6,077 research outputs found
The Dark Disk of the Milky Way
Massive satellite accretions onto early galactic disks can lead to the
deposition of dark matter in disk-like configurations that co-rotate with the
galaxy. This phenomenon has potentially dramatic consequences for dark matter
detection experiments. We utilize focused, high-resolution simulations of
accretion events onto disks designed to be Galaxy analogues, and compare the
resultant disks to the morphological and kinematic properties of the Milky
Way's thick disk in order to bracket the range of co-rotating accreted dark
matter. We find that the Milky Way's merger history must have been unusually
quiescent compared to median LCDM expectations and therefore its dark disk must
be relatively small: the fraction of accreted dark disk material near the Sun
is about 20% of the host halo density or smaller and the co-rotating dark
matter fraction near the Sun, defined as particles moving with a rotational
velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a
standard halo model. Such a dark disk could contribute dominantly to the low
energy (of order keV for a dark matter particle with mass 100 GeV) nuclear
recoil event rate of direct dectection experiments, but it will not change the
likelihood of detection significantly. These dark disks provide testable
predictions of weakly-interacting massive particle dark matter models and
should be considered in detailed comparisons to experimental data. Our findings
suggest that the dark disk of the Milky Way may provide a detectable signal for
indirect detection experiments, contributing up to about 25% of the dark matter
self-annihilation signal in the direction of the center of the Galaxy, lending
the signal a noticeably oblate morphology.Comment: 11 pages, 6 figures, 1 table; submitted to Ap
DC field induced enhancement and inhibition of spontaneous emission in a cavity
We demonstrate how spontaneous emission in a cavity can be controlled by the
application of a dc field. The method is specially suitable for Rydberg atoms.
We present a simple argument for the control of emission.Comment: 3-pages, 2figure. accepted in Phys. Rev.
High-fat meals rich in EPA plus DHA compared with DHA only have differential effects on postprandial lipemia and plasma 8-isoprostane F-2 alpha concentrations relative to a control high-oleic acid meal: a randomized controlled trial
Background: Eicosapentaenoic acid (EPA) plus docosahexaenoic acid (DHA) supplementation has beneficial cardiovascular effects, but postprandial influences of these individual fatty acids are unclear. Objectives: The primary objective was to determine the vascular effects of EPA + DHA compared with DHA only during postprandial lipemia relative to control highâoleic acid meals; the secondary objective was to characterize the effects of linoleic acidâenriched high-fat meals relative to the control meal. Design: We conducted a randomized, controlled, double-blind crossover trial of 4 high-fat (75-g) meals containing 1) highâoleic acid sunflower oil (HOS; control), 2) HOS + fish oil (FO; 5 g EPA and DHA), 3) HOS + algal oil (AO; 5 g DHA), and 4) highâlinoleic acid sunflower oil (HLS) in 16 healthy men (aged 35â70 y) with higher than optimal fasting triacylglycerol concentrations (mean ± SD triacylglycerol, 1.9 ± 0.5 mmol/L). Results: Elevations in triacylglycerol concentration relative to baseline were slightly reduced after FO and HLS compared with the HOS control (P < 0.05). The characteristic decrease from baseline in plasma nonesterified fatty acids after a mixed meal was inhibited after AO (Î 0â3 h, P < 0.05). HLS increased the augmentation index compared with the other test meals (P < 0.05), although the digital volume pulseâreflection index was not significantly different. Plasma 8-isoprostane F(2α) analysis revealed opposing effects of FO (increased) and AO (reduced) compared with the control (P < 0.05). No differences in nitric oxide metabolites were observed. Conclusions: These data show differential postprandial 8-isoprostane F(2α) responses to high-fat meals containing EPA + DHAârich fish oil compared with DHA-rich AO, but these differences were not associated with consistent effects on postprandial vascular function or lipemia. More detailed analyses of polyunsaturated fatty acidâderived lipid mediators are required to determine possible divergent functional effects of single meals rich in either DHA or EPA. This trial was registered at clinicaltrials.gov as NCT01618071
Electromagnetic Transition in Waveguide with Application to Lasers
The electromagnetic transition of two-level atomic systems in a waveguide is
calculated. Compared with the result in free space, the spontaneous emission
rate decrease because the phase space is smaller, and meanwhile, some resonance
appears in some cases. Moreover, the influence of non-uniform electromagnetic
field in a waveguide on absorption and stimulated emission is considered.
Applying the results to lasers, a method to enhance the laser power is
proposed.Comment: 4 pages, 2 figure
Diffuse Galactic Soft Gamma-Ray Emission
The Galactic diffuse soft gamma-ray (30-800 keV) emission has been measured
from the Galactic Center by the HIREGS balloon-borne germanium spectrometer to
determine the spectral characteristics and origin of the emission. The
resulting Galactic diffuse continuum is found to agree well with a single
power-law (plus positronium) over the entire energy range, consistent with RXTE
and COMPTEL/CGRO observations at lower and higher energies, respectively. We
find no evidence of spectral steepening below 200 keV, as has been reported in
previous observations. The spatial distribution along the Galactic ridge is
found to be nearly flat, with upper limits set on the longitudinal gradient,
and with no evidence of an edge in the observed region. The soft gamma-ray
diffuse spectrum is well modeled by inverse Compton scattering of interstellar
radiation off of cosmic-ray electrons, minimizing the need to invoke
inefficient nonthermal bremsstrahlung emission. The resulting power requirement
is well within that provided by Galactic supernovae. We speculate that the
measured spectrum provides the first direct constraints on the cosmic-ray
electron spectrum below 300 MeV.Comment: 26 pages, 7 figure, submitted to Ap
OSSE observations of galactic 511 keV annihilation radiation
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described
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