49,036 research outputs found
Strong GeV Emission Accompanying TeV Blazar H1426+428
For High frequency BL Lac objects (HBLs) like H1426+428, a significant
fraction of their TeV gamma-rays emitted are likely to be absorbed in
interactions with the diffuse IR background, yielding pairs. The
resulting pairs generate one hitherto undiscovered GeV emission by
inverse Compton scattering with the cosmic microwave background photons
(CMBPs). We study such emission by taking the 1998-2000 CAT data, the
reanalyzed 1999 & 2000 HEGRA data and the corresponding intrinsic spectra
proposed by Aharonian et al. (2003a). We numerically calculate the scattered
photon spectra for different intergalactic magnetic field (IGMF) strengths. If
the IGMF is about or weaker, there comes very strong GeV
emission, whose flux is far above the detection sensitivity of the upcoming
satellite GLAST! Considered its relatively high redshift (), the
detected GeV emission in turn provides us a valuable chance to calibrate the
poor known spectral energy distribution of the intergalactic infrared
background, or provides us some reliable constraints on the poorly known IGMF
strength.Comment: 5 pages, 1 figure. A&A in Pres
Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs
For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock
crosses the ejecta, usually the shocked regions are still precipitated by the
prompt MeV \gamma-ray emission. Because of the tight overlapping of the MeV
photon flow with the shocked regions, the optical depth for the GeV photons
produced in the shocks is very large. These high energy photons are absorbed by
the MeV photon flow and generate relativistic e^\pm pairs. These pairs
re-scatter the soft X-ray photons from the forward shock as well as the prompt
\gamma-ray photons and power detectable high energy emission, significant part
of which is in the sub-GeV energy range. Since the total energy contained in
the forward shock region and the reverse shock region are comparable, the
predicted sub-GeV emission is independent on whether the GRB ejecta are
magnetized (in which case the reverse shock IC and synchrotron self-Compton
emission is suppressed). As a result, a sub-GeV flash is a generic signature
for the GRB wind model, and it should be typically detectable by the future
{\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence
neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission
powered by the interaction of the shock accelerated protons with the
synchrotron photons in both the forward and reverse shock regions, there comes
another eV neutrino emission component powered by protons interacting
with the MeV photon flow. This last component has a similar spectrum to the one
generated in the internal shock phase, but the typical energy is slightly
lower.Comment: 7 pages, accepted for publication in Ap
Discovery of 14 radio pulsars in a survey of the Magellanic Clouds
A systematic survey of the Large and Small Magellanic Clouds for radio
pulsars using the Parkes radio telescope and the 20-cm multibeam receiver has
resulted in the discovery of 14 pulsars and the redetection of five of the
eight previously known spin-powered pulsars believed to lie in the Magellanic
Clouds. Of the 14 new discoveries, 12 are believed to lie within Clouds, three
in the Small Cloud and nine in the Large Cloud, bringing the total number of
known spin-powered pulsars in the Clouds to 20. Averaged over all positions
within the survey area, the survey had a limiting flux density of about 0.12
mJy. Observed dispersion measures suggest that the mean free electron density
in the Magellanic Clouds is similar to that in the disk of our Galaxy. The
observed radio luminosities have little or no dependence on pulsar period or
characteristic age and the differential luminosity function is consistent with
a power-law slope of -1 as is observed for Galactic pulsars.Comment: In press, Ap
Three-dimensional metamaterials with an ultra-high effective refractive index over broad bandwidth
The authors introduce a general mechanism, based on electrostatic and
magnetostatic considerations, for designing three-dimensional isotopic
metamaterials that possess an enhanced refractive index over an extremely large
frequency range. The mechanism allows nearly independent control of effective
electric permittivity and magnetic permeability without the use of resonant
elements
Recommended from our members
p22phox C242T Single-Nucleotide Polymorphism Inhibits Inflammatory Oxidative Damage to Endothelial Cells and Vessels.
BACKGROUND: The NADPH oxidase, by generating reactive oxygen species, is involved in the pathophysiology of many cardiovascular diseases and represents a therapeutic target for the development of novel drugs. A single-nucleotide polymorphism, C242T of the p22(phox) subunit of NADPH oxidase, has been reported to be negatively associated with coronary heart disease and may predict disease prevalence. However, the underlying mechanisms remain unknown. METHODS AND RESULTS: With the use of computer molecular modeling, we discovered that C242T single-nucleotide polymorphism causes significant structural changes in the extracellular loop of p22(phox) and reduces its interaction stability with Nox2 subunit. Gene transfection of human pulmonary microvascular endothelial cells showed that C242T p22(phox) significantly reduced Nox2 expression but had no significant effect on basal endothelial O2 (.-) production or the expression of Nox1 and Nox4. When cells were stimulated with tumor necrosis factor-α (or high glucose), C242T p22(phox) significantly inhibited tumor necrosis factor-α-induced Nox2 maturation, O2 (.-) production, mitogen-activated protein kinases and nuclear factor ÎșB activation, and inflammation (all P<0.05). These C242T effects were further confirmed using p22(phox) short-hairpin RNA-engineered HeLa cells and Nox2(-/-) coronary microvascular endothelial cells. Clinical significance was investigated by using saphenous vein segments from non-coronary heart disease subjects after phlebotomies. TT (C242T) allele was common (prevalence of â22%) and, in comparison with CC, veins bearing TT allele had significantly lower levels of Nox2 expression and O2 (.-) generation in response to high-glucose challenge. CONCLUSIONS: C242T single-nucleotide polymorphism causes p22(phox) structural changes that inhibit endothelial Nox2 activation and oxidative response to tumor necrosis factor-α or high-glucose stimulation. C242T single-nucleotide polymorphism may represent a natural protective mechanism against inflammatory cardiovascular diseases
Post-shock spikes: A new feature of proton and alpha enhancements associated with an interplanetary shock wave
Abrupt and prolonged enhancements in the intensities of 100 to approximately 2000 keV nucleon protons and alpha particles observed in interplanetary space are interpreted as particle populations confined between an interplanetary shock front and a magnetic field discontinuity. Prominent intensity spikes observed only below approximately 400 keV per charge for both protons and alpha particles several hours behind the shock front suggest that some fraction of the confined particles is accelerated by an energy per charge dependent process
Non-absorbing high-efficiency counter for itinerant microwave photons
Detecting an itinerant microwave photon with high efficiency is an
outstanding problem in microwave photonics and its applications. We present a
scheme to detect an itinerant microwave photon in a transmission line via the
nonlinearity provided by a transmon in a driven microwave resonator. With a
single transmon we achieve 84% distinguishability between zero and one
microwave photons and 90% distinguishability with two cascaded transmons by
performing continuous measurements on the output field of the resonator. We
also show how the measurement diminishes coherence in the photon number basis
thereby illustrating a fundamental principle of quantum measurement: the higher
the measurement efficiency, the faster is the decoherence
- âŠ