6,033 research outputs found
Diffusion in a crowded environment
We analyze a pair of diffusion equations which are derived in the infinite
system--size limit from a microscopic, individual--based, stochastic model.
Deviations from the conventional Fickian picture are found which ultimately
relate to the depletion of resources on which the particles rely. The
macroscopic equations are studied both analytically and numerically, and are
shown to yield anomalous diffusion which does not follow a power law with time,
as is frequently assumed when fitting data for such phenomena. These anomalies
are here understood within a consistent dynamical picture which applies to a
wide range of physical and biological systems, underlining the need for clearly
defined mechanisms which are systematically analyzed to give definite
predictions.Comment: 4 pages, 3 figures, minor change
XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495
We report the first detection of a sharp spectral feature in a Narrow-Line
Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop
in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any
detectable narrow Fe K alpha line emission. The energy of this feature suggests
a connection with the neutral iron K photoelectric edge, but the lack of any
obvious absorption in the spectrum at lower energies makes the interpretation
challenging. We explore two alternative explanations for this unusual spectral
feature: (i) partial covering absorption by clouds of neutral material and (ii)
ionised disc reflection with lines and edges from different ionisation stages
of iron blurred together by relativistic effects. We note that both models
require an iron overabundance to explain the depth of the feature. The X-ray
light curve shows strong and rapid variability, changing by a factor of four
during the observation. The source displays modest spectral variability which
is uncorrelated with flux.Comment: 5 pages incl. 6 figures, accepted for publication in MNRA
Trapping cold atoms using surface-grown carbon nanotubes
We present a feasibility study for loading cold atomic clouds into magnetic
traps created by single-wall carbon nanotubes grown directly onto dielectric
surfaces. We show that atoms may be captured for experimentally sustainable
nanotube currents, generating trapped clouds whose densities and lifetimes are
sufficient to enable detection by simple imaging methods. This opens the way
for a novel type of conductor to be used in atomchips, enabling atom trapping
at sub-micron distances, with implications for both fundamental studies and for
technological applications
Nucleon Spin Fluctuations and the Supernova Emission of Neutrinos and Axions
In the hot and dense medium of a supernova (SN) core, the nucleon spins
fluctuate so fast that the axial-vector neutrino opacity and the axion
emissivity are expected to be significantly modified. Axions with
m_a\alt10^{-2}\,{\rm eV} are not excluded by SN~1987A. A substantial transfer
of energy in neutrino-nucleon () collisions is enabled which may alter
the spectra of SN neutrinos relative to calculations where energy-conserving
collisions had been assumed near the neutrinosphere.Comment: 8 pages. REVTeX. 2 postscript figures, can be included with epsf.
Small modifications of the text, a new "Note Added", and three new
references. To be published in Phys. Rev. Let
The Spin Periods and Rotational Profiles of Neutron Stars at Birth
We present results from an extensive set of one- and two-dimensional
radiation-hydrodynamic simulations of the supernova core collapse, bounce, and
postbounce phases, and focus on the protoneutron star (PNS) spin periods and
rotational profiles as a function of initial iron core angular velocity, degree
of differential rotation, and progenitor mass. For the models considered, we
find a roughly linear mapping between initial iron core rotation rate and PNS
spin. The results indicate that the magnitude of the precollapse iron core
angular velocities is the single most important factor in determining the PNS
spin. Differences in progenitor mass and degree of differential rotation lead
only to small variations in the PNS rotational period and profile. Based on our
calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming
angular momentum conservation, we estimate final neutron star rotation periods.
We find periods of one millisecond and shorter for initial central iron core
periods of below ~ 10 s. This is appreciably shorter than what previous studies
have predicted and is in disagreement with current observational data from
pulsar astronomy. After considering possible spindown mechanisms that could
lead to longer periods we conclude that there is no mechanism that can robustly
spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens
to hundreds of milliseconds observed for young pulsars. Our results indicate
that, given current knowledge of the limitations of neutron star spindown
mechanisms, precollapse iron cores must rotate with periods around 50-100
seconds to form neutron stars with periods generically near those inferred for
the radio pulsar population.Comment: 31 pages, including 20 color figures. High-resolution figures
available from the authors upon request. Accepted to Ap
A Fresh Look at Axions and SN 1987A
We re-examine the very stringent limits on the axion mass based on the
strength and duration of the neutrino signal from SN 1987A, in the light of new
measurements of the axial-vector coupling strength of nucleons, possible
suppression of axion emission due to many-body effects, and additional emission
processes involving pions. The suppression of axion emission due to nucleon
spin fluctuations induced by many-body effects degrades previous limits by a
factor of about 2. Emission processes involving thermal pions can strengthen
the limits by a factor of 3-4 within a perturbative treatment that neglects
saturation of nucleon spin fluctuations. Inclusion of saturation effects,
however, tends to make the limits less dependent on pion abundances. The
resulting axion mass limit also depends on the precise couplings of the axion
and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.Comment: 32 latex pages, 13 postscript figures included, uses revtex.sty,
submitted to Physical Review
Development of a network for the on-farm conservation of crop genetic resources: First results of a pilot project for the re-introduction of old Lactuca varieties to the market
In a pilot project, we examined the chance of maintaining plant genetic resources by commercial utilization of old varieties using Lactuca sativa as a model plant. Nine market gardens in the region of Berlin and Brandenburg cultivated 18 old varieties during four cultivation periods to test field performance. They supplied the products to the market in their customary manner to analyse marketing success. Seven of the market gardens practice organic horticulture. In a complementary field trial at Humboldt-Universität zu Berlin, we established data concerning the field performance of the varieties, analysed dry matter contents, nitrate and phenol concentrations, and observed shelf life for two days under simulated retail conditions (18°C, 80% rel. air humidity). Generally, yield was acceptable for market purposes. However, cultivation in autumn failed because of the cold climate. Biotic and abiotic factors like slugs or hail caused non-specific damages. Specific problems of particular varieties were less important. Based on the results of 2007, the varieties can be put preliminarily into three categories: suitable for on-farm conservation, suitable for home gardens, and varieties with contrasting results depending on the respective market garden.The nitrate concentrations of all varieties were clearly below the EU acceptable limit of 2500 mg/kg fresh weight of lettuce grown in the field. The phenol concentrations varied from 3.3 to 17.2 mg GAE/g dry weight. Generally, the cultivars had a reasonable shelf life of one to two days, however three varieties showed a better storability whereas four other cultivars deteriorated rapidly. Marketing success was good in Berlin City but poor in the countryside of Brandenburg. The regular customers of the market gardens in Berlin who prefer organic food are a promising target group for further stimulation of interest to buy rare crop varieties. The on-farm conservation of old varieties in market gardens requires relatively large quantities of seeds of good quality. However there might arise problems in seed supply as the VERN e.V. was confronted with bottleneck problems. Therefore, we organised a network of interested market gardens who take on maintenance and propagation of individual varieties. The network will be developed in co-operation with the VERN e.V. who will also process the seed as well as organise the exchange of the various varieties within the network. Further, the network will deal with problems concerning maintenance breeding and seed quality
Numerical Toy-Model Calculation of the Nucleon Spin Autocorrelation Function in a Supernova Core
We develop a simple model for the evolution of a nucleon spin in a hot and
dense nuclear medium. A given nucleon is limited to one-dimensional motion in a
distribution of external, spin-dependent scattering potentials. We calculate
the nucleon spin autocorrelation function numerically for a variety of
potential densities and distributions which are meant to bracket realistic
conditions in a supernova core. For all plausible configurations the width of
the spin-density structure function is found to be less than the temperature.
This is in contrast with a naive perturbative calculation based on the one-pion
exchange potential which overestimates the width and thus suggests a large
suppression of the neutrino opacities by nucleon spin fluctuations. Our results
suggest that it may be justified to neglect the collisional broadening of the
spin-density structure function for the purpose of estimating the neutrino
opacities in the deep inner core of a supernova. On the other hand, we find no
indication that processes such as axion or neutrino pair emission, which depend
on nucleon spin fluctuations, are substantially suppressed beyond the
multiple-scattering effect already discussed in the literature. Aside from
these practical conclusions, our model reveals a number of interesting and
unexpected insights. For example, the spin-relaxation rate saturates with
increasing potential strength only if bound states are not allowed to form by
including a repulsive core. There is no saturation with increasing density of
scattering potentials until localized eigenstates of energy begin to form.Comment: 14 latex pages in two-column format, 15 postscript figures included,
uses revtex.sty and epsf.sty. Submitted to Physical Review
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