The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays , extend \approx 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 distribution ( T _ { halo } \approx 2 \times 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 the 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 \approx 1 \times 10 ^ { -3 } cm ^ { -3 } and with log ( T ) \approx 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 } \times 10 ^ { 42 } erg s ^ { -1 } .
These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation .