What can Fermi LAT observation of the Galactic Centre tell us about its active past?

Abstract

The Fermi-LAT gamma-ray data in the inner Galaxy region show several prominent features possibly related to the past activity of the Milky Way's super massive black hole. At a large, 50 deg scale, the Fermi LAT revealed symmetric hour glass structures with hard energy spectra extending up to 100 GeV (and dubbed 'the Fermi bubbles'). More recently and closer to the Galactic centre, at the 10 deg scale, several groups have claimed evidence for excess gamma-ray emission that appears symmetric around the Galactic center and has an energy spectrum peaking at few GeVs. We explore here the possibility that this emission originates in inverse Compton emission from high-energy electrons produced in a short duration, burst-like event injecting 1052 - 1053 erg, roughly 106 yrs ago. Several lines of evidence suggest that a series of 'burst like' events happened in the vicinity of our black hole in the past and gamma-ray observations may offer a new view of that scenario.