Cosmic magnetic lenses

Departamento de Física Teórica y del Cosmos Universidad de Granada, 18071 Granada, Spain
e-mail: battaner@ugr.es; jcastellano@ugr.es; masip@ugr.es

Received: 31 August 2010
Accepted: 15 December 2010

Abstract

Aims. Magnetic fields play a critical role in the propagation of charged cosmic rays. We investigate here whether some particular field configurations supported by various astrophysical objects produce images on cosmic ray maps.

Methods. We consider a simple configuration, namely a constant azimuthal field in a disk-like object, which we identify as a cosmic magnetic lens. Such a configuration is typical of most spiral galaxies, and we hypothetize that it can also appear on smaller or larger scales.

Results. We show that the magnetic lens deflects cosmic rays in a regular geometrical pattern, very much like a gravitational lens deflects light but with some interesting differences. In particular, the lens acts effectively only in a definite region of the cosmic-ray spectrum, and it can be convergent or divergent depending on the (clockwise or counterclockwise) direction of the magnetic field and the (positive or negative) electric charge of the cosmic ray. We find that the image of a point-like monochromatic source may be one, two, or four points depending on the relative positions of the source, the observer, and the center of the lens. For a perfect alignment and a lens in the orthogonal plane, the image becomes a ring. We also show that the presence of a lens could introduce low-scale fluctuations and matter-antimatter asymmetries in the fluxes from distant sources.

Conclusions. The concept of cosmic magnetic lens that we introduce here may be useful for interpreting possible patterns observed in the cosmic ray flux at different energies.