In 2010, astronomers working with the Fermi Gamma-Ray Space Telescope unveiled a previously unseen structure centered in the Milky Way that spans 50,000 light-years that may be the remnant of an eruption from the supermassive black hole at the center of our galaxy. The structure spans more than half of the visible sky, a region roughly as large as the Milky Way itself, and it may be millions of years old, its origin an unsolved mystery.
In 2012, The Daily Galaxy reported that scientists were conducting more analyses to better understand how the never-before-seen structure was formed. The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way. The bubbles also appear to have well-defined edges. The structure’s shape and emissions suggest it was formed as a result of a large and relatively rapid energy release – the source of which remains a mystery.
The nature of Fermi bubbles is still unclear, however the location of these objects indicates their connection to past or present activity in the center of the galaxy, where our central Sag A* black hole of 106 solar masses is located. Modern models link the bubbles to star formation and/or an energy release in the Galactic center as a result of tidal disruption of stars during their accretion onto Sag A*. The bubbles are not considered to be unique phenomena observed only in the Milky Way and similar structures can be detected in other galactic systems with active nuclei.
Fermi bubbles promise to reveal deep secrets about the structure and history of our galaxy. We don’t know why the mass of the black hole in the center of the Milky Way is so small relative to other supermassive black holes, or how the interaction between this relatively small black hole and the Milky Way galaxy works. The bubbles provide a unique link for both how the black hole grew and how the energy injection from the black hole accretion process impacted the Milky Way as a whole.
In August of 2017, The Daily Galaxy reported that a team of scientists from Russia and China developed a model which explains the nature of high-energy cosmic rays (CRs) in our Galaxy. These CRs have energies exceeding those produced by supernova explosions by one or two orders of magnitude. The model focuses on the discovery of the giant structures called Fermi bubbles.
Hints of the Fermi bubbles’ edges shown above were first observed in X-rays by ROSAT, which operated in the 1990s. The gamma rays mapped by the Fermi Gamma-ray Space Telescope extend much farther from the galaxy’s plane.
One of the key problems in the theory of the origin of cosmic rays (high-energy protons and atomic nuclei) is their acceleration mechanism. The issue was addressed in the 1960s when scientists suggested that CRs are generated during supernova (SN) explosions in the Milky Way. A specific mechanism of charged particle acceleration by SN shock waves was proposed in 1977. Due to the limited lifetime of the shocks, it is estimated that the maximum energy of the accelerated particles cannot exceed 1014-1015 electronvolts.
The question arises of how to explain the nature of particles with energies above 1015 eV. A major breakthrough in researching the acceleration processes of such particles came when the Fermi Gamma-ray Space Telescope detected two gigantic structures emitting radiation in gamma-ray band in the central area of the Galaxy in November 2010.
The discovered structures are elongated and are symmetrically located in the Galactic plane perpendicular to its center, extending 50,000 light-years, or roughly half of the diameter of the Milky Way disk. These structures became known as Fermi bubbles. Later, the Planck telescope team discovered their emission in the microwave band.
An international team of astrophysicists have shown that X-ray and gamma-ray emission in these areas is due to various processes involving relativistic electrons accelerated by shock waves resulting from stellar matter falling into a black hole.
In this case, the shock waves should accelerate both protons and nuclei. However, in contrast to electrons, relativistic protons with bigger masses hardly lose their energy in the Galactic halo and can fill up the entire volume of the galaxy. The authors of the paper suggest that giant Fermi bubbles shock fronts can re-accelerate protons emitted by supernova to energies greatly exceeding 1015 eV.
The IceCube Observatory, situated in Antarctica at the Amundsen-Scott South Pole Station, uses a cubic kilometer of pure Antarctic water ice as a neutrino detector to capture evidence of a high-energy neutrino passing through the ice as it interacts with a water molecule, setting up a domino-like chain reaction that leads a telltale flash of light. To date, Ice Cube it has detected 10 of these ghostly cosmic messengers coming from roughly the direction of the two Fermi Bubbles, leading some astrophysicists to conjecture that some unknown subatomic interactions are occurring there.
IceCube Detects Big Bird
On Dec. 4, 2012, IceCube detected an event now known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV). To put that in perspective, it’s more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron. Big Bird was the highest-energy neutrino ever detected at the time.
Where did Big Bird come from? The best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons.
“It’s like a crime scene investigation”, says Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, “The case involves an explosion, a suspect, and various pieces of circumstantial evidence.”
The peculiar orientation of the Fermi Bubbles — extending evenly above and below our galactic center — is a strong clue that they might be tied our central supermassive black hole, known as Sagittarius A*. One conjecture is that a star wandered too close to Sag A* and was devoured, releasing its gravitational energy in a single violent episode, leading to the formation of the bubbles.
Sagittarius A*, is the most plausible source of the PeV protons,’ says Felix Aharonian (Max-Planck Institute for Nuclear Physics Heidelberg, MPIK, and Dublin Institute for Advanced Studies, DIAS), adding that, ‘Several possible acceleration regions can be considered, either in the immediate vicinity of the black hole, or further away, where a fraction of the material falling into the black hole is ejected back into the environment, thereby initiating the acceleration of particles.’
Was Sagittarius A* more active in the past?
‘If, however, Sagittarius A* was more active in the past,’ astrophysicist Christopher van Eldik explains, ‘then it could indeed be responsible for the bulk of today’s galactic cosmic rays that are observed on earth.’ If true, this would dramatically influence the century-old debate on the origins of galactic cosmic rays, as the theory that their components are primarily accelerated to PeV energies by remnants of supernovae – shock waves that occur after the explosion of massive stars – would have to be revised to take this into account.
Which throws cold water on the hypothesis that perhaps dozens or hundreds of the densely packed stars at the Milky Way’s core went supernova at around the same time, creating the plumes.
In August of 2016, The Daily Galaxy reported that our “Milky Way’s ‘Lurking, Dormant Monster’ –“Our Central Black Hole Currently Fails to Outshine a Single Star.” Up until this May, 2019, Sagittarius A* (Sgr A*), appeared like a massive, dormant volcano, a sleeping monster, a slumbering region of spacetime where gravity is so strong that “what goes into them does not come out.”
It appears that something disrupted Sgr A*’s slumber. Conjectures for its recent flaring range from data errors to SO-2, one of two stars that approach very closely to Sgr. A* in an elliptical orbit. Every 16 years, it’s at its closest. In the middle of 2018 was its last closest approach, when it was only 17 light-hours away from the black hole. Another strong possibility is the massive gas cloud known as G2 that might be drawn into Sgr. A*’s accretion disk causing it to flare brightly as it was heated, triggering a chain of events that caused or contributed to the May 2019 flaring.
It turns out that, approximately 300 years ago, Sagittarius A* let loose, expelling a massive energy flare. Data taken from 1994 to 2005 revealed that clouds of gas near the central black hole, known as Sagittarius B2, brightened and faded quickly in X-ray light. The X-rays were emanating from just outside the black hole, created by the buildup of matter piling up outside the black hole, which subsequently heats up and expels X-rays.
One possibility includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way’s black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way’s center several million years ago.
“In other galaxies, we see that starbursts can drive enormous gas outflows,” said David Spergel, a scientist at Princeton University in New Jersey. “Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics.”