Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow

¹ Institute for Theoretical Physics and Cosmology (TTK), RWTH Aachen University, Sommerfeldstr. 16, 52074 Aachen, Germany
e-mail: pmertsch@physik.rwth-aachen.de
² Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
³ Kavli Institute for Particle Astrophysics & Cosmology, 2575 Sand Hill Road, M/S 29, Menlo Park, CA 94025, USA
⁴ Department of Physics and Applied Physics, Stanford University, Stanford, CA 94305, USA

Received: 1 August 2018
Accepted: 7 December 2018

Abstract

The discovery of the Fermi bubbles – a huge bilobular structure seen in GeV gamma-rays above and below the Galactic centre – implies the presence of a large reservoir of high energy particles at ~10 kpc from the disk. The absence of evidence for a strong shock coinciding with the edge of the bubbles, and constraints from multi-wavelength observations point towards stochastic acceleration by turbulence as a likely mechanism of acceleration. We have investigated the time-dependent acceleration of electrons in a large-scale outflow from the Galactic centre. For the first time, we present a detailed numerical solution of the particle kinetic equation that includes the acceleration, transport and relevant energy loss processes. We also take into account the addition of shock acceleration of electrons at the bubble’s blast wave. Fitting to the observed spectrum and surface brightness distribution of the bubbles allows determining the transport coefficients, thereby shedding light on the origin of the Fermi bubbles.