912 research outputs found
Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock
Plasma instabilities (e.g., Buneman, Weibel and other two-stream
instabilities) excited in collisionless shocks are responsible for particle
(electron, positron, and ion) acceleration. Using a new 3-D relativistic
particle-in-cell code, we have investigated the particle acceleration and shock
structure associated with an unmagnetized relativistic electron-positron jet
propagating into an unmagnetized electron-positron plasma. The simulation has
been performed using a long simulation system in order to study the nonlinear
stages of the Weibel instability, the particle acceleration mechanism, and the
shock structure. Cold jet electrons are thermalized and slowed while the
ambient electrons are swept up to create a partially developed hydrodynamic
(HD) like shock structure. In the leading shock, electron density increases by
a factor of 3.5 in the simulation frame. Strong electromagnetic fields are
generated in the trailing shock and provide an emission site. We discuss the
possible implication of our simulation results within the AGN and GRB context.Comment: 4 pages, 3 figures, ApJ Letters, in pres
Magnetic Field Generation in Core-Sheath Jets via the Kinetic Kelvin-Helmholtz Instability
We have investigated magnetic field generation in velocity shears via the
kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet
core and stationary plasma sheath. Our three-dimensional particle-in-cell
simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15
for both electron-proton and electron-positron plasmas. For electron-proton
plasmas we find generation of strong large-scale DC currents and magnetic
fields which extend over the entire shear-surface and reach thicknesses of a
few tens of electron skin depths. For electron-positron plasmas we find
generation of alternating currents and magnetic fields. Jet and sheath plasmas
are accelerated across the shear surface in the strong magnetic fields
generated by the kKHI. The mixing of jet and sheath plasmas generates
transverse structure similar to that produced by the Weibel instability.Comment: 28 pages, 12 figures, in press, ApJ, September 10, 201
Radiation spectra and polarization in magnetar bursts
We present Monte Carlo simulations of radiative transfer in magnetar
atmospheres. We include the effects of vacuum polarization, electron and proton
scattering, and free-free absorption. Simulations are performed for the
atmosphere model with the magnetic field perpendicular and also tilted with
respect to the neutron star surface, and we show that the average spectrum does
not strongly depend on the orientation of the magnetic field. We investigate
the region of the parameter space where the vacuum absorption-like feature
appears in the spectrum and we analyze the shape of the proton cyclotron line.
Our results indicate that the existence of the vacuum polarization feature
should be a general attribute of soft gamma-ray repeaters burst spectra,
provided that the energy release takes place at the sufficiently dense region,
and the atmosphere scaleheight is large enough. We discuss the existence of
such a feature in recent observational data on these sources.Comment: submitted to Ap
Radiation from relativistic jets in turbulent magnetic fields
Using our new 3-D relativistic electromagnetic particle (REMP) code
parallelized with MPI, we have investigated long-term particle acceleration
associated with an relativistic electron-positron jet propagating in an
unmagnetized ambient electron-positron plasma. The simulations have been
performed using a much longer simulation system than our previous simulations
in order to investigate the full nonlinear stage of the Weibel instability and
its particle acceleration mechanism. Cold jet electrons are thermalized and
ambient electrons are accelerated in the resulting shocks. The acceleration of
ambient electrons leads to a maximum ambient electron density three times
larger than the original value. Behind the bow shock in the jet shock strong
electromagnetic fields are generated. These fields may lead to the afterglow
emission. We have calculated the time evolution of the spectrum from two
electrons propagating in a uniform parallel magnetic field to verify the
technique.Comment: 3 pages, 2 figures, submitted for the Proceedings of The Sixth
Huntsville Gamma-Ray Burst Symposium 2008, Huntsville, AL, October 20-23,
200
Mixed quark-nucleon phase in neutron stars and nuclear symmetry energy
The influence of the nuclear symmetry energy on the formation of a mixed
quark-nucleon phase in neutron star cores is studied. We use simple
parametrizations of the nuclear matter equation of state, and the bag model for
the quark phase. The behavior of nucleon matter isobars, which is responsible
for the existence of the mixed phase, is investigated. The role of the nuclear
symmetry energy changes with the value of the bag constant B. For lower values
of B the properties of the mixed phase do not depend strongly on the symmetry
energy. For larger B we find that a critical pressure for the first quark
droplets to form is strongly dependent on the nuclear symmetry energy, but the
pressure at which last nucleons disappear is independent of it.Comment: 12 pages, 16 figures, Phys. Rev. C in pres
New Relativistic Particle-In-Cell Simulation Studies of Prompt and Early Afterglows from GRBs
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and microquasars commonly exhibit power-law emission spectra.
Recent PIC simulations of relativistic electron-ion (or electron-positron) jets
injected into a stationary medium show that particle acceleration occurs within
the downstream jet. In collisionless, relativistic shocks, particle (electron,
positron, and ion) acceleration is due to plasma waves and their associated
instabilities (e.g., the Weibel (filamentation) instability) created in the
shock region. The simulations show that the Weibel instability is responsible
for generating and amplifying highly non-uniform, small-scale magnetic fields.
These fields contribute to the electron's transverse deflection behind the jet
head. The resulting "jitter" radiation from deflected electrons has different
properties compared to synchrotron radiation, which assumes a uniform magnetic
field. Jitter radiation may be important for understanding the complex time
evolution and/or spectra in gamma-ray bursts, relativistic jets in general, and
supernova remnants.Comment: : 4 pages, 1 figure and 1 table, typos are corrected, submitted for
the Proceedings of The 4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, German
Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability
We have investigated generation of magnetic fields associated with velocity
shear between an unmagnetized relativistic jet and an unmagnetized sheath
plasma. We have examined the strong magnetic fields generated by kinetic shear
(Kelvin-Helmholtz) instabilities. Compared to the previous studies using
counter-streaming performed by Alves et al. (2012), the structure of KKHI of
our jet-sheath configuration is slightly different even for the global
evolution of the strong transverse magnetic field. In our simulations the major
components of growing modes are the electric field and the magnetic
field . After the component is excited, an induced
electric field becomes significant. However, other field components
remain small. We find that the structure and growth rate of KKHI with mass
ratios and are similar.
In our simulations saturation in the nonlinear stage is not as clear as in
counter-streaming cases. The growth rate for a mildly-relativistic jet case
() is larger than for a relativistic jet case
().Comment: 6 pages, 6 figures, presented at Dynamical processes in space plasmas
II, Isradinamic 2012, in press, ANGEO. arXiv admin note: text overlap with
arXiv:1303.256
The transport of cosmic rays in self-excited magnetic turbulence
The process of diffusive shock acceleration relies on the efficacy with which
hydromagnetic waves can scatter charged particles in the precursor of a shock.
The growth of self-generated waves is driven by both resonant and non-resonant
processes. We perform high-resolution magnetohydrodynamic simulations of the
non-resonant cosmic-ray driven instability, in which the unstable waves are
excited beyond the linear regime. In a snapshot of the resultant field,
particle transport simulations are carried out. The use of a static snapshot of
the field is reasonable given that the Larmor period for particles is typically
very short relative to the instability growth time. The diffusion rate is found
to be close to, or below, the Bohm limit for a range of energies. This provides
the first explicit demonstration that self-excited turbulence reduces the
diffusion coefficient and has important implications for cosmic ray transport
and acceleration in supernova remnants.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Radiation from accelerated particles in relativistic jets with shocks, shear-flow, and reconnection
We have investigated particle acceleration and shock structure associated
with an unmagnetized relativistic jet propagating into an unmagnetized plasma.
Strong magnetic fields generated in the trailing jet shock lead to transverse
deflection and acceleration of the electrons. We have self-consistently
calculated the radiation from the electrons accelerated in the turbulent
magnetic fields. We find that the synthetic spectra depend on the bulk Lorentz
factor of the jet, the jet temperature, and the strength of the magnetic fields
generated in the shock. We have also begun study of electron acceleration in
the strong magnetic fields generated by kinetic shear (Kelvin-Helmholtz)
instabilities. Our calculated spectra should lead to a better understanding of
the complex time evolution and/or spectral structure from gamma-ray bursts,
relativistic jets, and supernova remnants.Comment: 6 pages, 4 figures, 2012 Fermi Symposium proceedings - eConf C12102
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