Fermi bubbles around the M31 galaxy

Abstract

We have searched for an extended high-energy gamma-ray emission around the nearby giant spiral Andromeda galaxy M31 using almost 7 years of Fermi LAT data at energies above 300 MeV. The presence of a diffuse gamma-ray halo with total photon flux 2:6±0:6×10−9 cm−2 s−1, corresponding to a luminosity (0.3-100 GeV) of (3:2±0:6)×1038 erg s−1 (for a distance of 780 kpc) was found at a 5.3σ confidence level. The best-fit halo template corresponds to two 6-7.5 kpc bubbles symmetrically located perpendicular to the M31 galactic disc, similar to the ‘Fermi bubbles’ found around the Milky Way centre.