39,037 research outputs found
Time lag between prompt optical emission and gamma-rays in GRBs
The prompt optical emission contemporaneous with the -rays from
-ray bursts (GRBs) carries important information on the central engine
and explosion mechanism. We study the time lag between prompt optical emission
and -rays in GRB 990123 and GRB 041219a, which are the only two GRBs
had been detected at optical wavelengths during the ascending burst phase.
Assuming profiles of prompt optical light curves are the same as the prompt
-rays, we simulate optical light curves with different time lags and
compare them with the observed optical flux. Then the best fit time lag and its
error are determined by chi-squared values. We find that time lags between
prompt optical emission and -rays in GRB host galaxy rest-frames are
consistent in the two GRBs, which is s for GRB 990123 and s
for GRB 041219a. This result is consistent with a common origin of prompt
optical and -ray emissions in the two GRBs. Based on synchrotron
cooling model, we also derive the parameters for the two GRBs.Comment: 4 pages, 3 figures; accepted for publication in A&
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Phenotypic and Genotypic Characteristics of Methicillin-Resistant Staphylococcus aureus (MRSA) Related to Persistent Endovascular Infection.
Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus infection and correlates with poor clinical outcomes. MRSA isolates from patients with PB differ significantly from those of resolving bacteremia (RB) with regard to several in vitro phenotypic and genotypic profiles. For instance, PB strains exhibit less susceptibility to cationic host defense peptides and vancomycin (VAN) killing under in vivo-like conditions, greater damage to endothelial cells, thicker biofilm formation, altered growth rates, early activation of many global virulence regulons (e.g., sigB, sarA, sae and agr) and higher expression of purine biosynthesis genes (e.g., purF) than RB strains. Importantly, PB strains are significantly more resistant to VAN treatment in experimental infective endocarditis as compared to RB strains, despite similar VAN minimum inhibitory concentrations (MICs) in vitro. Here, we review relevant phenotypic and genotypic characteristics related to the PB outcome. These and future insights may improve our understanding of the specific mechanism(s) contributing to the PB outcome, and aid in the development of novel therapeutic and preventative measures against this life-threatening infection
Hybrid quantum device based on NV centers in diamond nanomechanical resonators plus superconducting waveguide cavities
We propose and analyze a hybrid device by integrating a microscale diamond
beam with a single built-in nitrogen-vacancy (NV) center spin to a
superconducting coplanar waveguide (CPW) cavity. We find that under an ac
electric field the quantized motion of the diamond beam can strongly couple to
the single cavity photons via dielectric interaction. Together with the strong
spin-motion interaction via a large magnetic field gradient, it provides a
hybrid quantum device where the dia- mond resonator can strongly couple both to
the single microwave cavity photons and to the single NV center spin. This
enables coherent information transfer and effective coupling between the NV
spin and the CPW cavity via mechanically dark polaritons. This hybrid
spin-electromechanical de- vice, with tunable couplings by external fields,
offers a realistic platform for implementing quantum information with single NV
spins, diamond mechanical resonators, and single microwave photons.Comment: Accepted by Phys. Rev. Applie
Multi-wavelength variability properties of Fermi blazar S5 0716+714
S5 0716+714 is a typical BL Lacertae object. In this paper we present the
analysis and results of long term simultaneous observations in the radio,
near-infrared, optical, X-ray and -ray bands, together with our own
photometric observations for this source. The light curves show that the
variability amplitudes in -ray and optical bands are larger than those
in the hard X-ray and radio bands and that the spectral energy distribution
(SED) peaks move to shorter wavelengths when the source becomes brighter, which
are similar to other blazars, i.e., more variable at wavelengths shorter than
the SED peak frequencies. Analysis shows that the characteristic variability
timescales in the 14.5 GHz, the optical, the X-ray, and the -ray bands
are comparable to each other. The variations of the hard X-ray and 14.5 GHz
emissions are correlated with zero-lag, so are the V band and -ray
variations, which are consistent with the leptonic models. Coincidences of
-ray and optical flares with a dramatic change of the optical
polarization are detected. Hadronic models do not have the same nature
explanation for these observations as the leptonic models. A strong optical
flare correlating a -ray flare whose peak flux is lower than the
average flux is detected. Leptonic model can explain this variability
phenomenon through simultaneous SED modeling. Different leptonic models are
distinguished by average SED modeling. The synchrotron plus synchrotron
self-Compton (SSC) model is ruled out due to the extreme input parameters.
Scattering of external seed photons, such as the hot dust or broad line region
emission, and the SSC process are probably both needed to explain the
-ray emission of S5 0716+714.Comment: 43 pages, 13 figures, 3 tables, to be appeared in Ap
Probing Extragalactic Dust through Nearby Gamma-ray Burst Afterglows
doi: 10.1088/0004-637X/710/1/648The quantities and wavelength dependencies of the dust extinction along the lines of sight toward 33 nearby gamma-ray bursts (GRBs) with redshifts z < 2 are derived from fitting their afterglow spectral energy distributions. Unlike previous studies which often assume a specific extinction law like that of the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC/SMC), our approach—we call it the "Drude" approach—is more flexible in determining the true wavelength dependence of the extinction (while the shape of the extinction curve inferred from that relying on a priori assumption of a template extinction law is, of course, fixed). The extinction curves deduced from the Drude approach display a wide diversity of shapes, ranging from relatively flat curves to curves which are featureless and steeply rise toward the far-ultraviolet, and from curves just like that of the MW, LMC, and SMC to curves resembling that of the MW and LMC but lacking the 2175 Å bump. The visual extinction AV derived from the Drude approach is generally larger by a factor of ~2-5 than that inferred by assuming a SMC-type template extinction law. Consistent with previous studies, the extinction-to-gas ratio is mostly smaller than that of the MW, and does not seem to correlate with the shape of the extinction curve. It is shown that the standard silicate-graphite interstellar grain model closely reproduces the extinction curves of all 33 GRBs host galaxies. For these 33 bursts at z < 2, we find no evidence for the evolution of the dust extinction, dust sizes, and relative abundances of silicate to graphite on redshifts.We are supported in part by a NASA/Swift Theory Program, a NASA/Chandra Theory Program, and the NSFC Outstanding Oversea Young Scholarship
On Dust Extinction of Gamma-ray Burst Host Galaxies
Although it is well recognized that gamma-ray burst (GRB) afterglows are
obscured and reddened by dust in their host galaxies, the wavelength-dependence
and quantity of dust extinction are still poorly known. Current studies on this
mostly rely on fitting the afterglow spectral energy distributions (SEDs) with
template extinction models. The inferred extinction (both quantity and
wavelength-dependence) and dust-to-gas ratios are often in disagreement with
that obtained from dust depletion and X-ray spectroscopy studies. We argue that
this discrepancy could result from the prior assumption of a template
extinction law. We propose an analytical formula to approximate the GRB host
extinction law. With the template extinction laws self-contained, and the
capability of revealing extinction laws differing from the conventional ones,
it is shown that this is a powerful approach in modeling the afterglow SEDs to
derive GRB host extinction.Comment: 9 pages, 4 figures; The Astrophysical Journal, in press (2008 Oct 1
issue
Density-dependent deformed relativistic Hartree-Bogoliubov theory in continuum
The deformed relativistic Hartree-Bogoliubov theory in continuum with the
density-dependent meson-nucleon couplings is developed. The formulism is
briefly presented with the emphasis on handling the density-dependent
couplings, meson fields, and potentials in axially deformed system with partial
wave method. Taking the neutron-rich nucleus Mg as an example, the newly
developed code is verified by the spherical relativistic continuum
Hartree-Bogoliubov calculations, where only the spherical components of the
densities are considered. When the deformation is included self-consistently,
it is shown that the spherical components of density-dependent coupling
strengths are dominant, while the contributions from low-order deformed
components are not negligible.Comment: 5 pages, 3 figures, and 1 tabl
An adaptive dwell time scheduling model for phased array radar based on three-way decision
Real-time resource allocation is crucial for phased array radars to undertake multi-task with limited resources such as in the situation of multi-target tracking, in which targets need to be prioritized so that resources can be allocated accordingly and effectively. In this paper, a three-way decision-based model is proposed for adaptive scheduling of phased radar dwell time. Using the model, the threat posed by a target is measured by an evaluation function, and therefore, a target is assigned to one of the three possible decision regions, i.e., positive region, negative region, and boundary region. A different region has a various priority in terms of resource demand, and as such, a different radar resource allocation decision is applied to each region to satisfy different tracking accuracy of multi-target. In addition, the dwell time scheduling model can be further optimized by implementing a strategy for determining a proper threshold of three-way decision making to optimize the thresholds adaptively in real-time. The advantages and the performance of the proposed model has been verified by experimental simulations with comparison to the traditional two-way decision model and the three-way decision model without threshold optimization. The experiential results have demonstrated that the performance of the proposed model has a certain advantage in detecting high threat targets.
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