The Fermi bubbles revisited

¹ Max-Planck-Institut für Kernphysik, PO Box 103980 69029 Heidelberg Germany
e-mail: bixian85@pmo.ac.cn
² Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, 2 Dublin, Ireland
⁴ Gran Sasso Science Institute, 7 viale Francesco Crispi, 67100 L’Aquila, Italy
⁵ Research School of Astronomy and Astrophysics, Australian National University, 2611 Canberra, Australia

Received: 3 February 2014
Accepted: 19 April 2014

Abstract

We analyze 60 months of all-sky data from the Fermi-LAT. The Fermi bubble structures discovered previously are clearly revealed by our analysis. With more data, hence better statistics, we can now divide each bubble into constant longitude slices to investigate their gross γ-ray spectral morphology. While the detailed spectral behavior of each slice derived in our analysis is somewhat dependent on the assumed background model, we find, robustly, a relative deficit in the flux at low energies (i.e., hardening) toward the top of the south bubble. In neither bubble does the spectrum soften with longitude. The morphology of the Fermi bubbles is also revealed to be energy-dependent: at high energies they are more extended. We conclude from the gamma-ray spectrum at high latitudes that a low energy break in the parent cosmic ray population is required in both leptonic and hadronic models. We briefly discuss possible leptonic and hadronic interpretations of this phenomenology.