2019ApJ...886...45B

There is compelling evidence for a highly energetic Seyfert explosion (10^56–57 erg) that occurred in the Galactic center a few million years ago. The clearest indications are the X-ray/γ-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 Hα 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, Si IV/Si II) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward 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° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D >= 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated Hα emission and H ionization fraction (x_e >= 0.5), enhanced CIV/CII and Si IV/Si II ratios, and high CIV and Si IV 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 Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud-halo interaction) is too weak to explain the Magellanic Stream's ionization. Instead, 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 that 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/γ-ray bubbles in leptonic jet/wind models (≃2-8 Myr).