988 research outputs found
Confronting QCD Instantons with HERA Data
The sensitivity of existing HERA data on the hadronic final state in
deep-inelastic scattering (DIS) to processes induced by QCD instantons is
systematically investigated. The maximally allowed fraction of such processes
in DIS is found to be on the percent level in the kinematic domain 0.0001 <
x-Bjorken < 0.01 and 5 < Q squared < 100 GeV squared. The best limits are
obtained from the multiplicity distribution.Comment: 5 pages, latex, entire paper w. tex, style and figure
The Dark Matter Annihilation Signal from Galactic Substructure: Predictions for GLAST
We present quantitative predictions for the detectability of individual
Galactic dark matter subhalos in gamma-rays from dark matter pair annihilations
in their centers. Our method is based on a hybrid approach, employing the
highest resolution numerical simulations available (including the recently
completed one billion particle Via Lactea II simulation) as well as analytical
models for the extrapolation beyond the simulations' resolution limit. We
include a self-consistent treatment of subhalo boost factors, motivated by our
numerical results, and a realistic treatment of the expected backgrounds that
individual subhalos must outshine. We show that for reasonable values of the
dark matter particle physics parameters (M_X ~ 50 - 500 GeV and ~
10^-26 - 10^-25 cm^3/s) GLAST may very well discover a few, even up to several
dozen, such subhalos, at 5 sigma significance, and some at more than 20 sigma.
We predict that the majority of luminous sources would be resolved with GLAST's
expected angular resolution. For most observer locations the angular
distribution of detectable subhalos is consistent with a uniform distribution
across the sky. The brightest subhalos tend to be massive (median Vmax of 24
km/s) and therefore likely hosts of dwarf galaxies, but many subhalos with Vmax
as low as 5 km/s are also visible. Typically detectable subhalos are 20 - 40
kpc from the observer, and only a small fraction are closer than 10 kpc. The
total number of observable subhalos has not yet converged in our simulations,
and we estimate that we may be missing up to 3/4 of all detectable subhalos.Comment: 19 pages, 12 figures, ApJ accepted, a version with higher resolution
figures can be downloaded from
http://www.sns.ias.edu/~mqk/transfer/VL2_GLAST_predictions.pd
Formation and evolution of galaxy dark matter halos and their substructure
We use the ``Via Lactea'' simulation to study the co-evolution of a Milky
Way-size LambdaCDM halo and its subhalo population. While most of the host halo
mass is accreted over the first 6 Gyr in a series of major mergers, the
physical mass distribution [not M_vir(z)] remains practically constant since
z=1. The same is true in a large sample of LambdaCDM galaxy halos. Subhalo mass
loss peaks between the turnaround and virialization epochs of a given mass
shell, and declines afterwards. 97% of the z=1 subhalos have a surviving bound
remnant at the present epoch. The retained mass fraction is larger for
initially lighter subhalos: satellites with maximum circular velocities Vmax=10
km/s at z=1 have today about 40% of their mass back then. At the first
pericenter passage a larger average mass fraction is lost than during each
following orbit. Tides remove mass in substructure from the outside in, leading
to higher concentrations compared to field halos of the same mass. This effect,
combined with the earlier formation epoch of the inner satellites, results in
strongly increasing subhalo concentrations towards the Galactic center. We
present individual evolutionary tracks and present-day properties of the likely
hosts of the dwarf satellites around the Milky Way. The formation histories of
``field halos'' that lie today beyond the Via Lactea host are found to strongly
depend on the density of their environment. This is caused by tidal mass loss
that affects many field halos on eccentric orbits.Comment: 20 pages, 18 figures. Figures 6,7 and 8 corrected in this version,
for details see the erratum in ApJ 679, 1680 and
http://www.ucolick.org/~diemand/vl/publ/vlevolerr.pdf. Data, movies and
images are available at http://www.ucolick.org/~diemand/vl
Galactic Substructure and Dark Matter Annihilation in the Milky Way Halo
We study the effects of substructure on the rate of dark-matter annihilation
in the Galactic halo. We use an analytic model for substructure that can extend
numerical simulation results to scales too small to be resolved by the
simulations. We first calibrate the analytic model to numerical simulations,
and then determine the annihilation boost factor, for standard WIMP models as
well as those with Sommerfeld (or other) enhancements, as a function of
Galactocentric radius in the Milky Way. We provide an estimate of the
dependence of the gamma-ray intensity of WIMP annihilation as a function of
angular distance from the Galactic center. This methodology, coupled with
future numerical simulation results can be a powerful tool that can be used to
constrain WIMP properties using Fermi all-sky data.Comment: 10 pages, 7 figures, submitted to Phys. Rev. D; added a few
reference
Carbon Ignition in Type Ia Supernovae: II. A Three-Dimensional Numerical Model
The thermonuclear runaway that culminates in the explosion of a Chandrasekhar
mass white dwarf as a Type Ia supernova begins centuries before the star
actually explodes. Here, using a 3D anelastic code, we examine numerically the
convective flow during the last minute of that runaway, a time that is crucial
in determining just where and how often the supernova ignites. We find that the
overall convective flow is dipolar, with the higher temperature fluctuations in
an outbound flow preferentially on one side of the star. Taken at face value,
this suggests an asymmetric ignition that may well persist in the geometry of
the final explosion. However, we also find that even a moderate amount of
rotation tends to fracture this dipole flow, making ignition over a broader
region more likely. Though our calculations lack the resolution to study the
flow at astrophysically relevant Rayleigh numbers, we also speculate that the
observed dipolar flow will become less organized as the viscosity becomes very
small. Motion within the dipole flow shows evidence of turbulence, suggesting
that only geometrically large fluctuations (~1 km) will persist to ignite the
runaway. We also examine the probability density function for the temperature
fluctuations, finding evidence for a Gaussian, rather than exponential
distribution, which suggests that ignition sparks may be strongly spatially
clustered.Comment: 16 pages, 9 figures, submitted to ApJ. A high resolution version of
this paper, as well as movies, can be found at
http://www.ucolick.org/~mqk/Carbo
MEMS-based Speckle Spectrometer
We describe a new concept for a MEMS-based active spatial filter for
astronomical spectroscopy. The goal of this device is to allow the use of a
diffraction-limited spectrometer on a seeing limited observation at improved
throughput over a comparable seeing-limited spectrometer, thus reducing the
size and cost of the spectrometer by a factor proportional to r0/D (For the
case of a 10 meter telescope this size reduction will be approximately a factor
of 25 to 50). We use a fiber-based integral field unit (IFU) that incorporates
an active MEMS mirror array to feed an astronomical spectrograph. A fast camera
is used in parallel to sense speckle images at a spatial resolution of lambda/D
and at a temporal frequency greater than that of atmospheric fluctuations. The
MEMS mirror-array is used as an active shutter to feed speckle images above a
preset intensity threshold to the spectrometer, thereby increasing the
signal-to-noise ratio (SNR) of the spectrogram. Preliminary calculations
suggests an SNR improvement of a factor of about 1.4. Computer simulations have
shown an SNR improvement of 1.1, but have not yet fully explored the parameter
space.Comment: 11 pages, 5 figures, presented at SPIE Astronomical Telescopes and
Instrumentation, 24 - 31 May 2006, Orlando, Florida US
All-electrical time-resolved spin generation and spin manipulation in n-InGaAs
We demonstrate all-electrical spin generation and subsequent manipulation by
two successive electric field pulses in an n-InGaAs heterostructure in a
time-resolved experiment at zero external magnetic field. The first electric
field pulse along the crystal axis creates a current induced spin
polarization (CISP) which is oriented in the plane of the sample. The
subsequent electric field pulse along [110] generates a perpendicular magnetic
field pulse leading to a coherent precession of this spin polarization with
2-dimensional electrical control over the final spin orientation. Spin
precession is probed by time-resolved Faraday rotation. We determine the
build-up time of CISP during the first field pulse and extract the spin
dephasing time and internal magnetic field strength during the spin
manipulation pulse.Comment: 5 pages, 4 figure
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