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 $\tilde B/\langle B\rangle \sim 1$ 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

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 $N$-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 $\gamma$-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) $\gamma$-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 $P(\nu)$ for pure jitter radiation is well-described by a sharply broken power-law: $P(\nu)\propto\nu^1$ for $\nu<\nu_j$ and $P(\nu)\propto\nu^{-(p-1)/2}$ for $\nu>\nu_j$, where $p$ is the electron power-law index and $\nu_j$ 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

A key to the spectral variability of prompt GRBs

We demonstrate that the rapid spectral variability of prompt GRBs is an inherent property of radiation emitted from shock-generated, highly anisotropic small-scale magnetic fields. We interpret the hard-to-soft evolution and the correlation of the soft index $\alpha$ with the photon flux observed in GRBs as a combined effect of temporal variation of the shock viewing angle and relativistic aberration of an individual thin, instantaneously illuminated shell. The model predicts that about a quarter of time-resolved spectra should have hard spectra, violating the synchrotron $\alpha=-2/3$ limit. The model also naturally explains why the peak of the distribution of $\alpha$ is at $\alpha\sim-1$. The presence of a low-energy break in the jitter spectrum at oblique angles also explains the appearance of a soft X-ray component in some GRBs and their paucity. We emphasize that our theory is based solely on the first principles and contains no ad hoc (phenomenological) assumptions.Comment: 4 pages, Proceedings of "Swift-05" meeting, Washington, D