2016ApJ...829....9M

The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend ~10 kpc vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo ({T}_halo~2×{10}^{6} K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between 10^6 and 10^8 K, with potential X-ray signatures. In this work, we constrain the bubbles' thermal gas structure by modeling O vii and O viii emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with n~1×{10}^{-3} cm^-3 and with log(T) ≃ 6.60-6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of {490}_{-77}^{+230} km s^-1, {4.3}_{-1.4}^{+0.8} Myr, and {2.3}_{-0.9}^{+5.1}×{10}^{42} ergs^-1. These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.