554 research outputs found
Millimetre-VLBI Monitoring of AGN with Sub-milliarcsecond Resolution
Global millimetre VLBI allows detailed studies of the most central jet
regions of AGN with unprecedent spatial resolution of a few 100-1000
Schwartzschild radii to be made. Study of these regions will help to answer the
question how the highly relativistic AGN jets are launched and collimated.
Since the early 1990s, bright mm-sources have been observed with global 3 mm
VLBI. Here we present new images from an ongoing systematic analysis of the
available observations. In particular, we focus on the structure and structural
evolution of the best observed AGN jets, taking 3C 454.3 as a characteristic
example. This core-dominated and highly variable quasar shows a complex
morphology with individual jet components accelerating superluminally towards
the outer structure. We briefly discuss the X-ray properties of 3C 454.3 and
present its radio- to X-ray large-scale brightness distribution.Comment: 4 pages, 4 figures, Proceedings of the 7th EVN Symposium held in
Toledo, Spain in October 2004, needs evn2004.cl
Ferredoxin:NADP(H) Oxidoreductase Abundance and Location Influences Redox Poise and Stress Tolerance
In linear photosynthetic electron transport, ferredoxin:NADP(H) oxidoreductase (FNR) transfers electrons from ferredoxin (Fd) to NADP(+). Both NADPH and reduced Fd (Fd(red)) are required for reductive assimilation and light/dark activation/deactivation of enzymes. FNR is therefore a hub, connecting photosynthetic electron transport to chloroplast redox metabolism. A correlation between FNR content and tolerance to oxidative stress is well established, although the precise mechanism remains unclear. We investigated the impact of altered FNR content and localization on electron transport and superoxide radical evolution in isolated thylakoids, and probed resulting changes in redox homeostasis, expression of oxidative stress markers, and tolerance to high light in planta. Our data indicate that the ratio of Fd(red) to FNR is critical, with either too much or too little FNR potentially leading to increased superoxide production, and perception of oxidative stress at the level of gene transcription. In FNR overexpressing plants, which show more NADP(H) and glutathione pools, improved tolerance to high-light stress indicates that disturbance of chloroplast redox poise and increased free radical generation may help “prime” the plant and induce protective mechanisms. In fnr1 knock-outs, the NADP(H) and glutathione pools are more oxidized relative to the wild type, and the photoprotective effect is absent despite perception of oxidative stress at the level of gene transcription
A Possible Periodicity in the Radio Lightcurves of 3C454.3
During the period 1966.5 - 2006.2 the 15GHz and 8GHz lightcurves of 3C454.3
(z=0.859) show a qsasi-periodicity of ~12.8 yr (~6.9 yr in the rest frame of
the source) with a double-bump structure. This periodic behaviour is
interpreted in terms of a rotating double-jet model in which the two jets are
created from the black holes in a binary system and rotate with the period of
the orbital motion. The periodic variations in the radio fluxes of 3C454.3 are
suggested to be mainly due to the lighthouse effects (or the variation in
Doppler boosting) of the precessing jets which are caused by the orbital
motion. In addition, variations in the mass-flow rates accreting onto the black
holes may be also involved.Comment: 15 pages, 11 figure
Exploring Curved Superspace
We systematically analyze Riemannian manifolds M that admit rigid
supersymmetry, focusing on four-dimensional N=1 theories with a U(1)_R
symmetry. We find that M admits a single supercharge, if and only if it is a
Hermitian manifold. The supercharge transforms as a scalar on M. We then
consider the restrictions imposed by the presence of additional supercharges.
Two supercharges of opposite R-charge exist on certain fibrations of a
two-torus over a Riemann surface. Upon dimensional reduction, these give rise
to an interesting class of supersymmetric geometries in three dimensions. We
further show that compact manifolds admitting two supercharges of equal
R-charge must be hyperhermitian. Finally, four supercharges imply that M is
locally isometric to M_3 x R, where M_3 is a maximally symmetric space.Comment: 39 pages; minor change
Milestones in the Observations of Cosmic Magnetic Fields
Magnetic fields are observed everywhere in the universe. In this review, we
concentrate on the observational aspects of the magnetic fields of Galactic and
extragalactic objects. Readers can follow the milestones in the observations of
cosmic magnetic fields obtained from the most important tracers of magnetic
fields, namely, the star-light polarization, the Zeeman effect, the rotation
measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio
polarization observations, as well as the newly implemented sub-mm and mm
polarization capabilities.
(Another long paragraph is omitted due to the limited space here)Comment: Invited Review (ChJA&A); 32 pages. Sorry if your significant
contributions in this area were not mentioned. Published pdf & ps files (with
high quality figures) now availble at http://www.chjaa.org/2002_2_4.ht
Gravity duals of supersymmetric gauge theories on three-manifolds
We study gravity duals to a broad class of N=2 supersymmetric gauge theories
defined on a general class of three-manifold geometries. The gravity
backgrounds are based on Euclidean self-dual solutions to four-dimensional
gauged supergravity. As well as constructing new examples, we prove in general
that for solutions defined on the four-ball the gravitational free energy
depends only on the supersymmetric Killing vector, finding a simple closed
formula when the solution has U(1) x U(1) symmetry. Our result agrees with the
large N limit of the free energy of the dual gauge theory, computed using
localization. This constitutes an exact check of the gauge/gravity
correspondence for a very broad class of gauge theories with a large N limit,
defined on a general class of background three-manifold geometries.Comment: 74 pages, 2 figures; v2: minor change
Potassium tert-Butoxide-Catalyzed Dehydrogenative C–H Silylation of Heteroaromatics: A Combined Experimental and Computational Mechanistic Study
We recently reported a new method for the direct dehydrogenative C–H silylation of heteroaromatics utilizing Earth-abundant potassium tert-butoxide. Herein we report a systematic experimental and computational mechanistic investigation of this transformation. Our experimental results are consistent with a radical chain mechanism. A trialkylsilyl radical may be initially generated by homolytic cleavage of a weakened Si–H bond of a hypercoordinated silicon species as detected by IR, or by traces of oxygen which can generate a reactive peroxide by reaction with (KOt-Bu)_4 as indicated by density functional theory (DFT) calculations. Radical clock and kinetic isotope experiments support a mechanism in which the C–Si bond is formed through silyl radical addition to the heterocycle followed by subsequent β-hydrogen scission. DFT calculations reveal a reasonable energy profile for a radical mechanism and support the experimentally observed regioselectivity. The silylation reaction is shown to be reversible, with an equilibrium favoring products due to the generation of H_2 gas. In situ NMR experiments with deuterated substrates show that H_2 is formed by a cross-dehydrogenative mechanism. The stereochemical course at the silicon center was investigated utilizing a ^2H-labeled silolane probe; complete scrambling at the silicon center was observed, consistent with a number of possible radical intermediates or hypercoordinate silicates
Holographic renormalization and supersymmetry
Holographic renormalization is a systematic procedure for regulating
divergences in observables in asymptotically locally AdS spacetimes. For dual
boundary field theories which are supersymmetric it is natural to ask whether
this defines a supersymmetric renormalization scheme. Recent results in
localization have brought this question into sharp focus: rigid supersymmetry
on a curved boundary requires specific geometric structures, and general
arguments imply that BPS observables, such as the partition function, are
invariant under certain deformations of these structures. One can then ask if
the dual holographic observables are similarly invariant. We study this
question in minimal N = 2 gauged supergravity in four and five dimensions. In
four dimensions we show that holographic renormalization precisely reproduces
the expected field theory results. In five dimensions we find that no choice of
standard holographic counterterms is compatible with supersymmetry, which leads
us to introduce novel finite boundary terms. For a class of solutions
satisfying certain topological assumptions we provide some independent tests of
these new boundary terms, in particular showing that they reproduce the
expected VEVs of conserved charges.Comment: 70 pages; corrected typo
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