2,770 research outputs found
Transport of Cosmic Rays in Chaotic Magnetic Fields
The transport of charged particles in disorganised magnetic fields is an
important issue which concerns the propagation of cosmic rays of all energies
in a variety of astrophysical environments, such as the interplanetary,
interstellar and even extra-galactic media, as well as the efficiency of Fermi
acceleration processes. We have performed detailed numerical experiments using
Monte-Carlo simulations of particle propagation in stochastic magnetic fields
in order to measure the parallel and transverse spatial diffusion coefficients
and the pitch angle scattering time as a function of rigidity and strength of
the turbulent magnetic component. We confirm the extrapolation to high
turbulence levels of the scaling predicted by the quasi-linear approximation
for the scattering frequency and parallel diffusion coefficient at low
rigidity. We show that the widely used Bohm diffusion coefficient does not
provide a satisfactory approximation to diffusion even in the extreme case
where the mean field vanishes. We find that diffusion also takes place for
particles with Larmor radii larger than the coherence length of the turbulence.
We argue that transverse diffusion is much more effective than predicted by the
quasi-linear approximation, and appears compatible with chaotic magnetic
diffusion of the field lines. We provide numerical estimates of the Kolmogorov
length and magnetic line diffusion coefficient as a function of the level of
turbulence. Finally we comment on applications of our results to astrophysical
turbulence and the acceleration of high energy cosmic rays in supernovae
remnants, in super-bubbles, and in jets and hot spots of powerful
radio-galaxies.Comment: To be published in Physical Review D, 20 pages 9 figure
Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C345
We investigate whether a pc-scale jet of 3C345 is dominated by a normal
plasma or an electron-positron plasma. We present a general condition that a
jet component becomes optically thick for synchrotron self-absorption, by
extending the method originally developed by Reynolds et al. The general
condition gives a lower limit of the electron number density, with the aid of
the surface brightness condition, which enables us to compute the magnetic
field density. Comparing the lower limit with another independent constraint
for the electron density that is deduced from the kinetic luminosity, we can
distinguish the matter content. We apply the procedure to the five components
of 3C345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes
at the peak frequencies were obtainable from literature. Evaluating the
representative values of Doppler beaming factors by their equipartition values,
we find that all the five components are likely dominated by an
electron-positron plasma. The conclusion does not depend on the lower cutoff
energy of the power-law distribution of radiating particles.Comment: 17 page
Rocaglates induce gain-of-function alterations to eIF4A and eIF4F
Rocaglates are a diverse family of biologically active molecules that have gained tremendous interest in recent years due to their promising activities in pre-clinical cancer studies. As a result, this family of compounds has been significantly expanded through the development of efficient synthetic schemes. However, it is unknown whether all of the members of the rocaglate family act through similar mechanisms of action. Here, we present a comprehensive study comparing the biological activities of >200 rocaglates to better understand how the presence of different chemical entities influences their biological activities. Through this, we find that most rocaglates preferentially repress the translation of mRNAs containing purine-rich 5' leaders, but certain rocaglates lack this bias in translation repression. We also uncover an aspect of rocaglate mechanism of action in which the pool of translationally active eIF4F is diminished due to the sequestration of the complex onto RNA.P50 GM067041 - NIGMS NIH HHS; R24 GM111625 - NIGMS NIH HHS; R35 GM118173 - NIGMS NIH HHSPublished versio
Relativistic parsec-scale jets: II. Synchrotron emission
We calculate the optically thin synchrotron emission of fast electrons and
positrons in a spiral stationary magnetic field and a radial electric field of
a rotating relativistic strongly magnetized force-free jet consisting of
electron-positron pair plasma. The magnetic field has a helical structure with
a uniform axial component and a toroidal component that is maximal inside the
jet and decreasing to zero towards the boundary of the jet. Doppler boosting
and swing of the polarization angle of synchrotron emission due to the
relativistic motion of the emitting volume are calculated. The distribution of
the plasma velocity in the jet is consistent with the electromagnetic field
structure. Two spatial distributions of fast particles are considered: uniform,
and concentrated in the vicinity of the Alfven resonance surface. The latter
distribution corresponds to the regular acceleration by an electromagnetic wave
in the vicinity of its Alfven resonance surface inside the jet. The
polarization properties of the radiation have been obtained and compared with
the existing VLBI polarization measurements of parsec-scale jets in BL Lac
sources and quasars. Our results give a natural explanation of the observed
bimodality in the alignment between the electric field vector of the polarized
radiation and the projection of the jet axis on the plane of the sky. We
interpret the motion of bright knots as a phase velocity of standing spiral
eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of
polarization and the velocity of the observed proper motion of bright knots
depend upon the angular rotational velocity of the jet. The observed
polarizations and velocities of knots indicate that the magnetic field lines
are bent in the direction opposite to the direction of the jet rotation.Comment: 14 pages, 5 figures, Astron. Astroph. in pres
Mass-losing accretion discs around supermassive black holes
We study the effects of outflow/wind on the gravitational stability of
accretion discs around supermassive black holes using a set of analytical
steady-state solutions. Mass-loss rate by the outflow from the disc is assumed
to be a power-law of the radial distance and the amount of the energy and the
angular momentum which are carried away by the wind are parameterized
phenomenologically. We show that the mass of the first clumps at the
self-gravitating radius linearly decreases with the total mass-loss rate of the
outflow. Except for the case of small viscosity and high accretion rate,
generally, the self-gravitating radius increases as the amount of mass-loss by
the outflow increases. Our solutions show that as more angular momentum is lost
by the outflow, then reduction to the mass of the first clumps is more
significant.Comment: Accepted for publication in Astrophysics & Space Scienc
Twinkling pulsar wind nebulae in the synchrotron cut-off regime and the gamma-ray flares in the Crab Nebula
Synchrotron radiation of ultra-relativistic particles accelerated in a pulsar
wind nebula may dominate its spectrum up to gamma-ray energies. Because of the
short cooling time of the gamma-ray emitting electrons, the gamma-ray emission
zone is in the immediate vicinity of the acceleration site. The particle
acceleration likely occurs at the termination shock of the relativistic striped
wind, where multiple forced magnetic field reconnections provide strong
magnetic fluctuations facilitating Fermi acceleration processes. The
acceleration mechanisms imply the presence of stochastic magnetic fields in the
particle acceleration region, which cause stochastic variability of the
synchrotron emission. This variability is particularly strong in the steep
gamma-ray tail of the spectrum, where modest fluctuations of the magnetic field
lead to strong flares of spectral flux. In particular, stochastic variations of
magnetic field, which may lead to quasi-cyclic gamma-ray flares, can be
produced by the relativistic cyclotron ion instability at the termination
shock. Our model calculations of the spectral and temporal evolution of
synchrotron emission in the spectral cut-off regime demonstrate that the
intermittent magnetic field concentrations dominate the gamma-ray emission from
highest energy electrons and provide fast, strong variability even for a
quasi-steady distribution of radiating particles. The simulated light curves
and spectra can explain the very strong gamma-ray flares observed in the Crab
nebula and the lack of strong variations at other wavelengths. The model
predicts high polarization in the flare phase, which can be tested with future
polarimetry observations.Comment: 5 pages, 3 figures, MNRAS in pres
Global climate models violate scaling of the observed atmospheric variability
We test the scaling performance of seven leading global climate models by
using detrended fluctuation analysis. We analyse temperature records of six
representative sites around the globe simulated by the models, for two
different scenarios: (i) with greenhouse gas forcing only and (ii) with
greenhouse gas plus aerosol forcing. We find that the simulated records for
both scenarios fail to reproduce the universal scaling behavior of the observed
records, and display wide performance differences. The deviations from the
scaling behavior are more pronounced in the first scenario, where also the
trends are clearly overestimated.Comment: Accepted for publishing in Physical Review Letter
Magnetic shear-driven instability and turbulent mixing in magnetized protostellar disks
Observations of protostellar disks indicate the presence of the magnetic
field of thermal (or superthermal) strength. In such a strong magnetic field,
many MHD instabilities responsible for turbulent transport of the angular
momentum are suppressed. We consider the shear-driven instability that can
occur in protostellar disks even if the field is superthermal. This instability
is caused by the combined influence of shear and compressibility in a
magnetized gas and can be an efficient mechanism to generate turbulence in
disks. The typical growth time is of the order of several rotation periods.Comment: 8 pages, 6 figures, A&A to appea
IJTC2009-15253 FILM FORMING CHARACTERISTICS OF OIL-IN-WATER EMULSIONS IN ELASTOHYDRODYNAMIC CONTACTS
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