Something in the Milky Way is shooting out unexplained bursts of energy that are hundreds of times more powerful than any human-made particle accelerator on Earth.
Now, with the help of a huge telescope located at high altitudes in Tibet, scientists have discovered the most energetic examples of these bursts ever detected, which could help to resolve a long-standing cosmic mystery. The most energetic ray the team detected is a world record (well, technically a galaxy record), and represents the most powerful particle accelerator ever observed, whether natural or created by scientists.
For decades, scientists have been awestruck by diffuse streaks of energetic gamma rays with mysterious origins. Theories suggest that these bursts are caused by cataclysmic events, such as exploding stars or material ejected by the tidal forces of supermassive black holes. These tumultuous explosions spew out cosmic rays, which are streams of protons and other atomic debris. When ultra-high energy cosmic rays interact with interstellar gas within a galaxy, it sets off nuclear reactions that produce the gamma ray bursts that scientists have long observed.
As observatories that focus on the high-energy universe have steadily improved, these diffuse gamma ray bursts have been detected at progressively greater energies. This week marked a new record in this effort: Scientists reported “for the first time, the long-awaited detection of diffuse gamma rays with energies between 100 TeV and 1 PeV in the Galactic disk,” according to a study published on Monday in Physical Review Letters.
To put that in perspective, the most powerful particle accelerator on Earth, CERN’s Large Hadron Collider in Geneva, blasts out beams at 6.5 teraelectron volts (TeV). The most energetic gamma ray burst observed in the new study reached an astonishing 957 TeV. The results hint at the existence of so-called “PeVatrons,” cosmic particle super-accelerators of unknown origin that may produce energetic bursts that exceed 1 PeV, a value 1,000 times more energetic than a TeV.
The new study described ultra-high-energy gamma rays observed with the Tibet ASγ experiment, an array on the Tibetan Plateau that covers nearly a square kilometer and that includes underground particle detectors. Twenty-three of the diffuse gamma rays were above 398 TeV and the one that approached 1 PeV was more than twice as energetic as the previous record-holder.
The Tibet ASγ collaboration was able to track down that now-dethroned gamma ray to its source in the Crab Nebula, which is the gassy remnant of a supernova. However, that case was an outlier; in general, it’s tricky to figure out what exactly might be causing these energetic light sources.
Ultra-high-energy cosmic rays carry a charge, which makes them vulnerable to getting sent on circuitous twists and turns by all the various magnetic fields within the galaxy. Once the cosmic rays transform into gamma rays, their paths across the Milky Way become more straightforward, but by that point, it’s challenging to follow their tracks back to a source.
Complicating matters further, it’s likely that many speculative PeVatrons don’t even exist anymore. The Tibet ASγ collaboration plans to follow up on this study by investigating whether any of the long-sought accelerators that created the observed bursts might still be alive and kicking out cosmic rays, or if their trails have now run cold for good.
"From dead PeVatrons, which are extinct like dinosaurs, we can only see the footprint—the cosmic rays they produced over a few million years, spread over the galactic disk," said study coauthor Masato Takita, a professor at the University of Tokyo’s Institute for Cosmic Ray Research, in a statement.
"But if we can locate real, active PeVatrons, we can study many more questions," he added.