Imperial astronomers of the Song Dynasty in China noticed an unusual star lighting up the eastern sky shortly before daybreak on July 4, A.D. 1054. They wrote in notes to the emperor, “It’s as brilliant as Venus, with pointed beams in all four directions and a reddish-white hue.” The glow was created by an explosion triggered by the spectacular death of a star situated 6,500 light-years distant in the constellation of Taurus. It was visible to the naked eye throughout the day for over a month. The Crab Nebula, one of the most magnificent and well-studied phenomena in the sky, is one of its remnants.
The Crab Nebula has long been recognized as a very active astrophysical phenomenon that emits radiation ranging from radio waves to gamma rays. However, scientists have discovered that it is considerably more energetic than previously assumed. 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 using an array of state-of-the-art detectors on the eastern edges of the Tibetan Plateau, indicating that it is so energetic that it poses potential challenges to classical physics theories.
THE UNIVERSAL ACCELERATOR
Since 2019, the Large High Altitude Air Shower Observatory (LHAASO), which is located 4,410 meters above sea level on the picturesque Haizi Mountain, has observed tens of thousands of highly powerful photons from the Crab Nebula. The observatory also made it feasible to quantify the nebula’s energy spectrum—how many photons of each level of energy it emits—for the first time in the higher end of the range, between 0.3 and 1.1 PeV. “The LHAASO discoveries are significant because they recorded the spectrum of the Crab Nebula in a new energy domain that no prior instrument has explored,” says Rene Ong, an astronomer at the University of California, Los Angeles who was not involved in the study.
The two photons with the greatest energy ever recorded from the Crab Nebula, one at 0.88 PeV, which the researchers had previously published in a Nature publication, and the other at 1.1 PeV, which was disclosed in the current study, are particularly fascinating to experimentalists and theorists alike. The minuscule particles had ten times the energy of a Ping-Pong ball bouncing off a paddle when they landed on Earth.
“From any point of view, these occurrences are extraordinary and nearly beyond imagination,” says Felix Aharonian of the Dublin Institute for Advanced Studies and the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, who is a co-author of the new article.
What is the mechanism through which the Crab Nebula accelerates these particles? The nebula’s heart contains a pulsar, an incredibly dense neutron star spinning 30 times per second, which was born in a supernova explosion almost 1,000 years ago. According to LHAASO’s principal investigator Cao Zhen of the Chinese Academy of Sciences’ Institute of High Energy Physics, the pulsar’s rotation generates an outward wind of pairs of electrons and their antimatter counterparts, positrons, which interact with the surrounding nebula to create shock waves and a natural particle accelerator. When speeding particles gather enough energy to escape, some collide with massless, low-temperature photons from the cosmic microwave background, passing a large portion of their energy onto these light particles. The photons then scatter, with some of them coming directly for Earth, carrying vital information about the Crab Nebula.
For decades, scientists have been monitoring high-energy particles from the Crab Nebula, but none had been this powerful. With an observatory on Spain’s Canary Islands, scientists saw photons with a mass of 75 trillion electron volts (TeV) in the early 2000s. Tibet AS-gamma, a Japanese-Chinese experiment, recently captured photons with energy of up to 450 TeV.
According to experts, the initial electron from the Crab Nebula must have been around 2.3 PeV to transmit a record-breaking 1.1-PeV photon to Earth. This energy is around 20,000 times more than what an electron accelerator on Earth can produce. And physicists anticipate the particles in the nebula to lose energy fast because electrons produce so-called synchrotron radiation as they move along curved pathways, which causes them to cool down. The energy they lose will eventually outweigh the energy they get from the accelerator. “However, the pulsar is roughly the same size as our biggest collider,” Cao explains. “The Crab Nebula must have some extraordinary technique to enhance acceleration while minimizing energy loss.”
According to Aharonian, the 2.3-PeV electron scenario is “allowed by classical electrodynamics and ideal magnetohydrodynamics but quite close to the theoretical limit.” The efficiency of acceleration is close to 100 percent. Given that the pulsar’s spin is the only source of energy and that the acceleration process is so complicated, he adds, “it’s truly astonishing nature’s accelerator operates at such high efficiency, as if it were an excellently constructed machine, except that no one created it.”
An “air shower” occurs when an extremely high-energy particle collides with Earth’s atmosphere, causing a cascade of secondary particles. These air shower events are recorded by ground-based detectors like LHAASO, which can then reconstruct the type, energy, and route of the initial particles, which are otherwise too elusive to track.
The LHAASO is one of the world’s largest and most sensitive devices. It is made up of three detector arrays that cover a total area of 1.3 square kilometers. The Square Kilometer Array is the biggest, with over 1,100 underground muon detectors and 6,000 aboveground counters to collect cosmic rays and gamma rays. The Water Cherenkov Detector Array, the second array, searches for high-energy gamma rays using large water ponds and light-activated scintillators. Finally, the experiment employs 18 Cherenkov telescopes with large fields of vision to detect blue radiation known as Cherenkov light that is generated during air showers.
People warned Cao in 2009 that he might not be able to see anything if he built LHAASO. “There was a widespread idea that the energy spectrum of our galaxy has a ‘cutoff’ at about 100 TeV, which appeared to be a theoretical ceiling,” he remembers.
“However, I didn’t buy it. My job as an experimentalist is to try new things, and LHAASO would head straight for the unexplored regime beyond 100 TeV.” Construction of the observatory began in 2017. It commenced operations two years later when LHAASO was still in the early stages of development. Cao and his colleagues reported a dozen PeV-level gamma-ray sources around the galaxy using data from the first few months, nearly tripling the total number of such sources identified to date. He claims, “Our data conclusively proved there is no such limit at 100 TeV.” “Instead, like in the case of the Crab Nebula, the energy spectrum continues to extend forward to and beyond 1 PeV.”
The results were not simple to come by, especially because China was a latecomer to gamma-ray astronomy. Cao recalls learning to put up China’s first gamma-ray detectors in a peach yard in suburban Beijing in 1986 as an undergraduate student. On the opposite side of the Pacific Ocean, the late astronomer and Nobel laureate James Cronin were preparing to discover PeV gamma-rays in the Utah deserts with a project named CASA-MIA (Chicago Air Shower Array–Michigan Muon Array). CASA-MIA was the largest and most ambitious experiment at the time to examine gamma rays with an energy of more than 100 TeV.
Unfortunately, during its five-year investigation, it found none. “CASA-MIA was extremely sensitive at the time, but it wasn’t enough to accomplish the job,” says Ong, who was on the CASA-MIA team at the time. Until LHAASO, no one tried that approach again. The new observatory has all of the features of CASA-MIA, plus a larger and better surface array, significantly improved muon detectors, a well-designed architecture, and a higher altitude. “That’s why it worked,” adds Ong. “It gives me tremendous pleasure to know that someone has taken up what we had worked on for ten years and done a fantastic job with it.”
VISIONING THE FUTURE
Cao concedes that data on the PeV-level acceleration occurring inside the Crab Nebula is currently restricted to two photons. The team expects to validate its results in a few years because LHAASO is intended to identify at least one or two such occurrences every year.
LHAASO will need to collaborate with other detectors to solve the ultimate questions concerning cosmic accelerators and cosmic rays. Though strong enough to dominate its energy band for years to come, the experiment has limited angular resolution and sky coverage, and it lacks immediate detection capability. It will collaborate with the forthcoming Cherenkov Telescope Array (CTA), a worldwide initiative to detect high-energy gamma rays in and out of our galaxy using more than 100 telescopes in the Northern and Southern hemispheres. CTA, unlike LHAASO, will employ imaging atmospheric Cherenkov telescopes and will be a valuable addition to that observatory. “To truly narrow down the origin of cosmic rays, LHAASO and CTA will need to work together for a decade or so,” says Ong, a CTA co-spokesperson. Cao adds LHAASO is open to collaborating with other experiments from across the world. The team has already inked partnerships with many observatories, including Russia’s Baikal Gigaton Volume Detector and Arizona’s Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS has begun following up on some of the sources mentioned in LHAASO’s earlier Nature publication.
By the end of this month, LHAASO will have completed the last phase of its construction. “The work has just begun, but it is already extremely impressive,” adds Aharonian. According to him, the experiment shows China’s rapid ascent in contemporary astrophysics as an ancient astronomical superpower. Because of its highly-trained young scientists and economic strength, as well as its government’s readiness to invest in fundamental science, the country is in an excellent position to achieve world-leading astrophysics research, he notes. “LHAASO is only one initiative that demonstrates how today’s China can do science in a timely and cost-effective manner,” adds Aharonian.