1,379 research outputs found
Multifractals Competing with Solitons on Fibonacci Optical Lattice
We study the stationary states for the nonlinear Schr\"odinger equation on
the Fibonacci lattice which is expected to be realized by Bose-Einstein
condensates loaded into an optical lattice. When the model does not have a
nonlinear term, the wavefunctions and the spectrum are known to show fractal
structures. Such wavefunctions are called critical. We present a phase diagram
of the energy spectrum for varying the nonlinearity. It consists of three
portions, a forbidden region, the spectrum of critical states, and the spectrum
of stationary solitons. We show that the energy spectrum of critical states
remains intact irrespective of the nonlinearity in the sea of a large number of
stationary solitons.Comment: 5 pages, 4 figures, major revision, references adde
Particle Acceleration, Magnetic Field Generation, and Associated Emission in Collisionless Relativistic Jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray
bursts (GRBs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations using injected relativistic electron-ion
(electro-positron) jets show that acceleration occurs within the downstream
jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
other two-streaming instability, and the Weibel instability) created in the
shocks are responsible for particle (electron, positron, and ion) acceleration.
The simulation results show that the Weibel instability is responsible for
generating and amplifying highly nonuniform, small-scale magnetic fields. These
magnetic fields contribute to the electron's transverse deflection behind the
jet head. The ``jitter'' radiation from deflected electrons has different
properties than synchrotron radiation which assumes a uniform magnetic field.
This jitter radiation may be important to understanding the complex time
evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and
supernova remnants.Comment: 4 pages, 3 figures, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007.
Fig. 3 is replaced by the correct versio
3-D GRMHD and GRPIC Simulations of Disk-Jet Coupling and Emission
We investigate jet formation in black-hole systems using 3-D General
Relativistic Particle-In-Cell (GRPIC) and 3-D GRMHD simulations. GRPIC
simulations, which allow charge separations in a collisionless plasma, do not
need to invoke the frozen condition as in GRMHD simulations. 3-D GRPIC
simulations show that jets are launched from Kerr black holes as in 3-D GRMHD
simulations, but jet formation in the two cases may not be identical.
Comparative study of black hole systems with GRPIC and GRMHD simulations with
the inclusion of radiate transfer will further clarify the mechanisms that
drive the evolution of disk-jet systems.Comment: 3 pages, 1 figure, Proceedings of the Eleventh Marcel Grossmann
Meeting on General Relativity, edited by H. Kleinert, R.T. Jantzen and R.
Ruffini, World Scientific, Singapore, 200
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: 19 pages,7 figures, contributed talk at Seventh European Workshop on
Collisionless Shocks, Paris, 7- 9 November 2007. High resolution version can
be obtained at http://gammaray.nsstc.nasa.gov/~nishikawa/shockws07.pd
Particle acceleration in electron-ion jets
Weibel instability created in collisionless shocks is 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 electron-ion jet fronts propagating
into an ambient plasma without initial magnetic fields with a longer simulation
system in order to investigate nonlinear stage of the Weibel instability and
its acceleration mechanism. The current channels generated by the Weibel
instability induce the radial electric fields. The z component of the Poynting
vector (E x B) become positive in the large region along the jet propagation
direction. This leads to the acceleration of jet electrons along the jet. In
particular the E x B drift with the large scale current channel generated by
the ion Weibel instability accelerate electrons effectively in both parallel
and perpendicular directions.Comment: 2 pages, 1 figure, Proceedings for Astrophysical Sources of High
Energy Particles and Radiation, AIP proceeding Series, eds . T. Bulik, G.
Madejski and B. Ruda
Radiation from relativistic jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations of relativistic electron-ion
(electron-positron) jets injected into a stationary medium show that particle
acceleration occurs within the downstream jet. In the presence of relativistic
jets, instabilities such as the Buneman instability, other two-streaming
instability, and the Weibel (filamentation) instability create collisionless
shocks, which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability is
responsible for generating and amplifying highly nonuniform, small-scale
magnetic fields. These magnetic fields contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons in small-scale magnetic fields has different properties than
synchrotron radiation which is calculated in a uniform magnetic field. This
jitter radiation, a case of diffusive synchrotron radiation, may be important
to understand the complex time evolution and/or spectral structure in gamma-ray
bursts, relativistic jets, and supernova remnants.Comment: 8 pages,3 figures, accepted for the Proceedings of Science of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, April 22
to 25, 200
Deterministic Weak Localization in Periodic Structures
The weak localization is found for perfect periodic structures exhibiting
deterministic classical diffusion. In particular, the velocity autocorrelation
function develops a universal quantum power law decay at 4 times Ehrenfest
time, following the classical stretched-exponential type decay. Such
deterministic weak localization is robust against weak enough randomness (e.g.,
quantum impurities). In the 1D and 2D cases, we argue that at the quantum limit
states localized in the Bravis cell are turned into Bloch states by quantum
tunnelling.Comment: 5 pages, 2 figure
Binary collisions of charged particles in a magnetic field
Binary collisions between charged particles in an external magnetic field are
considered in second-order perturbation theory, starting from the unperturbed
helical motion of the particles. The calculations are done with the help of an
improved binary collisions treatment which is valid for any strength of the
magnetic field, where the second-order energy and velocity transfers are
represented in Fourier space for arbitrary interaction potentials. The energy
transfer is explicitly calculated for a regularized and screened potential
which is both of finite range and non-singular at the origin, and which
involves as limiting cases the Debye (i.e., screened) and Coulomb potential.
Two distinct cases are considered in detail. (i) The collision of two identical
(e.g., electron-electron) particles; (ii) and the collision between a
magnetized electron and an uniformly moving heavy ion. The energy transfer
involves all harmonics of the electron cyclotron motion. The validity of the
perturbation treatment is evaluated by comparing with classical trajectory
Monte--Carlo calculations which also allows to investigate the strong
collisions with large energy and velocity transfer at low velocities. For large
initial velocities on the other hand, only small velocity transfers occur.
There the non-perturbative numerical classical trajectory Monte--Carlo results
agree excellently with the predictions of the perturbative treatment.Comment: submitted to Phys. Rev.
General Relativistic MHD Simulations of Jet Formation
We have performed 3-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of jet formation from an accretion disk with/without initial perturbation around a rotating black hole. We input a sinusoidal perturbation (m = 5 mode) in the rotation velocity of the accretion disk. The simulation results show the formation of a relativistic jet from the accretion disk. Although the initial perturbation becomes weakened by the coupling among different modes, it survives and triggers lower modes. As a result, complex non-axisymmetric density structure develops in the disk and the jet. Newtonian MHD simulations of jet formation with a non-axisymmetric mode show the growth of the m = 2 mode but GRMHD simulations cannot see the clear growth of the m = 2 mode
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