Researchers Discover Origin of Fermi Bubbles and Galactic Center X-ray Outflows

Saturday, May 16, 2020 - 10:57

Researchers at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences have presented a new model that, for the first time, simultaneously explains the origins of both the Fermi bubbles and the galactic center biconical X-ray structure.

Based on the model created by the researchers, the two structures are essentially the same phenomenon and was caused by the forward shock driven by a pair of jets emanating from Sagittarius A* (Sgr A*) – the supermassive black hole lurking at the galactic center—about five million years ago, phys.org reports.

Fermi bubbles are two colossal blobs filled with very hot gas, cosmic rays and magnetic fields. Although they cannot be seen with the naked eye, they are very bright in diffuse gamma-ray emissions. In gamma rays, the Fermi bubbles have very sharp edges and the edges coincide well with an X-ray structure called the galactic center biconical X-ray structure.

Seeing the very similar edges of Fermi bubbles and the galactic center biconical X-ray structure, the SHAO researchers realized these structures might share the same origin. Furthermore, the biconical X-ray structure could be naturally explained by the shock-compressed thin shell of hot thermal gas driven by a past energy outburst from the galactic center.

Based on the research, published in The Astrophysical Journal, the new model indicates that the total energy injected during the Fermi bubble event by the supermassive black hole is close to that released by about 20,000 supernovae. The total matter consumed by Sgr A* during this event is about 100 solar masses.

In this model, the edge of the Fermi bubbles is the forward shock driven by a pair of jets emanating from Sgr A* about five million years ago. "One good thing about this model is that the energy and age of the Fermi bubbles can be constrained by the X-ray observations quite well," said corresponding author Guo Fulai. The age of the bubbles inferred in this study is also consistent with that derived from recent ultraviolet observations of some high velocity clouds along many sightlines towards the bubble region.

"Another very interesting thing that we found in our study is that if the bubbles and the biconical X-ray structure share the same origin, they are very unlikely to be produced by star formation or black hole winds," said Guo. Near the galactic center, the biconical X-ray structure has a very narrow base, while the forward shock produced by star formation or black hole winds can easily propagate to large distances, leading to a base much wider than observed.

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