A little before sunrise on July 4, A.D. 1054, imperial astronomers of the Song Dynasty in China spotted an unknown star lighting up the eastern sky. “It’s as bright as Venus, with pointed rays in all four directions and a reddish-white color,” they wrote in notes delivered to the emperor. The glow, which remained visible to the naked eye during the day for almost a month, was from an explosion caused by the spectacular death of a star located 6,500 light-years away in the constellation of Taurus. Its relics are known today as the Crab Nebula, one of the most beautiful and well-studied objects in the sky.
Scientists have long known the Crab Nebula as a very energetic astrophysical object beaming off radiation ranging from radio waves to gamma rays. But recently, scientists discovered it is even more energetic than they thought. Using an array of state-of-the-art detectors on the eastern edges of the Tibetan Plateau, a team reported in Science this week that it had detected light particles with energies up to more than a quadrillion electron volts (1 PeV) from the famous supernova remnant, indicating that it is so energetic that it poses potential challenges to classical theories of physics.
The Cosmic Accelerator
Sitting 4,410 meters above sea level on the beautiful Haizi Mountain, the Large High Altitude Air Shower Observatory (LHAASO) has detected tens of thousands of very energetic photons from the Crab Nebula since 2019. And for the first time, the observatory made it possible to accurately measure the nebula’s energy spectrum—how many photons of each level of energy it emits—in the higher end of the range, between 0.3 and 1.1 PeV. “The LHAASO results are important because they measured the spectrum of the Crab Nebula in a new energy regime not explored by any previous instrument,” says Rene Ong, an astrophysicist at the University of California, Los Angeles, who was not involved in the research.
Particularly intriguing to experimentalists and theorists alike are the two photons carrying the highest energies ever detected from the Crab Nebula: one at 0.88 PeV, which the team had previously reported in a Nature paper, and the other at 1.1 PeV, which was revealed in the latest study. The tiny particles arrived at Earth with 10 times the energy of a Ping-Pong ball bouncing off a paddle.
“These events are extreme and almost beyond imagination from any point of view,” says Felix Aharonian, a co-author of the new paper at the Dublin Institute for Advanced Studies and the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
How is the Crab Nebula accelerating these particles? Born in the supernova explosion observed nearly 1,000 years ago, the nebula’s heart harbors a pulsar, an extremely dense neutron star spinning 30 times every second. The rotation of the pulsar generates an outward wind made of pairs of electrons and their antimatter counterparts, positrons, which then interact with the surrounding nebula to create shock waves and a natural particle accelerator, according to LHAASO’s principal investigator Cao Zhen of the Institute of High Energy Physics at the Chinese Academy of Sciences. When accelerated particles finally gain the energy to escape, some bump into massless, low-temperature photons from the cosmic microwave background and pass a significant part of their energy on to these particles of light. The photons then dash outward, with some heading straight to Earth, bringing with them important information about the Crab Nebula itself.
Scientists have been observing these high-energy particles from the Crab Nebula for decades, though none had been this energetic. In the early 2000s, scientists observed photons of 75 trillion electron volts (TeV) with an observatory on Spain’s Canary Islands. More recently, a Japanese-Chinese experiment called Tibet AS-gamma caught photons with energies of up to 450 TeV.
To send a record-breaking 1.1-PeV photon to Earth, the original electron from the Crab Nebula must have been about 2.3 PeV, scientists estimate. This energy is about 20,000 times what can be achieved by an electron accelerator on Earth. And physicists would expect the particles in the nebula to lose energy quickly because when electrons travel along curved paths, they release so-called synchrotron radiation, causing them to cool down. At some point, the energy they lose will exceed the energy they gain from the accelerator. “But the pulsar is just about the size of our largest collider,” Cao says. “There must be an incredible mechanism in the Crab Nebula to maximize acceleration against energy loss.”
So far, the 2.3-PeV electron scenario is “allowed by classical electrodynamics and ideal magnetohydrodynamics but very, very close to the theoretical limit,” Aharonian says. The acceleration efficiency is close to 100 percent. Considering the fact that the rotation of the pulsar is the only energy source and that the acceleration process is so complex, “it’s really surprising nature’s accelerator works at such high efficiency, as if it was an ideally designed machine,” he says, “except that no one really designed it.”
When a very high-energy particle strikes Earth’s atmosphere, it triggers a cascade of secondary particles in an event known as an “air shower.” Ground-based detectors such as LHAASO record these air shower events and can then reconstruct the type, energy and trajectory of the primary particles, which are otherwise too elusive to trace.
LHAASO is one of the largest and most sensitive instruments of its kind. Sprawling over a total area of 1.3 square kilometers, it consists of three arrays of detectors. The largest is the Square Kilometer Array, with some 6,000 aboveground counters and more than 1,100 subsurface muon detectors to catch cosmic rays and gamma rays. The second array, the Water Cherenkov Detector Array, uses huge water ponds and light-activated scintillators to look for high-energy gamma rays. Finally, the experiment uses 18 wide-field-of-view Cherenkov telescopes for detecting blue radiation called Cherenkov light that is emitted during air showers.
When Cao first proposed building LHAASO in 2009, people told him he might not be able to see anything. “There was a popular belief that there’s a ‘cutoff’ in the energy spectrum of our galaxy at around 100 TeV, which seemed to be a theoretical ceiling,” he recalls. “But I didn’t buy it. As an experimentalist, my mission is to experiment, and LHAASO would go exactly for the unknown regime beyond 100 TeV.” The observatory’s construction started in 2017. It began operations two years later, when LHAASO was not even half-complete. Using data from the first few months, Cao and his team reported a dozen PeV-level gamma-ray sources across the galaxy, almost doubling the total number of such sources discovered to date. “Our results clearly showed there is no such cutoff at 100 TeV,” he says. “Instead the energy spectrum keeps extending forward to, and beyond, 1 PeV, as in the case of the Crab Nebula.”
The results did not come easy, especially because China was a latecomer to the field of gamma-ray astronomy. Cao still remembers when he was an undergraduate student learning to set up China’s first gamma-ray detectors in a peach yard in suburban Beijing in 1986. On the other side of the Pacific Ocean at that time, the late astrophysicist and Nobel laureate James Cronin was already getting ready to detect PeV gamma rays via a project called CASA-MIA (the Chicago Air Shower Array–Michigan Muon Array) in the deserts of Utah. CASA-MIA was then the largest and most ambitious experiment to study gamma rays at energies above 100 TeV. Unfortunately, it did not detect any during its five years of observation. “CASA-MIA was very sensitive at the time, but it wasn’t sufficient to do the job,” says Ong, who was a part of the CASA-MIA team. No one tried that technique again until LHAASO. The new observatory is everything that CASA-MIA was, plus a bigger and better surface array, much better muon detectors, a cleverly designed layout and a higher altitude. “And that’s why it worked,” Ong says. “Personally, it’s extremely gratifying for me to see that someone took up what we had worked hard on for 10 years and did a really great job with it.”
Statistics about the PeV-level acceleration happening inside the Crab Nebula are so far limited to two photons, Cao admits. Because LHAASO is designed to detect at least one or two such events every year, however, the team hopes to confirm its findings in a couple of years.
To answer the ultimate questions about cosmic accelerators and cosmic rays, LHAASO will need to work with other detectors. The experiment, though powerful enough to dominate its energy band in years to come, suffers from relatively low angular resolution and sky coverage, and it lacks the ability of instantaneous detection. It will partner with the upcoming Cherenkov Telescope Array (CTA), a global effort to use more than 100 telescopes located in the Northern and Southern hemispheres to detect high-energy gamma rays in and out of our galaxy. Unlike LHAASO, CTA will use imaging atmospheric Cherenkov telescopes, and it will be highly complementary to that observatory. “LHAASO and CTA will need to run together for a decade or so to really pin down the origin of cosmic rays,” says Ong, who is a co-spokesperson of CTA. LHAASO is ready to collaborate with other experiments from around the world, Cao says. In fact, the team has already signed agreements with a number of observatories, including the Baikal Gigaton Volume Detector in Russia and the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona. VERITAS has started follow-up observations of some of the sources LHAASO reported in its previous Nature paper.
LHAASO will wrap up the last bit of its construction by the end of this month. “The work has just started, though it’s already very impressive,” Aharonian says. The experiment reflects the quick rise of China, an ancient astronomical powerhouse, in the modern astrophysics arena, he says. The nation is in a good position to accomplish world-leading astrophysics research because of its well-trained young scientists and economic power, along with its government’s willingness to invest in basic science, he observes. “LHAASO is just one project that shows how today’s China can do science in a timely and highly cost-efficient way,” Aharonian says.