In 2008, when the Fermi gamma-Ray Space Telescope got into the low Earth orbit, it opened our eyes to an entirely new Universe of high energy radiation.
The Fermi bubbles were one of the more fascinating discoveries. Big symmetrical blobs extending above and below the galactic plane, 25,000 light years from the Milky Way center on each side, exploding in gamma-Ray light.
In 2020, an additional surprise located a X Ray telescope named eROSITA: Including bigger bubbles, extending more than 45,000 light years, are emitted much less energetic X rays on each side of the galactic plane.
Researchers have since determined that both bubbles are most likely the result of some kind of bursts or outburst coming from the galactic center and the supermassive black hole therein. However, the mechanism which creates gamma and x radiation has been a bit more difficult to learn.
Now physicist Yutaka Fujita from the Tokyo Metropolitan University in Japan has come up with a single explanation that explains both sets of bubbles in one fell swoop, using simulations.
He has discovered that the X-ray emission is the product of a strong, quick moving wind that slams into the tenuous fuel filling the interstellar space, creating a shock wave that reverberates throughout the plasma, giving it that energizing shine.
The massive black hole that provides power to the center of the Milky Way (Sagittarius A *) is quite quiet when it comes to black holes . It’s very little feeding activity. It is classified as being “quiescent.” It has not always been that way, though. An active black hole additionally has effects that impact the area around it.
As the material drops toward the black hole, it warms up and blazes with light. Some of the material is channeled on the outside of the black hole along magnetic field lines, which serve like a synchrotron to speed particles to near light speeds. These’re launched from the black hole poles as potent jets of ionized plasma, pounding up to large numbers of light years in space.
There are also cosmic winds: Streams of charged particles are whipped up by the material orbiting the black hole, which then blitzes into space.
Sagittarius A * could be quiet at the moment, but that has not always been the case. If you look difficult enough, you can find relics of past activity like the Fermi bubbles lurking in space within the galactic plane. We can understand how and when this activity took place by studying these relics.
Fujita’s foray into the Fermi bubbles is based on data from the now retired Suzaku X-ray satellite, jointly run by NASA and also the Japanese Space Agency (JAXA). He took Suzaku observations of X-Ray structures associated with bubbles and carried out numerical simulations to try to reproduce them based on dark hole feeding procedures.
In the paper, he states, “a mixture of the density, temperature as well as shock age profiles of X-ray fuel may be used to identify the energy injection mechanisms.
By looking at the results of numerical simulations with observations, we point out that the bubbles had been produced by a rapid wind from the galactic center, because it generates a strong reverse shock, and reproduces the observed temperature peak there. “
The most probable scenario, he found, is a black hole wind blowing at a speed of 1,000 kilometers per second (621 miles) out of a previous feeding event which was metered out over the course of 10 million years and ended fairly recently. When the wind blows over, the charged particles collide with the interstellar medium, creating a shock wave which bounces back into the bubble. These reverse shock waves heat the substance which is inside the bubbles and causes it to glow.
Fujita’s numerical simulations precisely recreated the heat profile of the X-Ray structure.
Additionally, he checked out the potential for a single explosive eruption from the galactic center, and was not able to recreate the Fermi bubbles. This indicates that a slow, steady wind blowing from the galactic center was the most probable progenitor of the mysterious structures. And also the wind energy can only be attributed to Sagittarius A *, not to star formation — another phenomenon that creates cosmic winds.
“The wind might be the same as active galactic nuclei outflows frequently observed in other galaxies, and also believed to regulate the expansion of galaxies and their central black holes,” he wrote.
The paper has been published in the Monthly Notices of the Royal Astronomical Society.