6,905 research outputs found
Simulations of Electron Acceleration at Collisionless Shocks: The Effects of Surface Fluctuations
Energetic electrons are a common feature of interplanetary shocks and
planetary bow shocks, and they are invoked as a key component of models of
nonthermal radio emission, such as solar radio bursts. A simulation study is
carried out of electron acceleration for high Mach number, quasi-perpendicular
shocks, typical of the shocks in the solar wind. Two dimensional
self-consistent hybrid shock simulations provide the electric and magnetic
fields in which test particle electrons are followed. A range of different
shock types, shock normal angles, and injection energies are studied. When the
Mach number is low, or the simulation configuration suppresses fluctuations
along the magnetic field direction, the results agree with theory assuming
magnetic moment conserving reflection (or Fast Fermi acceleration), with
electron energy gains of a factor only 2 - 3. For high Mach number, with a
realistic simulation configuration, the shock front has a dynamic rippled
character. The corresponding electron energization is radically different:
Energy spectra display: (1) considerably higher maximum energies than Fast
Fermi acceleration; (2) a plateau, or shallow sloped region, at intermediate
energies 2 - 5 times the injection energy; (3) power law fall off with
increasing energy, for both upstream and downstream particles, with a slope
decreasing as the shock normal angle approaches perpendicular; (4) sustained
flux levels over a broader region of shock normal angle than for adiabatic
reflection. All these features are in good qualitative agreement with
observations, and show that dynamic structure in the shock surface at ion
scales produces effective scattering and can be responsible for making high
Mach number shocks effective sites for electron acceleration.Comment: 26 pages, 12 figure
Surface recombination measurements on III–V candidate materials for nanostructure light-emitting diodes
Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes
Assessing Alternatives for Directional Detection of a WIMP Halo
The future of direct terrestrial WIMP detection lies on two fronts: new, much
larger low background detectors sensitive to energy deposition, and detectors
with directional sensitivity. The former can large range of WIMP parameter
space using well tested technology while the latter may be necessary if one is
to disentangle particle physics parameters from astrophysical halo parameters.
Because directional detectors will be quite difficult to construct it is
worthwhile exploring in advance generally which experimental features will
yield the greatest benefits at the lowest costs. We examine the sensitivity of
directional detectors with varying angular tracking resolution with and without
the ability to distinguish forward versus backward recoils, and compare these
to the sensitivity of a detector where the track is projected onto a
two-dimensional plane. The latter detector regardless of where it is placed on
the Earth, can be oriented to produce a significantly better discrimination
signal than a 3D detector without this capability, and with sensitivity within
a factor of 2 of a full 3D tracking detector. Required event rates to
distinguish signals from backgrounds for a simple isothermal halo range from
the low teens in the best case to many thousands in the worst.Comment: 4 pages, including 2 figues and 2 tables, submitted to PR
Hysteresis phenomenon in deterministic traffic flows
We study phase transitions of a system of particles on the one-dimensional
integer lattice moving with constant acceleration, with a collision law
respecting slower particles. This simple deterministic ``particle-hopping''
traffic flow model being a straightforward generalization to the well known
Nagel-Schreckenberg model covers also a more recent slow-to-start model as a
special case. The model has two distinct ergodic (unmixed) phases with two
critical values. When traffic density is below the lowest critical value, the
steady state of the model corresponds to the ``free-flowing'' (or ``gaseous'')
phase. When the density exceeds the second critical value the model produces
large, persistent, well-defined traffic jams, which correspond to the
``jammed'' (or ``liquid'') phase. Between the two critical values each of these
phases may take place, which can be interpreted as an ``overcooled gas'' phase
when a small perturbation can change drastically gas into liquid. Mathematical
analysis is accomplished in part by the exact derivation of the life-time of
individual traffic jams for a given configuration of particles.Comment: 22 pages, 6 figures, corrected and improved version, to appear in the
Journal of Statistical Physic
A New WIMP Population in the Solar System and New Signals for Dark-Matter Detectors
We describe in detail how perturbations due to the planets can cause a
sub-population of WIMPs captured by scattering in surface layers of the Sun to
evolve to have orbits which no longer intersect the Sun. We argue that such
WIMPs, if their orbit has a semi-major axis less than 1/2 of Jupiter's, can
persist in the solar system for cosmological timescales. This leads to a new,
previously unanticipated WIMP population intersecting the Earth's orbit. The
WIMP-nucleon cross sections required for this population to be significant are
precisely those in the range predicted for SUSY dark matter, lying near the
present limits obtained by direct underground dark matter searches using
cyrogenic detectors. Thus, if a WIMP signal is observed in the next generation
of detectors, a potentially measurable signal due to this new population must
exist. This signal, lying in the keV range for Germanium detectors, would be
complementary to that of galactic halo WIMPs. A comparison of event rates,
anisotropies, and annual modulations would not only yield additional
confirmation that any claimed signal is indeed WIMP-based, but would also allow
one to gain information on the nature of the underlying dark matter model.Comment: Revtex, 37 pages including 6 figures, accepted by Phys. Rev D.
(version to be published, including changes made in response to referees
reports
A Mini-survey of X-ray Point Sources in Starburst and Non-Starburst Galaxies
We present a comparison of X-ray point source luminosity functions of 3
starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral
galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity
functions of the starbursts are flatter than those of the spiral galaxies; the
starbursts have relatively more sources at high luminosities. This trend
extends to early-type galaxies which have steeper luminosity functions than
spirals. We show that the luminosity function slope is correlated with 60
micron luminosity, a measure of star formation. We suggest that the difference
in luminosity functions is related to the age of the X-ray binary populations
and present a simple model which highlights how the shape of the luminosity
distribution is affected by the age of the underlying X-ray binary population.Comment: 8 pages, 4 figures. accepted for publication in Ap
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