New observatory makes a splash

High atop the Tibetan Plateau in southwestern China, a unique astronomical observatory has already made a noteworthy discovery, and it hasn’t even fully opened yet. Scientists working at the partially completed Large High Altitude Air Shower Observatory say they have picked up evidence of high-energy gamma rays from outer space.

Astrophysicist Cao Zhen and his colleagues documented the evidence in a May 17 article in Nature. If it proves true, it would reveal the most energetic gamma rays ever observed. Understanding those gamma rays may one day help scientists understand how matter spread throughout the universe.

The early success of LHAASO has impressed the astrophysics community. “These results are really stunning—some of the most exciting I have ever seen,” said Alan Watson, a resident astrophysicist at Pierre Auger Observatory in Argentina.

Chinese scientists began making observations from LHAASO in 2019. And while the full array of Cherenkov telescopes, muon detectors, and electromagnetic detectors isn’t scheduled for completion until later this year, Cao’s report demonstrates the capability of the $175 million observatory to investigate some of the universe’s most elusive phenomena.

Gamma rays, which sit at the edge of the electromagnetic spectrum, are the most energetic of nature’s known wavelengths. While radio waves emanating from an AM station create wavelengths measured in hundreds of meters, gamma ray wavelengths can be 10 trillion times smaller, as tiny as 0.01 nanometers.

On Earth, radioactive decay of certain elements or nuclear detonations can produce some types of gamma rays. But the ones that interest astrophysicists at LHAASO result from exploding stars, pulsars, and magnetars. When gamma rays enter Earth’s atmosphere and collide with particles, they produce a muon. These elementary particles emit flashes of dim blue light as they move through water.

The strange sensors of LHAASO are designed to detect those faintly glowing muons. The observatory’s array involves pools of purified water that suspend more than 3,000 beachball-sized cameras working in tandem to detect the particles. While other similar arrays exist in Antarctica, experts expect LHAASO’s to be the most sensitive gamma ray-detecting instrument in the world upon completion.

In the report in Nature, Cao’s team extrapolated from their analysis of the rays’ angle of entry into the pool, hypothesizing that the most powerful gamma rays came from within the Crab Nebula, the site of an ancient supernova and an incredibly powerful pulsar, and the Cygnus Cocoon, a stellar nursery 4,600 light years from the Sun.

Astrophysicists still don’t exactly know how such high-energy waves are created, but research at LHAASO may one day help scientists learn more. “The findings are extremely important and impressive,” Petra Huentemeye, spokesman for the rival High-Altitude Water Cherenkov Observatory in Mexico, told Science. “It’s a giant leap toward finally understanding the origin of the highest energy cosmic rays.”