20,436 research outputs found
The broad-band radio spectrum of LSI+61303 in outburst
Aims: Our aim is to explore the broad-band radio continuum spectrum of
LSI+61303 during its outbursts by employing the available set of secondary
focus receivers of the Effelsberg 100 m telescope. Methods: The clear
periodicity of the system LSI+61303 allowed observations to be scheduled
covering the large radio outburst in March-April 2012. We observed LSI+61303 on
14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32 GHz with a
cadence of about 12 hours followed by two additional observations several days
later. Based on these observations we obtained a total of 24 quasi-simultaneous
broad-band radio spectra. Results: During onset, the main flare shows an almost
flat broad-band spectrum, most prominently seen on March 27, 2012, where - for
the first time - a flat spectrum (alpha=0.00+/-0.07, S nu^alpha) is observed up
to 32 GHz (9 mm wavelength). The flare decay phase shows superimposed
'sub-flares' with the spectral index oscillating between -0.4 and -0.1 in a
quasi-regular fashion. Finally, the spectral index steepens during the decay
phase, showing optically thin emission with values alpha -0.5 to -0.7.
Conclusions: The radio characteristics of LSI+61303 compare well with those of
the microquasars XTE J1752-223 and Cygnus X-3. In these systems the flaring
phase is actually also composed of a sequence of outbursts with clearly
different spectral characteristics: a first outburst with a flat/inverted
spectrum followed by a bursting phase of optically thin emission.Comment: 4 pages, 3 figures, accepted for publication in A&
Comment on "Photoluminescence ring formation in coupled quantum wells: Excitonic versus ambipolar diffusion"
This is a comment on M. Stern, V. Garmider, E. Segre, M. Rappaport, V.
Umansky, Y. Levinson, and I. Bar-Joseph, Phys. Rev. Lett. 101, 257402 (2008)
The gravitational wave spectrum of non-axisymmetric, freely precessing neutron stars
Evidence for free precession has been observed in the radio signature of
several pulsars. Freely precessing pulsars radiate gravitationally at
frequencies near the rotation rate and twice the rotation rate, which for
rotation frequencies greater than Hz is in the LIGO band. In older
work, the gravitational wave spectrum of a precessing neutron star has been
evaluated to first order in a small precession angle. Here we calculate the
contributions to second order in the wobble angle, and we find that a new
spectral line emerges. We show that for reasonable wobble angles, the
second-order line may well be observable with the proposed advanced LIGO
detector for precessing neutron stars as far away as the galactic center.
Observation of the full second-order spectrum permits a direct measurement of
the star's wobble angle, oblateness, and deviation from axisymmetry, with the
potential to significantly increase our understanding of neutron star
structure.Comment: 22 pages, 1 figure. Minor changes in the text, typos correcte
Antioxidant status in acute stroke patients and patients at stroke risk
Background and Purpose: Antioxidant enzymes like copper/ zinc superoxide dismutase (SOD), catalase and gluthatione peroxidase (GSHPx) are part of intracellular protection mechanisms to overcome oxidative stress and are known to be activated in vascular diseases and acute stroke. We investigated the differences of antioxidant capacity in acute stroke and stroke risk patients to elucidate whether the differences are a result of chronic low availability in arteriosclerosis and stroke risk or due to changes during acute infarction. Methods: Antioxidant enzymes were examined in 11 patients within the first hours and days after acute ischemic stroke and compared to risk- and age-matched patients with a history of stroke in the past 12 months ( n = 17). Antioxidant profile was determined by measurement of glutathione (GSH), malondialdehyde (MDA), SOD, GSHPx and minerals known to be involved in antioxidant enzyme activation like selenium, iron, copper and zinc. Results: In comparison to stroke risk patients, patients with acute ischemic stroke had significant changes of the GSH system during the first hours and days after the event: GSH was significantly elevated in the first hours (p < 0.01) and GSHPx was elevated 1 day after the acute stroke (p < 0.05). Selenium, a cofactor of GSHPx, was decreased (p < 0.01). GSHPx levels were negatively correlated with National Institutes of Health Stroke Scale (NIHSS) scores on admission (r = - 0.84, p < 0.001) and NIHSS scores after 7 days ( r = - 0.63, p < 0.05). MDA levels showed a trend for elevation in the first 6 h after the acute stroke ( p = 0.07). No significant differences of SOD, iron, copper nor zinc levels could be identified. Conclusions: Differences of antioxidant capacity were found for the GSH system with elevation of GSH and GSHPx after acute stroke, but not for other markers. The findings support the hypothesis that changes of antioxidant capacity are part of acute adaptive mechanisms during acute stroke. Copyright (C) 2004 S. Karger AG, Basel
Exciton Mott transition in Si Revealed by Terahertz Spectroscopy
Exciton Mott transition in Si is investigated by using terahertz time-domain
spectroscopy. The excitonic correlation as manifested by the 1s-2p resonance is
observed above the Mott density. The scattering rate of charge carriers is
prominently enhanced at the proximity of Mott density, which is attributed to
the non-vanishing exciton correlation in the metallic electron-hole plasma.
Concomitantly, the signature of plasmon-exciton coupling is observed in the
loss function spectra.Comment: 5 pages, 3 figure
Measuring nonadiabaticity of molecular quantum dynamics with quantum fidelity and with its efficient semiclassical approximation
We propose to measure nonadiabaticity of molecular quantum dynamics
rigorously with the quantum fidelity between the Born-Oppenheimer and fully
nonadiabatic dynamics. It is shown that this measure of nonadiabaticity applies
in situations where other criteria, such as the energy gap criterion or the
extent of population transfer, fail. We further propose to estimate this
quantum fidelity efficiently with a generalization of the dephasing
representation to multiple surfaces. Two variants of the multiple-surface
dephasing representation (MSDR) are introduced, in which the nuclei are
propagated either with the fewest-switches surface hopping (FSSH) or with the
locally mean field dynamics (LMFD). The LMFD can be interpreted as the
Ehrenfest dynamics of an ensemble of nuclear trajectories, and has been used
previously in the nonadiabatic semiclassical initial value representation. In
addition to propagating an ensemble of classical trajectories, the MSDR
requires evaluating nonadiabatic couplings and solving the Schr\"{o}dinger (or
more generally, the quantum Liouville-von Neumann) equation for a single
discrete degree of freedom. The MSDR can be also used to measure the importance
of other terms present in the molecular Hamiltonian, such as diabatic
couplings, spin-orbit couplings, or couplings to external fields, and to
evaluate the accuracy of quantum dynamics with an approximate nonadiabatic
Hamiltonian. The method is tested on three model problems introduced by Tully,
on a two-surface model of dissociation of NaI, and a three-surface model
including spin-orbit interactions. An example is presented that demonstrates
the importance of often-neglected second-order nonadiabatic couplings.Comment: 14 pages, 4 figures, submitted to J. Chem. Phy
Realistic heterointerfaces model for excitonic states in growth-interrupted quantum wells
We present a model for the disorder of the heterointerfaces in GaAs quantum
wells including long-range components like monolayer island formation induced
by the surface diffusion during the epitaxial growth process. Taking into
account both interfaces, a disorder potential for the exciton motion in the
quantum well plane is derived. The excitonic optical properties are calculated
using either a time-propagation of the excitonic polarization with a
phenomenological dephasing, or a full exciton eigenstate model including
microscopic radiative decay and phonon scattering rates. While the results of
the two methods are generally similar, the eigenstate model does predict a
distribution of dephasing rates and a somewhat modified spectral response.
Comparing the results with measured absorption and resonant Rayleigh scattering
in GaAs/AlAs quantum wells subjected to growth interrupts, their specific
disorder parameters like correlation lengths and interface flatness are
determined. We find that the long-range disorder in the two heterointerfaces is
highly correlated, having rather similar average in-plane correlation lengths
of about 60 and 90 nm. The distribution of dephasing rates observed in the
experiment is in agreement with the results of the eigenstate model. Finally,
we simulate highly spatially resolved optical experiments resolving individual
exciton states in the deduced interface structure.Comment: To appear in Physical Review
Perioperative infection prophylaxis and risk factor impact in colon surgery
Background: A prospective observational study was undertaken in 2,481 patients undergoing elective colon resection in 114 German centers to identify optimal drug and dosing modalities and risk factors for postoperative infection. Methods: Patients were pair matched using six risk factors and divided into 672 pairs (ceftriaxone vs, other cephalosporins, group A) and 400 pairs (ceftriaxone vs. penicillins, group B). End points were local and systemic postoperative infection and cost effectiveness. Results: Local infection rates were 6.0 versus 6.5% (group A) and 4.0 versus 10.5% (group B); systemic infection rates in groups A and B were 4.9 versus 6.3% and 3.3 versus 10.5%, respectively. Ceftriaxone was more effective than penicillins overall (6.8 vs. 17.8%, p < 0.001). Length of postoperative hospital stay was 16.2 versus 16.9 days (group A) and 15.8 versus 17.6 days (group B). Of the six risk factors, age and concomitant disease were significant for systemic infection, and blood loss, rectum resection and immunosuppressive therapy were significant for local infection. Penicillin was a risk factor compared to ceftriaxone (p < 0.0001). Ceftriaxone saved Q160.7 versus other cephalosporins and O416.2 versus penicillins. Conclusion: Clinical and microbiological efficacy are responsible for the cost effectiveness of ceftriaxone for perioperative prophylaxis in colorectal surgery. Copyright (C) 2000 S. Karger AG, Basel
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