172 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
Synthetic Spectra from PIC Simulations of Relativistic Collisionless Shocks
We extract synthetic photon spectra from first-principles particle-in-cell
simulations of relativistic shocks propagating in unmagnetized pair plasmas.
The two basic ingredients for the radiation, namely accelerated particles and
magnetic fields, are produced self-consistently as part of the shock evolution.
We use the method of Hededal & Nordlund (2005) and compute the photon spectrum
via Fourier transform of the electric far-field from a large number of
particles, sampled directly from the simulation. We find that the spectrum from
relativistic collisionless shocks is entirely consistent with synchrotron
radiation in the magnetic fields generated by Weibel instability. We can
recover the so-called "jitter'' regime only if we artificially reduce the
strength of the electromagnetic fields, such that the wiggler parameter K = qB
lambda/mc^2 becomes much smaller than unity ("B" and "lambda" are the strength
and scale of the magnetic turbulence, respectively). These findings may place
constraints on the origin of non-thermal emission in astrophysics, especially
for the interpretation of the hard (harder than synchrotron) low-frequency
spectrum of Gamma-Ray Bursts.Comment: 5 pages, 3 figures, submitted to ApJ Letter
Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find small differences in the results for no ambient and
modest ambient magnetic fields. Simulations show that the Weibel instability
created in the collisionless shock front accelerates jet and ambient particles
both perpendicular and parallel to the jet propagation direction. The small
scale magnetic field structure generated by the Weibel instability is
appropriate to the generation of ``jitter'' radiation from deflected electrons
(positrons) as opposed to synchrotron radiation. The jitter radiation resulting
from small scale magnetic field structures may be important for understanding
the complex time structure and spectral evolution observed in gamma-ray bursts
or other astrophysical sources containing relativistic jets and relativistic
collisionless shocks.Comment: 6 pages, 1 figure, revised and accepted for Advances in Space
Research (35th COSPAR Scientific Assembly, Paris, 18-25 July 2004
In-situ Particle Acceleration in Collisionless Shocks
The outflows from gamma ray bursts, active galactic nuclei and relativistic
jets in general interact with the surrounding media through collisionless
shocks. With three dimensional relativistic particle-in-cell simulations we
investigate such shocks. The results from these experiments show that
small--scale magnetic filaments with strengths of up to percents of
equipartition are generated and that electrons are accelerated to power law
distributions N(E)~E^{-p} in the vicinity of the filaments through a new
acceleration mechanism. The acceleration is locally confined, instantaneous and
differs from recursive acceleration processes such as Fermi acceleration. We
find that the proposed acceleration mechanism competes with thermalization and
becomes important at high Lorentz factors.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004
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