The universe is permeated by a constant stream of fast charged particles, which are commonly called cosmic radiation. They move from star system to star system with energies larger than what the most powerful particle accelerator can produce, keeping their origin mysterious.
But now, thanks to 10 years worth of observations, we think we have found the source of cosmic rays in the galactic center: the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*). The cosmic rays are seen reaching an energy of a few petaelectronvolts (PeV), which is about 200 times the energy given to protons in the Large Hadron Collider.
One petaelectronvolt is a tiny measurement for our everyday scale of energies, and it corresponds to the acoustic energy of a whisper. But, if you imagine that packed into a subatomic particle, it is clear why cosmic rays are so energetic.
Scientists don’t know much about the origin of cosmic rays because they don’t follow a straight path in the cosmos. They are charged particles (mostly electrons and protons) and they are pushed and pulled by the magnetic fields of galaxies, stars, and planets.
For that reason, researchers used an indirect method to work out their origin. When the most energetic cosmic rays are produced, gamma-ray photons are emitted as well so finding one allows us to find the origin of the other. Researchers from 42 institutions used the H.E.S.S. observatory in Namibia to observe where very-high-energy gamma rays are coming from.
The results, published in Nature, highlight a significant source of these photons at the core of the Milky Way. There are several potential candidates, such as a compact cluster of massive stars, a supernova remnant and a pulsar wind nebula. But one suspect is more likely than the others.
“The supermassive black hole located at the center of the Galaxy, called Sgr A*, is the most plausible source of the PeV protons,” said Felix Aharonian, a member of the HESS collaboration, in a statement.
“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.”
While the discovery brings us closer to unmasking what’s behind cosmic rays, it doesn’t explain everything. Based on current observations only a fraction of these particles could come from the center.
“If, however, our central black hole was more active in the past,” the team added, “then it could indeed be responsible for the bulk of the Galactic cosmic rays that are observed today at the Earth.”
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