Gravito-magnetic amplification in cosmology

Magnetic fields interact with gravitational waves in various ways. We consider the coupling between the Weyl and the Maxwell fields in cosmology and study the effects of the former on the latter. The approach is fully analytical and the results are gauge-invariant. We show that the nature and the outcome of the gravito-magnetic interaction depends on the electric properties of the cosmic medium. When the conductivity is high, gravitational waves reduce the standard (adiabatic) decay rate of the B-field, leading to its superadiabatic amplification. In poorly conductive environments, on the other hand, Weyl-curvature distortions can result into the resonant amplification of large-scale cosmological magnetic fields. Driven by the gravitational waves, these B-fields oscillate with an amplitude that is found to diverge when the wavelengths of the two sources coincide. We present technical and physical aspects of the gravito-magnetic interaction and discuss its potential implications.Comment: Typos corrected, clarifications added, published in PR

Large-scale magnetic fields in cosmology

Despite the widespread presence of magnetic fields, their origin, evolution and role are still not well understood. Primordial magnetism sounds appealing but is not problem free. The magnetic implications for the large-scale structure of the universe still remain an open issue. This paper outlines the advantages and shortcomings of early-time magnetogenesis and the typical role of B-fields in linear structure-formation scenarios.Comment: Invited Talk (36th EPS Conference on Plasma Physics, 2009

The Parker Instability in 3-D: Corrugations and Superclouds Along the Carina-Sagittarius Arm

Here we present three-dimensional MHD models for the Parker instability in a thick magnetized disk, including the presence of a spiral arm. The $B$-field is assumed parallel to the arm, and the model results are applied to the optical segment of the Carina-Sagittarius arm. The characteristic features of the undular and interchange modes are clearly apparent in the simulations. The undular mode creates large gas concentrations distributed along the arm. This results in a clear arm/inter-arm difference: the instability triggers the formation of large interstellar clouds inside the arms, but generates only small structures with slight density enhancements in the inter-arm regions. The resulting clouds are distributed in an antisymmetric way with respect to the midplane, creating an azimuthal corrugation along the arm. For conditions similar to those of the optical segment of the Carina-Sagittarius arm, it has a wavelength of about 2.4 kpc. This structuring can explain the origin of both HI superclouds and the azimuthal corrugations in spiral arms. The wavelength matches the corrugation length derived with the young stellar groups located in the optical segment of the Carina-Sagittarius arm. Keywords: Galaxy: kinematics and dynamics -- Galaxy: structure -- Instabilities -- ISM: clouds -- ISM: magnetic fields -- ISM: structure -- MHDComment: 29 pages, 12 figures, Latex, Accepted by the Astrophysical Journa

Specificity of SPIO particles for characterization of liver hemangiomas using MRI

We investigated the specificity of superparamagnetic iron oxide (SPIO)â€"enhanced T1-weighted spin-echo (SE) magnetic resonance (MR) images for the characterization of liver hemangiomas. When imaging liver hemangiomas, which are the most frequent benign liver tumors, a method with very high specificity is required, which will obviate other studies, follow-up, or invasive diagnostic procedures such as percutaneous biopsy. Eighty-three lesions were examined by MR imaging at 1.5 T before and after intravenous injection of SPIO particles. Lesions were categorized as follows according to the final diagnosis: 37 hemangiomas, nine focal nodular hyperplasias (FNHs), 19 hepatocellular carcinomas (HCCs), and 18 metastases. Their signal intensity values were normalized to muscle and compared. The only lesions showing a significant increase in signal intensity ratio (lesion to muscle) on postcontrast T1-weighted SE images were hemangiomas (p < 0.001). The signal intensity ratio of hemangiomas increased on average by 70%. Based on receiver operating characteristic analysis and using a cutoff level of 50% signal increase, the specificity and sensitivity of SPIO-enhanced MR imaging for the characterization of hemangiomas would be 100% and 70%, respectively. The T1 effect of SPIO particles can help differentiate hemangiomas from other focal liver lesions such as FNHs, HCCs, and metastases and may obviate biopsy. When using SPIO particles for liver imaging, it is useful to add a T1-weighted sequence to T2-weighted images, thereby providing additional information for lesion characterizatio

A Magnetized Local Supercluster and the Origin of the Highest Energy Cosmic Rays

A sufficiently magnetized Local Supercluster can explain the spectrum and angular distribution of ultra-high energy cosmic rays. We show that the spectrum of extragalactic cosmic rays with energies below $\sim 10^{20}$ eV may be due to the diffusive propagation in the Local Supercluster with fields of $\sim 10^{-8} - 10^{-7}$ Gauss. Above $\sim 10^{20}$ eV, cosmic rays propagate in an almost rectilinear way which is evidenced by the change in shape of the spectrum at the highest energies. The fit to the spectrum requires that at least one source be located relatively nearby at $\sim 10-15$ Mpc away from the Milky Way. We discuss the origin of magnetic fields in the Local Supercluster and the observable predictions of this model.Comment: 11 pages, 2 figures, submitted to PR

Radio spectral study of the cluster of galaxies Abell 2255

Spectral index studies of halos, relics, and radio galaxies provide useful information on their origin and connection with merger processes. We present WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm, 85 cm, and 2 m. The spectral index images allowed us to study the integrated spectrum of halo and relic and to investigate the physical properties of the Beaver head-tail radio galaxy belonging to the cluster. In the radio halo, the spectral index is steeper at the center and flatter at the locations of the radio filaments, clearly detected at 25 cm. In the relics, the spectral index flattens, moving away from the cluster center. For the Beaver radio galaxy, the spectrum severely steepens from the head towards the end of the tail, because of the energy losses suffered by the relativistic particles. In the 2 m map, which is the first high-sensitivity image presented in the literature at such a long wavelength, a new Mpc-size emission region is detected between the known radio halo and the NW relic. Not detecting this feature in the more sensitive 85 cm observations implies that it must have a very steep spectrum (alpha <= -2.6). The observational properties of the radio halo suggest that either we are looking at a superposition of different structures (filaments in the foreground plus real halo in the background) seen in projection across the cluster center or that the halo is intrinsically peculiar. The newly detected extended region to the NW of the halo could be considered as an asymmetric extension of the halo itself. However, since radio halos are known in the literature as structures showing a regular morphology, the new feature could represent the first example of steep Mpc-size diffuse structures (MDS), detected around clusters at very low frequencies.Comment: 23 pages, 18 figures. A&A, in pres

Particle Diffusion and Acceleration by Shock Wave in Magnetized Filamentary Turbulence

We expand the off-resonant scattering theory for particle diffusion in magnetized current filaments that can be typically compared to astrophysical jets, including active galactic nucleus jets. In a high plasma beta region where the directional bulk flow is a free-energy source for establishing turbulent magnetic fields via current filamentation instabilities, a novel version of quasi-linear theory to describe the diffusion of test particles is proposed. The theory relies on the proviso that the injected energetic particles are not trapped in the small-scale structure of magnetic fields wrapping around and permeating a filament but deflected by the filaments, to open a new regime of the energy hierarchy mediated by a transition compared to the particle injection. The diffusion coefficient derived from a quasi-linear type equation is applied to estimating the timescale for the stochastic acceleration of particles by the shock wave propagating through the jet. The generic scalings of the achievable highest energy of an accelerated ion and electron, as well as of the characteristic time for conceivable energy restrictions, are systematically presented. We also discuss a feasible method of verifying the theoretical predictions. The strong, anisotropic turbulence reflecting cosmic filaments might be the key to the problem of the acceleration mechanism of the highest energy cosmic rays exceeding 100 EeV (10^{20} eV), detected in recent air shower experiments.Comment: 39 pages, 2 figures, accepted for publication in Ap

Report of the GDR working group on the R-parity violation

This report summarizes the work of the "R-parity violation group" of the French Research Network (GDR) in Supersymmetry, concerning the physics of supersymmetric models without conservation of R-parity at HERA, LEP, Tevatron and LHC and limits on R-parity violating couplings from various processes. The report includes a discussion of the recent searches at the HERA experiment, prospects for new experiments, a review of the existing limits, and also theoretically motivated alternatives to R-parity and a brief discussion on the implications of R-parity violation on the neutrino masses.Comment: 60 pages, LaTeX, 22 figures, 2 table