Astronomers said they have traced the origin of the most energetic particles in the universe to the supermassive black holes found in the centres of nearby galaxies.
The findings, published Thursday in the journal Science, could go a long way to solving a decades-long mystery as to the origin of high-energy cosmic rays, and shed light on the mysterious processes that occur on the edges of black holes.
"This result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy," said University of Leeds astronomer Alan Watson, a spokesperson for the Pierre Auger Collaboration, the international team of astronomers behind the study.
Cosmic rays are actually atomic particles — most often protons — that fly through space at high speeds and can collide with the Earth's upper atmosphere, sending a shower of secondary particles to the planet's surface.
Astronomers know that some of the cosmic rays that hit the Earth come from the Sun, but the point of origin for most of them is nearly impossible to determine because as they hurtle through space, their path can twist and turn in response to magnetic fields or lose energy after bouncing into other particles.
Some of these particles, however, hit our atmosphere at speeds far greater than most cosmic rays. Based on their collision speed, these cosmic rays are likely to have taken the most direct path to Earth, the authors contend.
Finding evidence of these highest-energy particles, however, has proven a challenging task because of their rarity. To catch evidence of them, astronomers at the Pierre Auger Observatory, located in Argentina, cast a wide net.
Using an array of 1,600 particle detectors placed 1.5 kilometres apart in a 3,000-square-kilometre grid, the astronomers collected data from the particle showers that result from the collision of cosmic rays and the atmosphere.
Since 2004, they've found evidence of only 27 cosmic ray events with energy levels above 5.7 x 1019electron volts, or 57 EeV, or about 100 million times higher than the energy levels created at the most powerful particle accelerators on Earth.
Most of these events, however, correspond with the known locations of active galactic nuclei, which are thought to be the sites of supermassive black holes that are devouring large amounts of matter.
"When you look at the highest-energy cosmic rays from the most violent sources, they point back to their sources. The challenge now is to record enough of these cosmic bullets to understand the processes that hurl them into space," said Paul Mantsch, project manager of the Pierre Auger Observatory, in a statement.
It's not the first study to suggest cosmic rays could begin near black holes, but it is the first to attempt to track their point of origin directly rather than through indirect measures.
A study published in May of this year in the Astrophysical Journal Letters suggested gamma rays — a wavelength of light — originating from the black hole at the centre of the Milky Way could be artifacts of previous collisions between cosmic rays and the cloud of gas thought to surround the black hole.
As University of Arizona professor David Ballantyne said at the time, "We know something is going on around the black hole, but we still haven't been able to provide a definitive answer."
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