  The Fermi satellite discovery of the gamma-ray emitting bubbles extending 50
deg (10 kpc) from the Galactic Centre has revitalized earlier claims that our
Galaxy has undergone an explosive episode in the recent past. We now explore a
new constraint on such activity. The Magellanic Stream is a clumpy gaseous
structure passing over the South Galactic Pole (SGP) at a distance of at least
50-100 kpc. Patchy H-alpha emission discovered along the Magellanic Stream over
the SGP is a factor of 5 too bright to have been produced by the Galactic
stellar population. Time-dependent models of Stream clouds exposed to a flare
in ionising photon flux show that the ionised gas must recombine and cool for a
time interval 0.6 - 2.9 Myr for the emitted H-alpha surface brightness to drop
to the observed level. A nuclear starburst is ruled out. Sgr A* is a more
likely candidate because it is two orders of magnitude more efficient at
converting gas to UV radiation. The central black hole (4 x 10^6 Msun) can
supply the required ionising luminosity with a fraction of the Eddington
accretion rate (3-30%). In support of nuclear activity, the H-alpha emission
along the Stream has a polar angle dependence peaking close to the SGP.
Moreover, it is now generally accepted that the Stream over the SGP must be
further than the Magellanic Clouds. At the lower halo gas densities, shocks
become too ineffective and are unlikely to give rise to a polar angle
dependence in the H-alpha emission. Thus it is likely that the Stream emission
arose from a `Seyfert flare' that was active 1-3 Myr ago, consistent with the
cosmic ray lifetime in the Fermi bubbles. Sgr A* activity today is greatly
suppressed (70-80 dB) relative to the Seyfert outburst...
