  There is compelling evidence for a highly energetic Seyfert explosion
(10^{56-57} erg) that occurred in the Galactic Centre a few million years ago.
The clearest indications are the x-ray/gamma-ray "10 kpc bubbles" identified by
the Rosat and Fermi satellites. In an earlier paper, we suggested another
manifestation of this nuclear activity, i.e. elevated H-alpha emission along a
section of the Magellanic Stream due to a burst (or flare) of ionizing
radiation from Sgr A*. We now provide further evidence for a powerful flare
event: UV absorption line ratios (in particular CIV/CII, SiIV/SiII) observed by
the Hubble Space Telescope reveal that some Stream clouds towards both galactic
poles are highly ionized by a source capable of producing ionization energies
up to at least 50 eV. We show how these are clouds caught in a beam of bipolar,
radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In
our model, the biconic axis is tilted by about 15 deg from the South Galactic
Pole with an opening angle of roughly 60 deg. For the Stream at such large
Galactic distances (D > 75 kpc), nuclear activity is a plausible explanation
for all of the observed signatures: elevated H-alpha emission and H ionization
fraction (X_e > 0.5), enhanced CIV/CII and SiIV/SiII ratios, and high CIV and
SiIV column densities. Wind-driven "shock cones" are ruled out because the
Fermi bubbles lose their momentum and energy to the Galactic corona long before
reaching the Stream. The nuclear flare event must have had a radiative UV
luminosity close to the Eddington limit (f_E ~ 0.1-1). Our time-dependent
Seyfert flare models adequately explain the observations and indicate the
Seyfert flare event took place T_o = 3.5 +/- 1 Myr ago. The timing estimates
are consistent with the mechanical timescales needed to explain the
x-ray/gamma-ray bubbles in leptonic jet/wind models (2-8 Myr).
