Magnetic Reconnection in Turbulent Plasmas and Gamma Ray Bursts

We discuss how the model of magnetic reconnection in the presence of turbulence proposed inLazarian & Vishniac 1999 makes the reconnection rate independent either of resistivity or microscopic plasma effects, but determined entirely by the magnetic field line wandering induced by turbulence. We explain that the model accounts for both fast and slow regimes of reconnection and that this property naturally induces flares of reconnection in low beta plasma environments. In addition, we show that the model involves volume reconnection which can convert a substantial part of the energy into energetic particles. It is important that the reconnection induces an efficient acceleration of the first order Fermi type. Finally, we relate the properties of the reconnection with the observed properties of gamma ray bursts and provide evidence supporting the explanation of gamma ray bursts based on energy release via reconnection.Comment: 12 pages, review talk at 5th International Symposium on High-Energy Gamma-Ray Astrophysics, Heidelber

Superdiffusion of Cosmic Rays: Implications for Cosmic Ray Acceleration

Diffusion of cosmic rays (CRs) is the key process of understanding their propagation and acceleration. We employ the description of spatial separation of magnetic field lines in MHD turbulence in Lazarian & Vishniac (1999) to quantify the divergence of magnetic field on scales less than the injection scale of turbulence and show this divergence induces superdiffusion of CR in the direction perpendicular to the mean magnetic field. The perpendicular displacement squared increases, not as distance $x$ along magnetic field, which is the case for a regular diffusion, but as the $x^{3}$ for freely streaming CRs. The dependence changes to $x^{3/2}$ for the CRs propagating diffusively along magnetic field. In the latter case we show that it is important to distinguish the perpendicular displacement in respect to the mean field and to the local magnetic field. We consider how superdiffusion changes the acceleration of CRs in shocks and show how it decreases efficiency of the CRs acceleration in perpendicular shocks. We also demonstrate that in the case when small-scale magnetic field is being generated in the pre-shock region, an efficient acceleration can take place for the CRs streaming without collisions along magnetic loops.Comment: 18 pages, 6 figures, erratum in eqs.(12,13) and table 1 included, ApJ accepte

Cosmic Ray transport in MHD turbulence: large and small scale interactions

Cosmic ray (CR) transport and acceleration is essential for many astrophysical problems, e.g., CMB foreground, ionization of molecular clouds and all high energy phenomena. Recent advances in MHD turbulence call for revisions in the paradigm of cosmic ray transport. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulation, in which turbulence is injected at large scale and cascades to to small scales. We shall address the issue of the transport of CRs, both parallel and perpendicular to the magnetic field and show that the issue of cosmic ray subdiffusion is only important for restricted cases when the ambient turbulence is far from that suggested by numerical simulations. Moreover, on scales less than injection scale of turbulence, CRs's transport becomes super-diffusive. We also shall discuss the nonlinear growth of kinetic gyroresonance instability of cosmic rays induced by large scale compressible turbulence. This gyroresonance of cosmic rays on turbulence is demonstrated an important scattering mechanism in addition to direct interaction with the compressible turbulence. The feedback of the instability on large scale turbulence cannot be neglected, and should be included in future simulations.Comment: 7 pages, 1 figure, submitted to ASTRONUM2011 conference proceedings, typos correcte

Observational signature of near-extremal Kerr-like black holes in a modified gravity theory at the Event Horizon Telescope

We study the shadows cast by near-extremal Kerr-MOG black holes for different values of the parameter in modified gravity (MOG). In particular, we consider an isotropic emitter orbiting near such black holes and analytically compute the positions, fluxes and redshift factors of their images. The size of the shadow decreases when the modified parameter is increased. For each shadow, the images of the emitter appear on a special part of the shadow which has a rich structure. The primary image and secondary images are similar to those produced for the near-extremal (high spin) Kerr black hole, but the near-extremal Kerr-MOG black hole can have a spin ($\hat{J}/M^2_{\alpha}$) which is finitely lower than 1. When the modified parameter is varied, the typical positions of the corresponding images do not change, nor does the typical redshift factor associated with the primary image. However, another typical redshift factor associated with the secondary image increases when the modified parameter is increased. We also find that the fluxes increase in that case. These images appear periodically with period greater than that of Kerr. This provides an alternative signature away from the Kerr case which may be tested by the Event Horizon Telescope.Comment: 16 pages, 5 figures and 1 table. Slight change in title , fixed typos, added references. Published in PR

Counterion correlations and attraction between like-charged macromolecules

A simple model is presented for the appearance of attraction between two like charged polyions inside a polyelectrolyte solution. The polyions are modeled as rigid cylinders in a continuum dielectric solvent. The strong electrostatic interactions between the polyions and the counterions results in counterion condensation. If the two polyions are sufficiently close to each other their layers of condensed counterions can become correlated resulting in attraction between the macromolecules. To explore the counterion induced attraction we calculate the correlation functions for the condensed counterions. It is found that the correlations are of very short range. For the parameters specific to the double stranded DNA, the correlations and the attraction appear only when the surface-to-surface separation is less than 7 /AA.Comment: 6 pages, 8 eps figures, RevTeX with epsfi

Synovial joint lubrication – does nature teach more effective engineering lubrication strategies?

Nature shows numerous examples of systems which show energy efficiency, elegance in their design and optimum use of materials. Biomimetics is an emerging field of research in engineering and successes have been documented in the diverse fields of robotics, mechanics, materials engineering and many more. To date little biomimetics research has been directed towards tribology in terms of transferring technologies from biological systems into engineering applications. The potential for biomimicry has been recognised in terms of replicating natural lubricants but this system reviews the potential for mimicking the synovial joint as an efficient and durable tribological system for potential engineering systems. The use of materials and the integration of materials technology and fluid/surface interactions are central to the discussion