Magnetic fields in cosmic particle acceleration sources

We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. We discuss a number of CR-driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review

Modeling high-speed reacting flows with variable turbulent Prandtl and Schmidt numbers

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.A turbulence model that considers the impact of compressibility effects on turbulence and allows for the calculation of the variable turbulent Prandtl (PrT) and Schmidt (ScT) numbers as part of the solution is presented. The model is based on modeling the slow part of pressure/scalar-gradient correlation depending on characteristic time of scalar variable fluctuations (the ratio of scalar variable variance to its dissipation rate) and on the assumption that the velocity fluctuations directed normal to the streamlines play a key role in turbulent mixing process. For the validation of the code the described numerical procedures are applied to a series of jet flow problems. These include supersonic turbulent jets of variable composition and high-speed chemically reacting coflows.dc201

Equilibration processes in the Warm-Hot Intergalactic Medium

The Warm-Hot Intergalactic Medium (WHIM) is thought to contribute about 40-50 % to the baryonic budget at the present evolution stage of the universe. The observed large scale structure is likely to be due to gravitational growth of density fluctuations in the post-inflation era. The evolving cosmic web is governed by non-linear gravitational growth of the initially weak density fluctuations in the dark energy dominated cosmology. Non-linear structure formation, accretion and merging processes, star forming and AGN activity produce gas shocks in the WHIM. Shock waves are converting a fraction of the gravitation power to thermal and non-thermal emission of baryonic/leptonic matter. They provide the most likely way to power the luminous matter in the WHIM. The plasma shocks in the WHIM are expected to be collisionless. Collisionless shocks produce a highly non-equilibrium state with anisotropic temperatures and a large differences in ion and electron temperatures. We discuss the ion and electron heating by the collisionless shocks and then review the plasma processes responsible for the Coulomb equilibration and collisional ionisation equilibrium of oxygen ions in the WHIM. MHD-turbulence produced by the strong collisionless shocks could provide a sizeable non-thermal contribution to the observed Doppler parameter of the UV line spectra of the WHIM.Comment: 13 pages, 4 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 8; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

Collective effects of stellar winds and unidentified gamma-ray sources

We study collective wind configurations produced by a number of massive stars, and obtain densities and expansion velocities of the stellar wind gas that is to be target, in this model, of hadronic interactions. We study the expected $\gamma$-ray emission from these regions, considering in an approximate way the effect of cosmic ray modulation. We compute secondary particle production (electrons from knock-on interactions and electrons and positrons from charged pion decay), and solve the loss equation with ionization, synchrotron, bremsstrahlung, inverse Compton, and expansion losses. We provide examples where configurations can produce sources for GLAST satellite, and the MAGIC, HESS, or VERITAS telescopes in non-uniform ways, i.e., with or without the corresponding counterparts. We show that in all cases we studied no EGRET source is expected

Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts

It is only now, with low-frequency radio telescopes, long exposures with high-resolution X-ray satellites and gamma-ray telescopes, that we are beginning to learn about the physics in the periphery of galaxy clusters. In the coming years, Sunyaev-Zeldovich telescopes are going to deliver further great insights into the plasma physics of these special regions in the Universe. The last years have already shown tremendous progress with detections of shocks, estimates of magnetic field strengths and constraints on the particle acceleration efficiency. X-ray observations have revealed shock fronts in cluster outskirts which have allowed inferences about the microphysical structure of shocks fronts in such extreme environments. The best indications for magnetic fields and relativistic particles in cluster outskirts come from observations of so-called radio relics, which are megaparsec-sized regions of radio emission from the edges of galaxy clusters. As these are difficult to detect due to their low surface brightness, only few of these objects are known. But they have provided unprecedented evidence for the acceleration of relativistic particles at shock fronts and the existence of muG strength fields as far out as the virial radius of clusters. In this review we summarise the observational and theoretical state of our knowledge of magnetic fields, relativistic particles and shocks in cluster outskirts.Comment: 34 pages, to be published in Space Science Review

Chaos in a low-order model of magnetoconvection

In the limit of tall, thin rolls, weakly nonlinear convection in a vertical magnetic field is described by an asymptotically exact third-order set of ordinary differential equations. These equations are shown here to have three codimension-two bifurcation points: a Takens-Bogdanov bifurcation, at which a gluing bifurcation is created; a point at which the gluing bifurcation is replaced by a pair of homoclinic explosions between which there are Lorenz-like chaotic trajectories; and a new type of bifurcation point at which the first of a cascade of period-doubling bifurcation lines originates. The last two bifurcation points are analysed in terms of a one-dimensional map. The equations also have a T-point, at which there is a heteroclinic connection between a saddle and a pair of saddle-foci; emerging from this point is a line of Shil'nikov bifurcations, involving homoclinic connections to a saddle-focus

High-Energy Cosmology: gamma rays and neutrinos from beyond the galaxy

Our knowledge of the high-energy universe is undergoing a period of rapid change as new astronomical detectors of high-energy radiation start to operate at their design sensitivities. Now is a boomtime for high-energy astrophysics, with new discoveries from Swift and HESS, results from MAGIC and VERITAS starting to be reported, the upcoming launches of the gamma-ray space telescopes GLAST and AGILE, and anticipated data releases from IceCube and Auger. A formalism for calculating statistical properties of cosmological gamma-ray sources is presented. Application is made to model calculations of the statistical distributions of gamma-ray and neutrino emission from (i) beamed sources, specifically, long-duration GRBs, blazars, and extagalactic microquasars, and (ii) unbeamed sources, including normal galaxies, starburst galaxies and clusters. Expressions for the integrated intensities of faint beamed and unbeamed high-energy radiation sources are also derived. A toy model for the background intensity of radiation from dark-matter annihilation taking place in the early universe is constructed. Estimates for the gamma-ray fluxes of local group galaxies, starburst, and infrared luminous galaxies are briefly reviewed. Because the brightest extragalactic gamma-ray sources are flaring sources, and these are the best targets for sources of PeV -- EeV neutrinos and ultra-high energy cosmic rays, rapidly slewing all-sky telescopes like MAGIC and an all-sky gamma-ray observatory beyond Milagro will be crucial for optimal science return in the multi-messenger age.Comment: 10 pages, 3 figs, accepted for publication in the Barcelona Conference on Multimessenger Astronomy; corrected eq. 27, revised Fig. 3, added 2 ref

Conformal Invariance and Degrees of Freedom in the QCD String

We demonstrate that the Hagedorn-like growth of the number of observed meson states can be used to constrain the degrees of freedom of the underlying effective QCD string. We find that the temperature relevant for such string theories is not given by the usual Hagedorn value $T_H\approx 160$ MeV, but is considerably higher. This resolves an apparent conflict with the results from a static quark-potential analysis, and suggests that conformal invariance and modular invariance are indeed reflected in the hadronic spectrum. We also find that the $D_\perp=2$ scalar string is in excellent agreement with data.Comment: 13 pages (Standard LaTeX); --> replaced version emphasizes new results, and agrees with version to appear in Physical Review Letters (Jan 1994

Study of BaTiO₃ ceramics doped with Mn and Ce or Nb and Sr

ESR study of BaTiO₃ ferroelectrics doped with cerium or with niobium and strontium, both types of samples being doped with manganese, was performed at room temperature. In the samples of the first type the most intensive line with g-factor 1.9612 was shown to originate from paramagnetic center Ti³⁺–Ce³⁺. The small intensity line was related to Fe³⁺–Vo center, which can be unavoidable impurity in BaTiO₃. In the samples doped with strontium, the six lines intensive spectrum, which corresponds to hyperfine structure of Mn²⁺, was observed. The intensity of the spectrum was shown to decrease with strontium concentration increase. This effect was supposed to be related to the decrease of the grain sizes that results in migration of manganese and, possibly, niobium ions. The influence of these impurities, of their charge states and positions in crystalline lattice of BaTiO₃ on the PTCR effect is discussed.Представлено результати ЕПР досліджень сегнетоелектричної кераміки BaTiO₃ двох серій: легованої церієм та марганцем або ніобієм, стронцієм та марганцем, при кімнатній температурі. Показано, що у зразках першої серії найбільш інтенсивна лінія з g-фактором 1.9612 належить до парамагнітного центру Ti³⁺–Ce³⁺. Лінії малої інтенсивності належать до центру Fe³⁺–Vo (пов’язаного з неконтрольованою домішкою заліза у зразках) та до іону Mn²⁺. В другій серії зразків спостерігався спектр, який відповідає надтонкій структурі іону Mn²⁺. Інтенсивність цього спектру зменшувалась із збільшенням концентрації стронцію. Можливо цей ефект пов’язаний із зменшенням розмірів зерен та міграцією марганцю та, можливо ніобію, у міжзерновий простір. Обговорюється вплив домішок, їх зарядового стану та позиції у кристалічній ґратці на ефект позитивного температурного коефіцієнту опору

Particle acceleration mechanisms

We review the possible mechanisms for production of non-thermal electrons which are responsible for non-thermal radiation in clusters of galaxies. Our primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering, that produce hard X-ray emission. We briefly review acceleration mechanisms and point out that in most astrophysical situations, and in particular for the intracluster medium, shocks, turbulence and plasma waves play a crucial role. We consider two scenarios for production of non-thermal radiation. The first is hard X-ray emission due to non-thermal Bremsstrahlung by nonrelativistic particles. Non-thermal tails are produced by accelerating electrons from the background plasma with an initial Maxwellian distribution. However, these tails are accompanied by significant heating and they are present for a short time of <10^6 yr, which is also the time that the tail will be thermalised. Such non-thermal tails, even if possible, can only explain the hard X-ray but not the radio emission which needs GeV or higher energy electrons. For these and for production of hard X-rays by the inverse Compton model, we need the second scenario where there is injection and subsequent acceleration of relativistic electrons. It is shown that a steady state situation, for example arising from secondary electrons produced from cosmic ray proton scattering by background protons, will most likely lead to flatter than required electron spectra or it requires a short escape time of the electrons from the cluster. An episodic injection of relativistic electrons, presumably from galaxies or AGN, and/or episodic generation of turbulence and shocks by mergers can result in an electron spectrum consistent with observations but for only a short period of less than one billion years.Comment: 22 pages, 5 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 11; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke