29 research outputs found
Non-relativistic Collisionless Shocks in Unmagnetized Electron-Ion Plasmas
We show that the Weibel-mediated collisionless shocks are driven at
non-relativistic propagation speed (0.1c < V < 0.45c) in unmagnetized
electron-ion plasmas by performing two-dimensional particle-in-cell
simulations. It is shown that the profiles of the number density and the mean
velocity in the vicinity of the shock transition region, which are normalized
by the respective upstream values, are almost independent of the upstream bulk
velocity, i.e., the shock velocity. In particular, the width of the shock
transition region is ~100 ion inertial length independent of the shock
velocity. For these shocks the energy density of the magnetic field generated
by the Weibel-type instability within the shock transition region reaches
typically 1-2% of the upstream bulk kinetic energy density. This mechanism
probably explains the robust formation of collisionless shocks, for example,
driven by young supernova remnants, with no assumption of external magnetic
field in the universe.Comment: 4 pages, 7 figures, accepted for publication in ApJ Letter
The filamentation instability driven by warm electron beams: Statistics and electric field generation
The filamentation instability of counterpropagating symmetric beams of
electrons is examined with 1D and 2D particle-in-cell (PIC) simulations, which
are oriented orthogonally to the beam velocity vector. The beams are uniform,
warm and their relative speed is mildly relativistic. The dynamics of the
filaments is examined in 2D and it is confirmed that their characteristic size
increases linearly in time. Currents orthogonal to the beam velocity vector are
driven through the magnetic and electric fields in the simulation plane. The
fields are tied to the filament boundaries and the scale size of the
flow-aligned and the perpendicular currents are thus equal. It is confirmed
that the electrostatic and the magnetic forces are equally important, when the
filamentation instability saturates in 1D. Their balance is apparently the
saturation mechanism of the filamentation instability for our initial
conditions. The electric force is relatively weaker but not negligible in the
2D simulation, where the electron temperature is set higher to reduce the
computational cost. The magnetic pressure gradient is the principal source of
the electrostatic field, when and after the instability saturates in the 1D
simulation and in the 2D simulation.Comment: 10 pages, 6 figures, accepted by the Plasma Physics and Controlled
Fusion (Special Issue EPS 2009
Large-scale magnetic field generation via the kinetic Kelvin-Helmholtz instability in unmagnetized scenarios
Collisionless plasma instabilities are fundamental in magnetic field
generation in astrophysical scenarios, but their role has been addressed in
scenarios where velocity shear is absent. In this work we show that velocity
shears must be considered when studying realistic astrophysical scenarios,
since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We
present the first self-consistent three-dimensional (3D) particle-in-cell (PIC)
simulations of the KHI in conditions relevant for unmagnetized relativistic
outflows with velocity shear, such as active galactic nuclei (AGN) and
gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC
magnetic field, which extends over the entire shear-surface, reaching
thicknesses of a few tens of electron skin depths, and persisting on
time-scales much longer than the electron time scale. This DC magnetic field is
not captured by MHD models since it arises from intrinsically kinetic effects.
Our results indicate that the KHI can generate intense magnetic fields yielding
equipartition values up to \epsilon_B/\epsilon_p ~ 10^-3 in the electron
time-scale. The KHI-induced magnetic fields have a characteristic structure
that will lead to a distinct radiation signature, and can seed the turbulent
dynamo amplification process. The dynamics of the KHI are relevant for
non-thermal radiation modeling and can also have a strong impact on the
formation of relativistic shocks in presence of velocity shears
GRB spectral parameters within the fireball model
Fireball model of the GRBs predicts generation of numerous internal shocks,
which then efficiently accelerate charged particles and generate magnetic and
electric fields. These fields are produced in the form of relatively
small-scale stochastic ensembles of waves, thus, the accelerated particles
diffuse in space due to interaction with the random waves and so emit so called
Diffusive Synchrotron Radiation (DSR) in contrast to standard synchrotron
radiation they would produce in a large-scale regular magnetic fields. In this
paper we present first results of comprehensive modeling of the GRB spectral
parameters within the fireball/internal shock concept. We have found that the
non-perturbative DSR emission mechanism in a strong random magnetic field is
consistent with observed distributions of the Band parameters and also with
cross-correlations between them; this analysis allowed to restrict GRB physical
parameters from the requirement of consistency between the model and observed
distributions.Comment: 14 pages, 17 figures, MNRAS in pres
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
Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock
The prompt emissions of gamma-ray bursts are seeded by radiating
ultrarelativistic electrons. Internal shocks propagating through a jet launched
by a stellar implosion, are expected to amplify the magnetic field & accelerate
electrons. We explore the effects of density asymmetry & a quasi-parallel
magnetic field on the collision of plasma clouds. A 2D relativistic PIC
simulation models the collision of two plasma clouds, in the presence of a
quasi-parallel magnetic field. The cloud density ratio is 10. The densities of
ions & electrons & the temperature of 131 keV are equal in each cloud. The mass
ratio is 250. The peak Lorentz factor of the electrons is determined, along
with the orientation & strength of the magnetic field at the cloud collision
boundary. The magnetic field component orthogonal to the initial plasma flow
direction is amplified to values that exceed those expected from shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification, caused by a current sheet which
develops in response to the differing deflection of the incoming upstream
electrons & ions. The electron deflection implies a charge separation of the
upstream electrons & ions; the resulting electric field drags the electrons
through the magnetic field, whereupon they acquire a relativistic mass
comparable to the ions. We demonstrate how a magnetic field structure
resembling the cross section of a flux tube grows in the current sheet of the
shock transition layer. Plasma filamentation develops, as well as signatures of
orthogonal magnetic field striping. Localized magnetic bubbles form. Energy
equipartition between the ion, electron & magnetic energy is obtained at the
shock transition layer. The electronic radiation can provide a seed photon
population that can be energized by secondary processes (e.g. inverse Compton).Comment: 12 pages, 15 Figures, accepted to A&
The Generation of Magnetic Fields and X-ray Observations
We show that strong magnetic fields can be generated at shock waves
associated with formation of galaxies or clusters of galaxies by the Weibel
instability, an instability in collisionless plasmas. The estimated strength of
the magnetic field generated through this mechanism is close to the order of
values observed in galaxies or clusters of galaxies at present, which indicates
that strong amplification of magnetic fields after formation of galaxies or
clusters of galaxies is not required. This mechanism could have worked even at
a redshift of ~10, and therefore the generated magnetic fields may have
affected the formation of stars at the early universe. This model will be
confirmed by future observations of nearby clusters of galaxies. In this
context, we also present the Japanese X-ray missions.Comment: Invited review, 6 pages, accepted by Astronomische Nachrichten
(proceedings of "The Origin and Evolution of Cosmic Magnetism", 29 August - 2
September 2005, Bologna, Italy
Eigenmodes and growth rates of relativistic current filamentation instability in a collisional plasma
I theoretically found eigenmodes and growth rates of relativistic current
filamentation instability in collisional regimes, deriving a generalized
dispersion relation from self-consistent beam-Maxwell equations. For
symmetrically counterstreaming, fully relativistic electron currents, the
collisional coupling between electrons and ions creates the unstable modes of
growing oscillation and wave, which stand out for long-wavelength
perturbations. In the stronger collisional regime, the growing oscillatory mode
tends to be dominant for all wavelengths. In the collisionless limit, those
modes vanish, while maintaining another purely growing mode that exactly
coincides with a standard relativistic Weibel mode. It is also shown that the
effects of electron-electron collisions and thermal spread lower the growth
rate of the relativistic Weibel instability. The present mechanisms of
filamentation dynamics are essential for transport of homogeneous electron beam
produced by the interaction of high power laser pulses with plasma.Comment: 44 pages, 12 figures. Accepted for publication in Phys. Rev.
Gamma-Ray Bursts: The Underlying Model
A pedagogical derivation is presented of the ``fireball'' model of gamma-ray
bursts, according to which the observable effects are due to the dissipation of
the kinetic energy of a relativistically expanding wind, a ``fireball.'' The
main open questions are emphasized, and key afterglow observations, that
provide support for this model, are briefly discussed. The relativistic outflow
is, most likely, driven by the accretion of a fraction of a solar mass onto a
newly born (few) solar mass black hole. The observed radiation is produced once
the plasma has expanded to a scale much larger than that of the underlying
``engine,'' and is therefore largely independent of the details of the
progenitor, whose gravitational collapse leads to fireball formation. Several
progenitor scenarios, and the prospects for discrimination among them using
future observations, are discussed. The production in gamma- ray burst
fireballs of high energy protons and neutrinos, and the implications of burst
neutrino detection by kilometer-scale telescopes under construction, are
briefly discussed.Comment: In "Supernovae and Gamma Ray Bursters", ed. K. W. Weiler, Lecture
Notes in Physics, Springer-Verlag (in press); 26 pages, 2 figure
Diffusive Synchrotron Radiation from Pulsar Wind Nebulae
Diffusive Synchrotron Radiation (DSR) is produced by charged particles as
they random walk in a stochastic magnetic field. The spectrum of the radiation
produced by particles in such fields differs substantially from those of
standard synchrotron emission because the corresponding particle trajectories
deviate significantly from gyration in a regular field. The Larmor radius,
therefore, is no longer a good measure of the particle trajectory. In this
paper we analyze a special DSR regime which arises as highly relativistic
electrons move through magnetic fields which have only random structure on a
wide range of spatial scales. Such stochastic fields arise in turbulent
processes, and are likely present in pulsar wind nebulae (PWNe). We show that
DSR generated by a single population of electrons can reproduce the observed
broad-band spectra of PWNe from the radio to the X-ray, in particular producing
relatively flat spectrum radio emission as is usually observed in PWNe. DSR can
explain the existence of several break frequencies in the broad-band emission
spectrum without recourse to breaks in the energy spectrum of the relativistic
particles. The shape of the radiation spectrum depends on the spatial spectrum
of the stochastic magnetic field. The implications of the presented DSR regime
for PWN physics are discussed.Comment: 15 pages, 5 figures, accepted to MNRA