6,201 research outputs found
Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?
We compute the effective values of apparent transverse velocity and flux
boosting factors for the VLBI radio knots of blazar jets, by integrating over
the angular distributions of these quantities across the widths of jets with
finite opening angles but constant velocities. For high bulk Lorentz factors
(Gamma > 10) variations across the jet can be quite large if the opening angle,
omega, is even a few degrees on sub-parsec scales. The resulting apparent
speeds are often much lower than those obtained from the usual analyses that
ignore the finite jet opening angles. We can thus reconcile the usually
observed subluminal or mildly superluminal speeds with the very high (>~ 20)
Gamma factors, required by the inverse Compton origin and rapid variability of
TeV fluxes, as well as by intraday radio variability. Thus it is possible to
associate the VLBI radio knots directly with shocks in the ultra-relativistic
main jet flow, without invoking very rapid jet deceleration on parsec scales,
or extremely unlikely viewing angles.Comment: 10 pages, 1 figure, to appear in ApJ Letters, Nov. 10 2004 issu
Coarse Graining RNA Nanostructures for Molecular Dynamics Simulations
A series of coarse-grained models have been developed for the study of the
molecular dynamics of RNA nanostructures. The models in the series have one to
three beads per nucleotide and include different amounts of detailed structural
information. Such a treatment allows us to reach, for the systems of thousands
of nucleotides, a time scale of microseconds (i.e. by three orders of magnitude
longer than in the full atomistic modelling) and thus to enable simulations of
large RNA polymers in the context of bionanotechnology. We find that the
3-beads-per-nucleotide models, described by a set of just a few universal
parameters, are able to describe different RNA conformations and are comparable
in structural precision to the models where detailed values of the backbone
P-C4' dihedrals taken from a reference structure are included. These findings
are discussed in the context of the RNA conformation classes
On the congruence subgroup problem: determination of the "Metaplectic Kernel"
This article does not have an abstract
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