Mysterious high-energy particles known as cosmic rays zip through space at a wide range of energies, some millions of times greater than those produced in the world’s most powerful atom smasher. Scientists have long thought cosmic rays from inside our galaxy come from supernova explosions, but a new study has fingered a second source: the supermassive black hole at the heart of the Milky Way. With this new result, the search for cosmic ray origins, which has frustrated scientists for more than 100 years, has taken an unexpected new twist.
“It’s very exciting,” says astrophysicist Andrew Taylor of the Dublin Institute for Advanced Studies. “This has probably shaken the field quite a lot. People will need to reassess their models.”
Cosmic rays pose a mystery for astronomers because they don’t follow a straight path through space. They get tugged and pushed by magnetic fields, so it is almost impossible to figure out where particular particles have come from. So instead, researchers have looked at gamma rays, high-energy photons that are thought to be produced at or near the source of the cosmic rays. Find out where the gamma rays come from, and you’ve probably found the source of cosmic rays.
Although many of the cosmic rays from within our galaxy appear to be blasted out from supernova explosions at blistering speeds, such explosions can’t explain the highest energy cosmic rays: those with energies measured in peta-electronvolts (PeV, or 1015 eV). (Here on Earth, 1 PeV is the total energy that the Large Hadron Collider can achieve when slamming together lead ions.)
“We don’t really know what’s going on,” says Werner Hofmann of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
The difficulty in studying both cosmic rays and their accompanying gamma rays, however, is that they get destroyed by colliding with atoms high in the atmosphere and never reach Earth’s surface. These collisions do, however, send showers of other particles raining down toward the surface. Astronomers can measure the spread of those particles with detectors on the ground, or capture flashes of light called Cherenkov radiation, which the particles give off as they decelerate in the atmosphere.
In the new study, Hofmann and colleagues used the High Energy Stereoscopic System (HESS), an array of five telescopes in Namibia, which can detect such radiation. HESS has been studying the galactic center for about a decade, Hofmann says, simply because it is an interesting source of gamma rays. In recent years, his team has carried out more detailed observations. And, as it reports online today in Nature, the distribution of gamma rays coming from around the galactic center is exactly what you would expect if some process, close to the black hole, is firing out protons with PeV energies.
Many of those protons may much later arrive at Earth as PeV cosmic rays, but some are colliding with gas molecules close to their source and producing gamma rays. It is these gamma rays HESS is able to pick up, revealing the origin of these superfast protons. “It really demonstrates that there is a central source [of protons],” Hofmann says.
“This is a great result. It’s very fascinating,” says astrophysicist Pasquale Blasi of the Arcetri Astrophysical Observatory in Florence, Italy. “For the very first time we have almost direct evidence of the acceleration of protons to these energies.” But he cautions that it has not yet been proved that these same protons make it all the way to Earth as cosmic rays. Over such a distance, there is a high probability that they can diffuse out of the halo of the galaxy and escape. There are ways to detect cosmic rays en route between the galactic center and here, but “we may need to think outside the box,” he says.
According to Hofmann, there are “very few clues about what the actual accelerator is.” One possibility the paper mentions is that very close to the black hole, where gas and dust are being sucked in by its gravity, the tangle of electric and magnetic fields in this superheated material is somehow whipping protons up to very high energy. The team will continue to monitor the galactic center’s gamma rays for insights. Any changes in luminosity over days, months, or years will provide some clues, as will the distribution of gamma rays around the black hole and a more detailed energy spectrum. “These could give a handle on the mechanism,” Hofmann says.
Ultimately, the answer may have to wait for the construction of a new detector, the Cherenkov Telescope Array (CTA), which will have more than 100 mirrors spread between sites in the Northern and Southern hemispheres and so will produce images with finer resolution than any available today. “CTA could resolve the size of the source: Is it really a point [source] or more extended?” Hofmann says.
Taylor points out that this result also bolsters a current theory for the source of the much rarer and even higher energy cosmic rays that have traveled across the vast reaches of space from distant galaxies. Theorists think they come from active galactic nuclei (AGNs), supermassive black holes that are consuming matter so fast that they heat up the in-falling gas and dust to colossal temperatures, making it shine brightly enough to be seen across the universe. If our relatively tame central black hole can produce cosmic rays, he says, “that strengthens the case that AGNs are the source of extragalactic cosmic rays.”