Fermi bubbles: High-latitude X-ray supersonic shell

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 northeast 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 ~7Myr 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.