Nested Fermi and eROSITA bubbles require very similar $\sim10^{56}$ erg collimated Galactic-center outbursts; their asymmetry indicates an eastern density gradient

Abstract

Observations indicate two nested pairs of extended bipolar bubbles emanating from the Milky-Way center - the $|b|\sim80^\circ$ latitude eROSITA bubbles (RBs), encompassing the smaller, $|b|\sim 50^{\circ}$ Fermi bubbles (FBs) - and classify the edges of both bubble pairs as strong forward shocks. Identifying each bubble pair as driven by a distinct, collimated outburst, we evolve these bubbles and constrain their origin using a stratified 1D model verified by a suite of 2D and 3D hydrodynamic simulations which reproduce X-ray observations. While the RBs are at the onset of slowdown, the FBs are still expanding ballistically into the RB-shocked medium. Observational constraints indicate that both RB and FB outbursts had (up to factor $\sim2$-$4$ uncertainties) $\sim4^\circ$ half-opening angles and $\sim 2000$ km s$^{-1}$ velocities $100$ pc from their base, carrying $\sim10^{56}$ erg. The FBs and RBs could thus arise from identical outbursts separated by $\sim10$ Myr; their longitudinal asymmetry favors an eastern ambient-density gradient over western wind suggestions.