In 2008, when the Fermi Gamma-Ray Space Telescope was placed into Earth orbit, a new universe of high-energy radiation was revealed to us. Among its most intriguing findings was the Fermi Bubbles, which are enormous, symmetrical blobs that stretch both above and below the galactic disk, 25,000 light-years upon every side from the heart of the Milky Way, and glow in gamma-ray radiation.
Then, in the year 2020, an X-ray telescope called eROSITA uncovered still another unexpected phenomenon: bubbles that were much larger than before, spanning over 45,000 light-years on either side of the galactic plane and releasing X-rays of a lower energy. In light of new evidence, scientists have determined that the two groups of bubbles are the consequence of explosions from the galactic core and the supermassive black hole there. It has proven more difficult to identify the source of the gamma and X-rays.
Now, a physicist from Tokyo Metropolitan University in Japan named Yutaka Fujita has used computer simulations to come up with a single hypothesis that accounts for both classes of bubbles simultaneously. According to his research, the X-ray emission is caused by a strong, fast-moving wind that crashes into the flimsy gas filling interstellar space, creating a shock wave that rebounds off the plasma and causes it to shine energetically.
Sagittarius A*, the supermassive black hole at the center of the Milky Way, is relatively quiet compared to other black holes. Minimal feeding activity earns it the “quiescent” status. That is not how things have always been. Furthermore, a black hole that is actively feeding off of matter may alter the surrounding region in a wide variety of ways.
Any matter that gets close enough to a black hole will heat up and radiate light as it descends into the abyss. A portion of the mass is diverted along magnetic field lines beyond the black hole, where they function as a synchrotron to speed up particles to almost the speed of light. These may extend for millions of light-years into space and are propelled by enormous jets of ionized plasma that are blasted from the black hole’s poles. Furthermore, there exist cosmic winds, which are jets of charged particles whipped up by the material surrounding the black hole and ejected into the void.
Even while Sagittarius A* is rather peaceful at the moment, that hasn’t always been the case. If one looks carefully enough, one may see remains of former activity, such as the Fermi bubbles, in the area around the galactic plane. These artifacts may tell us when and where the action took place, as well as provide clues about its nature.
Data from the decommissioned Suzaku X-ray satellite, which was managed by NASA and the Japanese Space Agency, is the basis for Fujita’s exploration of the Fermi bubbles (JAXA). He used Suzaku data on X-ray bubble formations to inform numerical models based on black hole feeding processes, with some success.
He concluded that the most plausible explanation is that the black hole is being buffeted by a wind traveling at a speed of 1,000 kilometers per second (621 mph) due to a recent feeding event that was spaced out over a period of 10 million years. When the charged particles in the wind hit the interstellar medium, they create a shock wave that is reflected back into the bubble as the wind continues to expand. The glowing substance within the bubbles is heated by the backward shock waves. Fujita’s numerical calculations successfully replicated the X-ray structure’s temperature profile.
He also tried to replicate the Fermi bubbles by assuming a single explosive eruption from the core of the galaxy, but was unsuccessful. This indicates that the most probable origin of the unknown structures was a slow, constant wind from the galactic center. Even while star formation also generates cosmic winds, the strength of the wind can only be ascribed to Sagittarius A*.
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