1,051 research outputs found
Influence of the photonuclear effect on electron-neutrino-induced electromagnetic cascades under the Landau-Pomeranchuk-Migdal regime in standard rock
The observation of earth skimming neutrinos has been proposed as a rather
sensitive method to detect ultra-high energy (UHE) cosmic neutrinos. Energetic
cosmic neutrinos can interact inside the rock and produce leptons via a charged
current interaction. In the case of an incoming electron neutrino undergoing a
charged current interaction, the produced UHE electron will induce an
underground electromagnetic shower. At high energy (above 7.7 TeV in standard
rock), such showers are subject to LPM (Landau, Pomeranchuk and Migdal)
suppression of the radiative processes cross sections (bremsstrahlung and pair
production). The consequence of this suppression is that showers are elongated.
This effect will increase the detection probability of such events allowing
deeper showers to emerge with detectable energies. On the other hand, the
photonuclear processes which are usually neglected in electromagnetic showers
with respect to radiative processes, turn out to become dominant in the LPM
regime and will reduce the shower length. In this work, we have performed a
complete Monte Carlo study of an underground shower induced by UHE electrons by
taking into account both the LPM suppression and the photonuclear interaction.
We will discuss the effects of both of these processes on the shower length and
on the detectability of such events by ground arrays or fluorescence
telescopes. We show that limits on neutrino fluxes that were obtained using
simulations that were obviously neglecting photonuclear processes are
overoptimistic and should be corrected.Comment: 6 pages, 7 figure
Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board INTEGRAL
Polarization measurements provide direct insight into the nature of
astrophysical processes. Unfortunately, only a few instruments are available
for this kind of measurements at gamma-ray energies, and the sources need to be
very bright. Gamma-Ray Bursts (GRBs) are ideal candidates due to their large
flux over limited time intervals, maximizing the available signal-to-noise
ratio. To date a few polarization measurements have been reported, claiming of
a high degree of polarization in the prompt emission of GRBs but with low
statistical evidence.
We used the IBIS telescope on board the INTEGRAL satellite to measure the
polarization of the prompt gamma-ray emission of the long and bright GRB
041219A in the 200-800 keV energy band. We find a variable degree of
polarization ranging from less than 4% over the first peak to 43+/-25% for the
whole second peak. Time resolved analysis of both peaks indicates a high degree
of polarization, and the null average polarization in the first peak can be
explained by the rapid variations observed in the polarization angle and
degree.
Our results are consistent with different models for the prompt emission of
GRBs at these energies, but they favor synchrotron radiation from a
relativistic outflow with a magnetic field which is coherent on an angular size
comparable with the angular size of the emitting region (~1/Gamma) . Indeed
this model has the best capabilities to maintain a high polarization level, and
to produce the observed variability.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journal Letter
The transmembrane protein bacterioopsin affects the polarity of the hydrophobic core of the host lipid bilayer
AbstractInfluence of the transmembrane protein bacterioopsin (the retinal-free form of bacteriorhodopsin) on the polarity of egg-phosphatidylcholine bilayers was studied by means of a steady-state and time-resolved fluorescence approach exploiting the solvatochromic properties of the 2-anthroyl fluorophore. Introduced in phosphatidylcholine molecules in the form of 8-(2-anthroyl)octanoic acid, this fluorophore probed the hydrocarbon core of the lipid bilayer. As previously shown (E. Pérochon et al., Biochemistry 31 (1992) 7672–7682), water molecules were detected in this region of the terminal part of the lipid acyl chains. Their number was considerably reduced upon addition of bacterioopsin to the lipids. This was assessed by a blue shift in the fluorescence emission spectra of the probe and a marked decrease in the fractional population of fluorophores interacting with water, to the benefit of those experiencing a hydrophobic environment. In agreement with current theories, this decrease in the hydration of the bilayer may be linked to an increase in the acyl chain order and a decrease in the lateral diffusion coefficient of lipids near the protein. The data obtained at high protein concentration accounts for a protein/lipid interface which is much less hydrated than the hydrophobic core of a protein-free lipid bilayer
ROP18 Is a Rhoptry Kinase Controlling the Intracellular Proliferation of Toxoplasma gondii
Toxoplasma gondii is an obligate intracellular parasite for which the discharge of apical organelles named rhoptries is a key event in host cell invasion. Among rhoptry proteins, ROP2, which is the prototype of a large protein family, is translocated in the parasitophorous vacuole membrane during invasion. The ROP2 family members are related to protein-kinases, but only some of them are predicted to be catalytically active, and none of the latter has been characterized so far. We show here that ROP18, a member of the ROP2 family, is located in the rhoptries and re-localises at the parasitophorous vacuole membrane during invasion. We demonstrate that a recombinant ROP18 catalytic domain (amino acids 243–539) possesses a protein-kinase activity and phosphorylate parasitic substrates, especially a 70-kDa protein of tachyzoites. Furthermore, we show that overexpression of ROP18 in transgenic parasites causes a dramatic increase in intra-vacuolar parasite multiplication rate, which is correlated with kinase activity. Therefore, we demonstrate, to our knowledge for the first time, that rhoptries can discharge active protein-kinases upon host cell invasion, which can exert a long-lasting effect on intracellular parasite development and virulence
Synergistic role of micronemal proteins in Toxoplasma gondii virulence
Apicomplexan parasites invade cells by a unique mechanism involving discharge of secretory vesicles called micronemes. Microneme proteins (MICs) include transmembrane and soluble proteins expressing different adhesive domains. Although the transmembrane protein TRAP and its homologues are thought to bridge cell surface receptors and the parasite submembranous motor, little is known about the function of other MICs. We have addressed the role of MIC1 and MIC3, two soluble adhesins of Toxoplasma gondii, in invasion and virulence. Single deletion of the MIC1 gene decreased invasion in fibroblasts, whereas MIC3 deletion had no effect either alone or in the mic1KO context. Individual disruption of MIC1 or MIC3 genes slightly reduced virulence in the mouse, whereas doubly depleted parasites were severely impaired in virulence and conferred protection against subsequent challenge. Single substitution of two critical amino acids in the chitin binding–like (CBL) domain of MIC3 abolished MIC3 binding to cells and generated the attenuated virulence phenotype. Our findings identify the CBL domain of MIC3 as a key player in toxoplasmosis and reveal the synergistic role of MICs in virulence, supporting the idea that parasites have evolved multiple ligand–receptor interactions to ensure invasion of different cells types during the course of infection
Splendeur du béton : les prédécesseurs et l'oeuvre d'Auguste Perret
Referències bibliogràfiques. Índe
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