_II),
extending with non-negligible densities out to the virial radius of the Milky
Way. We show that cosmic ray collisions with this circum-galactic gas should be
expected to provide a significant flux of gamma rays, on the order of 10% of
the observed isotopic gamma ray background at energies above 1 GeV. In
addition, gamma rays originating from the extended H_II halos of other galaxies
along a given line-of-sight should contribute to this background at a similar
level.Comment: 6 pages, 3 figures, submitted to APJ, comments welcom
The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations
Long-lived >100 MeV emission has been a common feature of most Fermi-LAT
detected gamma-ray bursts (GRBs), e.g., detected up to ~10^3s in long GRBs
080916C and 090902B and ~10^2s in short GRB 090510. This emission is consistent
with being produced by synchrotron emission of electrons accelerated to high
energy by the relativistic collisionless shock propagating into the weakly
magnetized medium. Here we show that this high-energy afterglow emission
constrains the preshock magnetic field to satisfy 1(n/1cc)^{9/8}
mG<B<10^2(n/1cc)^{3/8}mG, where n is the preshock density, more stringent than
the previous constraint by X-ray afterglow observations on day scale. This
suggests that the preshock magnetic field is strongly amplified, most likely by
the streaming of high energy shock accelerated particles.Comment: 9 pages, JCAP accepte
Interlayer Registry Determines the Sliding Potential of Layered Metal Dichalcogenides: The case of 2H-MoS2
We provide a simple and intuitive explanation for the interlayer sliding
energy landscape of metal dichalcogenides. Based on the recently introduced
registry index (RI) concept, we define a purely geometrical parameter which
quantifies the degree of interlayer commensurability in the layered phase of
molybdenum disulphide (2HMoS2). A direct relation between the sliding energy
landscape and the corresponding interlayer registry surface of 2H-MoS2 is
discovered thus marking the registry index as a computationally efficient means
for studying the tribology of complex nanoscale material interfaces in the
wearless friction regime.Comment: 13 pages, 7 figure
Cover to Volume 3
The fibroblast mitogen platelet-derived growth factor -BB (PDGF-BB) induces a transient expression of the orphan nuclear receptor NR4A1 (also named Nur77, TR3 or NGFIB). The aim of the present study was to investigate the pathways through which NR4A1 is induced by PDGF-BB and its functional role. We demonstrate that in PDGF-BB stimulated NIH3T3 cells, the MEK1/2 inhibitor CI-1040 strongly represses NR4A1 expression, whereas Erk5 downregulation delays the expression, but does not block it. Moreover, we report that treatment with the NF-κB inhibitor BAY11-7082 suppresses NR4A1 mRNA and protein expression. The majority of NR4A1 in NIH3T3 was found to be localized in the cytoplasm and only a fraction was translocated to the nucleus after continued PDGF-BB treatment. Silencing NR4A1 slightly increased the proliferation rate of NIH3T3 cells; however, it did not affect the chemotactic or survival abilities conferred by PDGF-BB. Moreover, overexpression of NR4A1 promoted anchorage-independent growth of NIH3T3 cells and the glioblastoma cell lines U-105MG and U-251MG. Thus, whereas NR4A1, induced by PDGF-BB, suppresses cell growth on a solid surface, it increases anchorage-independent growth
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