The Double-Episode Jet Genesis of the eROSITA and Fermi Bubbles

Abstract

The Fermi and eROSITA bubbles are giant gamma-ray and X-ray lobes in the Milky Way, extending up to $\sim$50° and ~$\sim$80° in galactic latitude, respectively, yet their origins remain debated. Using three-dimensional magnetohydrodynamic simulations, we investigate a scenario in which two temporally separated episodes of active galactic nucleus (AGN) jets launched from the Galactic center produce the bubbles, with each structure bounded by a forward shock. Our simulations reveal that the first jet pair, launched 15 Myr ago, forms the outer eROSITA bubbles (extending to $\sim$18 kpc), while the second, launched 5 Myr ago, creates the nested Fermi bubbles ($\sim$10 kpc height). This model broadly reproduces the observed elongated morphology, multi-band X-ray surface brightness distribution, O VIII/O VII line ratios, radio ridge structures, and gamma-ray emissions of the bubbles. Cosmic-ray electrons are accelerated \textit{in situ} at the shock fronts, explaining the sharp edges and nearly uniform gamma-ray surface brightness distribution of Fermi bubbles. The results suggest that the eROSITA and Fermi bubbles encode a time-resolved record of episodic AGN activity in the Galactic center, providing a physically motivated framework for interpreting their multi-wavelength properties.