9,901 research outputs found
Radiation Spectral Synthesis of Relativistic Filamentation
Radiation from many astrophysical sources, e.g. gamma-ray bursts and active
galactic nuclei, is believed to arise from relativistically shocked
collisionless plasmas. Such sources often exhibit highly transient spectra
evolving rapidly, compared with source lifetimes. Radiation emitted from these
sources is typically associated with non-linear plasma physics, complex field
topologies and non-thermal particle distributions. In such circumstances a
standard synchrotron paradigm may fail to produce accurate conclusions
regarding the underlying physics. Simulating spectral emission and spectral
evolution numerically in various relativistic shock scenarios is then the only
viable method to determine the detailed physical origin of the emitted spectra.
In this Letter we present synthetic radiation spectra representing the early
stage development of the filamentation (streaming) instability of an initially
unmagnetized plasma, which is relevant for both collisionless shock formation
and reconnection dynamics in relativistic astrophysical outflows, as well as
for laboratory astrophysics experiments. Results were obtained using a highly
efficient "in situ" diagnostics method, based on detailed particle-in-cell
modeling of collisionless plasmas. The synthetic spectra obtained here are
compared with those predicted by a semi-analytical model for jitter radiation
from the filamentation instability, the latter including self-consistent
generated field topologies and particle distributions obtained from the
simulations reported upon here. Spectra exhibit dependence on the presence - or
absence - of an inert plasma constituent, when comparing baryonic plasmas (i.e.
containing protons) with pair plasmas. The results also illustrate that
considerable care should be taken when using lower-dimensional models to obtain
information about the astrophysical phenomena generating observed spectra.Comment: 5 pages, 5 figures, accepted in Astrophysical Journal Letter
Angular Dependence of Jitter Radiation Spectra from Small-Scale Magnetic Turbulence
Jitter radiation is produced by relativistic electrons moving in turbulent
small-scale magnetic fields such as those produced by streaming Weibel-type
instabilities at collisionless shocks in weakly magnetized media. Here we
present a comprehensive study of the dependence of the jitter radiation spectra
on the properties of, in general, anisotropic magnetic turbulence. We have
obtained that the radiation spectra do reflect, to some extent, properties of
the magnetic field spatial distribution, yet the radiation field is anisotropic
and sensitive to the viewing direction with respect to the field anisotropy
direction. We explore the parameter space of the magnetic field distribution
and its effect on the radiation spectrum. Some important results include: the
presence of the harder-than-synchrotron segment below the peak frequency at
some viewing angles, the presence of the high-frequency power-law tail even for
a monoenergetic distribution of electrons, the dependence of the peak frequency
on the field correlation length rather than the field strength, the strong
correlation of the spectral parameters with the viewing angle. In general, we
have found that even relatively minor changes in the magnetic field properties
can produce very significant effects upon the jitter radiation spectra. We
consider these results to be important for accurate interpretation of prompt
gamma-ray burst spectra and possibly other sources.Comment: 75 pages, 29 figures, submitted to Ap
Magnetic fields and cosmic rays in GRBs. A self-similar collisionless foreshock
Cosmic rays accelerated by a shock form a streaming distribution of outgoing
particles in the foreshock region. If the ambient fields are negligible
compared to the shock and cosmic ray energetics, a stronger magnetic field can
be generated in the shock upstream via the streaming (Weibel-type) instability.
Here we develop a self-similar model of the foreshock region and calculate its
structure, e.g., the magnetic field strength, its coherence scale, etc., as a
function of the distance from the shock. Our model indicates that the entire
foreshock region of thickness , being comparable
to the shock radius in the late afterglow phase when ,
can be populated with large-scale and rather strong magnetic fields (of
sub-gauss strengths with the coherence length of order )
compared to the typical interstellar medium magnetic fields. The presence of
such fields in the foreshock region is important for high efficiency of Fermi
acceleration at the shock. Radiation from accelerated electrons in the
foreshock fields can constitute a separate emission region radiating in the
UV/optical through radio band, depending on time and shock parameters. We also
speculate that these fields being eventually transported into the shock
downstream can greatly increase radiative efficiency of a gamma-ray burst
afterglow shock.Comment: 10 pages, 1 figure. Submitted to Ap
Asymmetric diffusion of cosmic rays
Cosmic ray propagation is diffusive because of pitch angle scattering by
waves. We demonstrate that if the high-amplitude magnetohydrodynamic turbulence
with is present on top of the mean field
gradient, the diffusion becomes asymmetric. As an example, we consider the
vertical transport of cosmic rays in our Galaxy propagating away from a
point-like source. We solve this diffusion problem analytically using a
one-dimensional Markov chain analysis. We obtained that the cosmic ray density
markedly differs from the standard diffusion prediction and has a sizable
effect on their distribution throughout the galaxy. The equation for the
continuous limit is also derived, which shows limitations of the
convection-diffusion equation.Comment: 6 pages, 7 figures. Submitted to Physics of Plasma
On NP-Hardness of the Paired de Bruijn Sound Cycle Problem
The paired de Bruijn graph is an extension of de Bruijn graph incorporating
mate pair information for genome assembly proposed by Mevdedev et al. However,
unlike in an ordinary de Bruijn graph, not every path or cycle in a paired de
Bruijn graph will spell a string, because there is an additional soundness
constraint on the path. In this paper we show that the problem of checking if
there is a sound cycle in a paired de Bruijn graph is NP-hard in general case.
We also explore some of its special cases, as well as a modified version where
the cycle must also pass through every edge.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
Gamma-ray Burst Prompt Emission: Jitter Radiation in Stochastic Magnetic Field Revisited
We revisit the radiation mechanism of relativistic electrons in the
stochastic magnetic field and apply it to the high-energy emissions of
gamma-ray bursts (GRBs). We confirm that jitter radiation is a possible
explanation for GRB prompt emission in the condition of a large electron
deflection angle. In the turbulent scenario, the radiative spectral property of
GRB prompt emission is decided by the kinetic energy spectrum of turbulence.
The intensity of the random and small-scale magnetic field is determined by the
viscous scale of the turbulent eddy. The microphysical parameters
and can be obtained. The acceleration and cooling timescales are
estimated as well. Due to particle acceleration in magnetized filamentary
turbulence, the maximum energy released from the relativistic electrons can
reach a value of about eV. The GeV GRBs are possible sources of
high-energy cosmic-ray.Comment: ApJ accepted, commments are welcom
Metrizable DH-spaces of the first category
We show that if a separable space X has a meager open subset containing a
copy of the Cantor set 2^\omega, then X has types of countable dense
subsets. We suggest a generalization of the \lambda-set for non-separable
spaces. Let X be an h-homogeneous \Lambda-set. Then X is densely homogeneous
and (X \setminus A) is homeomorphic to X for every \sigma-discrete A \subset X
Third Order ODEs Systems and Its Characteristic Connections
We compute the characteristic Cartan connection associated with a system of
third order ODEs. Our connection is different from Tanaka normal one, but still
is uniquely associated with the system of third order ODEs. This allows us to
find all fundamental invariants of a system of third order ODEs and, in
particular, determine when a system of third order ODEs is trivializable. As
application differential invariants of equations on circles in
are computed
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