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

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.

💡 Research Summary

This paper investigates a two‑episode active‑galactic‑nucleus (AGN) jet scenario for the origin of the Milky Way’s eROSITA and Fermi bubbles using three‑dimensional magnetohydrodynamic (MHD) simulations. The authors model two pairs of kinetic‑energy‑dominated jets launched along the Galactic rotation axis: the first pair is injected 15 Myr ago with a total energy of 3.46 × 10⁵⁵ erg, and the second pair follows 10 Myr later (i.e., 5 Myr ago) with 1.10 × 10⁵⁵ erg. Each jet episode lasts 1 Myr. The simulations produce two distinct forward shocks. The older, outer shock expands to ≈ 18 kpc (≈ 15 kpc in the north) and defines the eROSITA bubbles, while the younger, inner shock reaches ≈ 10 kpc and delineates the Fermi bubbles.

Thermodynamic analysis shows that the outer shock heats ambient halo gas to 0.25–0.30 keV, forming a broad, low‑density shell that cools adiabatically behind the front. The inner shock re‑compresses this material, raising its temperature to 0.30–0.40 keV and creating a denser, narrower shell. Magnetic fields are amplified to ≈ 1 µG in the shocked regions, and internal energy density rises sharply at the shock fronts.

Synthetic all‑sky X‑ray maps are generated for the ROSAT bands (0.11–2.04 keV). In the softest band (0.11–0.284 keV) the model reproduces the observed high‑latitude brightness and the strong low‑latitude attenuation due to HI absorption. In the medium (0.44–1.21 keV) and hard (0.73–2.04 keV) bands, two separate emission layers appear, corresponding to the two forward shocks, matching the observed morphology of the eROSITA and Fermi bubbles. The O VIII/O VII line‑ratio map shows an enhancement inside the eROSITA bubbles with a Mach number < 1.5, consistent with a dynamical age of ~15 Myr.

For the gamma‑ray emission, a toy model assumes the cosmic‑ray electron (CRe) energy density scales with the thermal energy density. Inverse‑Compton scattering of the interstellar radiation field by CRe accelerated in situ at the inner shock yields a 1–5 GeV surface‑brightness distribution that reproduces the sharp edges and near‑uniform intensity of the observed Fermi bubbles. The required CRe energy is ≈ 9.5 × 10⁵² erg, with spectral indices p ≈ 2.2 inside the bubbles and p ≈ 2.4 outside. This scenario resolves the cooling‑time problem that plagued models relying on aged electrons from a single outburst.

Polarized synchrotron emission at 30 GHz is also computed. The resulting Stokes Q and U maps display ridge‑like features aligned with the shock‑compressed bubble edges and reproduce the large‑scale antisymmetric polarization lobes seen by Planck and WMAP. The combination of toroidal and poloidal magnetic components naturally explains the observed polarization pattern.

The authors argue that the double‑episode jet model simultaneously accounts for the size, age, morphology, X‑ray line diagnostics, radio polarization, and gamma‑ray characteristics of both bubble systems—something single‑episode models struggle to achieve. They suggest that the eROSITA and Fermi bubbles constitute a time‑resolved record of episodic AGN activity in the Galactic center, with the older jet providing a reservoir of low‑energy electrons responsible for the high‑latitude radio “SPASS” lobes, while the younger jet supplies freshly accelerated electrons that power the gamma‑ray emission. The work offers a physically motivated framework for interpreting multi‑wavelength observations and motivates future high‑resolution studies to further constrain the energetics and timing of the two jet events.