Abstract
Strong magnetic fields are found in several circumstances in the Universe : firstly, in stars and, among them, the most condensed : neutron stars and magnetars. In neutron stars (NS), the main source of energy is the magnetic field, of the order of B =109 - 1012 G. They derive their energy from accretion, rotation and residual internal heat. One out of every 10 000supernovaexplosions of type IIproduces a magnetar. Magnetars have extra-ordinary fields of B =1014 - 1015 G, compared with thestrongest man-madefields B~ 5105 G (stationary) or ~107 G (for a few milliseconds). Strong fields also exist near the active nuclei of galaxies, and even at the center of our Milky Way, where field lines are traced by fine filaments of synchrotron emission. In spiral galaxies, density waves produce shocks that strengthen the Bfieldalong the arms. The amplitude of the field can be measured by the degree of polarization of the radiation and the Faraday rotation. The magnetic field is responsible for collimating the radio jets. The transverse polarization measured there shows that the field structure Bis helical : the accretion disk from which the jet originates certainly imparts its rotation to the field lines B, frozen in the disk. The radio jet can be thought of as a coaxial cable, where the current flows in one direction at the center, then in the opposite direction in the sheath around it. In the case of AGNs, the current flows towards the nucleus on a parsec scale. Active nuclei are veritable cosmic batteries. They could thus create fields and propagate them throughout space via jets. In galaxy clusters, mergers create shocks that re-accelerate relativistic electrons : these are radio relics. The large-scale magnetic field remains confined to the cosmic filaments of the web, as indicated by the trajectory of high-energy cosmic rays. Its amplitude is constrained by CMB photons. The origin of the cosmic field could be inflation, or the Biermann battery during reionization. The latter creates strong density and temperature gradients, which generate currents, and hence dynamo fields. Weibel instability, due to opposing currents, could also contribute to this phenomenon.
