Formation of large-scale magnetic structures associated with the Fermi bubbles

Abstract

The Fermi bubbles are part of a complex region of the Milky Way. This region presents broadband extended non-thermal radiation, apparently coming from a physical structure rooted in the Galactic Centre and with a partly-ordered magnetic field threading it. We explore the possibility of an explosive origin for the Fermi bubble region to explain its morphology, in particular that of the large-scale magnetic fields, and provide context for the broadband non-thermal radiation. We perform 3D magnetohydrodynamical simulations of an explosion from a few million years ago that pushed and sheared a surrounding magnetic loop, anchored in the molecular torus around the Galactic Centre. Our results can explain the formation of the large-scale magnetic structure in the Fermi bubble region. Consecutive explosive events may match better the morphology of the region. Faster velocities at the top of the shocks than at their sides may explain the hardening with distance from the Galactic Plane found in the GeV emission. In the framework of our scenario, we estimate the lifetime of the Fermi bubbles as $2\times10^6$ yr, with a total energy injected in the explosion(s) $> 10^{55}$ ergs. The broadband non-thermal radiation from the region may be explained by leptonic emission, more extended in radio and X-rays, and confined to the Fermi bubbles in gamma rays.