2018MNRAS.480..223K

The nature of the bipolar, γ-ray Fermi bubbles (FB) is still unclear, in part because their faint, high-latitude X-ray counterpart has until now eluded a clear detection. We stack ROSAT data at varying distances from the FB edges, thus boosting the signal and identifying an expanding shell behind the south-west, south-east, and north-west edges, albeit not in the dusty north-east sector near Loop I. A Primakoff-like model for the underlying flow is invoked to show that the signals are consistent with halo gas heated by a strong, forward shock to ∼ keV temperatures. Assuming ion-electron thermal equilibrium then implies a ∼10⁵⁶ erg event near the Galactic centre ∼7 Myr ago. However, the reported high absorption-line velocities suggest a preferential shock-heating of ions, and thus more energetic (∼10⁵⁷ erg), younger (<=3 Myr) FBs.