Ultra-high-energy particles from just outside enormous, active black holes in nearby galaxies travel as far as 250 million light years to make it all the way to Earth, an international team of 400 physicists and astronomers from 17 countries reports in the Nov. 9 issue of the journal Science.
“This is the dawn of a new type of astronomy, the beginning of ultra-high-energy-charged particle astronomy,” said physics professor Katsushi Arisaka, who led the UCLA research group. “This is the first instance of identifying astrophysical objects with charged particles that come to Earth at very high energies.
“The high-energy particles come once per square mile per century,” Arisaka said. “So we had to build an observatory in Argentina that covers an area the size of the entire city of Los Angeles, which took a decade.”
“The energy we observe is spectacular,” said research co-author Alexander Kusenko, UCLA professor of physics and astronomy. “A single particle carries as much energy as a bullet from a rifle or a tennis ball off Roger Federer’s racket.”
These particles, called ultra-high-energy cosmic rays, have puzzled scientists for decades. Typically, they cannot travel far because they are intercepted by cosmic microwave background radiation produced in the Big Bang, which permeates the universe. When a particle interacts with the background radiation, it produces additional particles and its energy is diffused.
“If you collide two cars, you get only parts from those cars,” Kusenko said, “but if you collide two elementary particles, you can produce other elementary particles. It’s like if you have enough energy, you can collide two cars and produce two buses and three motorcycles.” Cosmic rays, discovered 95 years ago, are very energetic particles coming from space into Earth’s atmosphere. Cosmic rays of lower energy have their origin in the sun; those of higher energy come from within our galaxy. But the highest energy rays — those with ultra-high energies — have generally been thought to come from outside our galaxy, although no sources had been identified, said Graciela Gelmini, UCLA professor of theoretical physics and a co-author of the Science paper. Such rays are subatomic particles, most likely protons or other nuclei, with energies much higher than any man-made particle accelerators can produce, Gelmini said.
Black holes are collapsed stars so dense that nothing, not even light, can escape their gravitational pull after entering the black hole’s horizon. However, before matter crosses the black hole’s horizon, it heats up to extremely high temperatures. Particles hit other particles at very high velocities and some get shot out and make it all the way to Earth, said Kusenko, who specializes in particle physics, high-energy astrophysics and cosmology.
The team of scientists used the Pierre Auger Observatory in Argentina, the largest cosmic ray observatory in the world. They report that active galactic nuclei powered by “supermassive” black holes are the most likely sources of the highest-energy cosmic rays that hit Earth. Active galactic nuclei devour stars, dust and gas and generate tremendous amounts of energy. They have long been considered sites where high-energy particle production might take place. While most galaxies, including ours, have black holes at their centers, only a fraction of all galaxies have active galactic nuclei, Gelmini said.
When the highest-energy cosmic rays smash into the upper atmosphere, they create a cascade of billions of secondary particles known as an “air shower,” which can spread across 15 square miles as the particles reach the Earth’s surface.
When the particles interact with the upper atmosphere, they produce secondary particles that interact with other nuclei and produce more charged particles. “It’s like an avalanche,” Gelmini said.
“The particles fall harmlessly when they make it to Earth,” Kusenko said.
The Pierre Auger Observatory records cosmic ray showers through an array of 1,600 particle detectors placed about one mile apart in a grid spread across 1,200 square miles. Twenty-four specially designed telescopes record the emission of fluorescent light from the air shower. The combination of particle detectors and fluorescence telescopes provides a powerful instrument for this research, Arisaka said.
While much progress has been made in nearly a century of research in understanding cosmic rays with low to moderate energies, those with extremely high energies remain mysterious. Physicists can reconstruct the energy of the original particle and the direction from which it travelled.