3,024 research outputs found
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
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
Self-Interacting Dark Matter with Flavor Mixing
The crisis of both cold and collisional dark matter (DM) models is resolved
by postulating flavor mixing of DM particles. Flavor-mixed particles segregate
in the gravitational field to form dark halos composed of heavy mass
eigenstates. Since these particles are mixed in the interaction basis, elastic
collisions convert some of heavy eigenstates into light ones which leave the
halo. This annihilation-like process will soften dense central cusps of halos.
The proposed model accumulates most of the attractive features of
self-interacting and annihilating DM models, but does not suffer from their
severe drawbacks. This model is natural does not require fine tuning.Comment: TeX, 4 pages. Submitted to PRL. Other works are at
http://www.cita.utoronto.ca/~medvedev
Cosmological Simulations of Multi-Component Cold Dark Matter
The nature of dark matter is unknown. A number of dark matter candidates are
quantum flavor-mixed particles but this property has never been accounted for
in cosmology. Here we explore this possibility from the first principles via
extensive -body cosmological simulations and demonstrate that the
two-component dark matter model agrees with observational data at all scales.
Substantial reduction of substructure and flattening of density profiles in the
centers of dark matter halos found in simulations can simultaneously resolve
several outstanding puzzles of modern cosmology. The model shares the "why
now?" fine-tuning caveat pertinent to all self-interacting models. Predictions
for direct and indirect detection dark matter experiments are made.Comment: 6 pages, 4 figure
Physics of Collisionless GRB Shocks and Their Radiation Properties
We present a theory of ultrarelativistic collisionless shocks based on the
relativistic kinetic two-stream instability. We demonstrate that the shock
front is unstable to the generation of small-scale, randomly tangled magnetic
fields. These fields are strong enough to scatter the energetic incoming (in
the shock frame) protons and electrons over pitch angle and, therefore, to
convert their kinetic energy of bulk motion into heat with very high
efficiency. This validates the use of MHD approximation and the shock jump
conditions in particular. The effective collisions are also necessary for the
diffusive Fermi acceleration of electrons to operate and produce an observed
power-law. Finally, these strong (sub-equipartition) magnetic fields are also
required for the efficient synchrotron-type radiation emission from the shocks.
The predicted magnetic fields have an impact on polarization properties of
the observed radiation (e.g., a linear polarization from a jet-like ejecta and
polarization scintillations in radio for a spherical one) and on its spectrum.
We present an analytical theory of jitter radiation, which is emitted when the
magnetic field is correlated on scales smaller then the gyration (Larmor)
radius of the accelerated electrons. A composite jitter+synchrotron model of
GRB -ray emission from internal shocks is capable of resolving many
puzzles of GRB spectra, such as the violation of the ``line of death'', sharp
spectral breaks, and multiple spectral components seen in some bursts (good
examples are GRB910503, GRB910402, etc.). We stress that simultaneous detection
of both spectral components opens a way to a precise diagnostics of the
conditions in GRB shocks. We also discuss the relation of our results to other
systems, such as internal shocks in blazars, radio lobes, and supernova shocks.Comment: 5 pages, 4 figures. To appear in proceedings of the Gamma 2001
symposiu
Generation of Magnetic Fields and Jitter Radiation in GRBs. I. Kinetic Theory
We present a theory of generation of strong (sub-equipartition) magnetic
fields in relativistic collisionless GRB shocks. These fields produced by the
kinetic two-stream instability are tangled on very small spatial scales. This
has a clear signature in the otherwise synchrotron(-self-Compton) -ray
spectrum. Second, we present an analytical theory of jitter radiation, which is
emitted when the correlation length of the magnetic field is smaller then the
gyration (Larmor) radius of the accelerated electrons. We demonstrate that the
spectral power for pure jitter radiation is well-described by a
sharply broken power-law: for and
for , where is the electron
power-law index and is the jitter break, which is independent of the
magnetic field strength and depends on the shock energetics and kinematics.
Here we mostly focus on the first problem. The radiation theory and comparison
with observations will be discussed in the forthcoming publications.Comment: 3 pages with 2 eps figures, aipproc.sty. To appear in Proceedings of
the 20th Texas Symposium on Relativistic Astrophysics, Austin, Texas, 2000,
edited by J. Craig Wheeler and Hugo Martel (American Institute of Physics
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