Just before sunrise on July 4, 1054 AD, imperial astronomers of the Song Dynasty discovered an unknown star in China that lit up the eastern sky. “It is as bright as Venus, with sharp rays in all four directions and a reddish-white color,” they wrote in notes to the emperor. The glow, which remained visible to the naked eye for almost a month, came from an explosion caused by the spectacular death of a star in Taurus, 6,500 light-years away. 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 high-energy astrophysical object that emits radiation from. radiates Radio waves to gamma rays. But recently scientists discovered that it is even more energetic than they thought. Using a series of state-of-the-art detectors on the eastern edge of the Tibetan Plateau, a team in Science reported this week that there were light particles with energies of up to a quadrillion electron volts (1 PeV) from the famous supernova remnant, indicating that it is so energetic that he potentially questions the classical theories of physics.
The cosmic accelerator
The Large High Altitude Air Shower Observatory (LHAASO) is located at 4,410 meters above sea level on the beautiful Haizi Mountain and has detected tens of thousands of very high-energy photons from the Crab Nebula since 2019 – they measure exactly the energy spectrum of the nebula – how many photons of each energy level it emits – at the upper 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 that no previous instrument explored,” says Rene Ong, an astrophysicist at the University of California, Los Angeles, who is not involved in the research was.
Particularly fascinating for experimentalists and theorists are the two photons that carry the highest energies ever discovered by the Crab Nebula: one at 0.88 PeV, which the team previously reported on in a Nature paper, and the other at 1.1 PeV, that in the latest study. The tiny particles reached the earth with ten times the energy of a table tennis ball bouncing off a paddle.
“These events are extreme and hardly imaginable in every respect,” says Felix Aharonian, co-author of the new paper at the Dublin Institute for Advanced Studies and at the Max Planck Institute for Nuclear Physics in Heidelberg.
How does the Crab Nebula accelerate these particles? Born in the supernova explosion observed nearly 1,000 years ago, the heart of the nebula is home to a pulsar, an extremely dense neutron star that spins 30 times per second. The pulsar’s rotation creates an outward wind of electron pairs and their antimatter counterparts, positrons, which then interact with the surrounding nebula to create shock waves and a natural particle accelerator, according to Cao Zhen of the Institute of High Energy Physics at the Chinese Academy of Sciences Sciences. When accelerated particles finally gain the energy to escape, some encounter massless, low-temperature photons from the cosmic microwave background and pass on a significant portion of their energy to these light particles. The photons then shoot outwards, some of them directly towards 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, although none was so energetic. In the early 2000s, scientists at an observatory on the Spanish Canary Islands observed photons of 75 trillion electron volts (TeV). Recently, a Sino-Japanese experiment called Tibet captured AS-gamma photons with energies up to 450 TeV.
Scientists estimate that to send a record-breaking photon of 1.1 PeV to Earth, the original electron in the Crab Nebula must have been around 2.3 PeV. This energy is about 20,000 times what an electron accelerator can achieve on earth. And physicists would expect the particles in the nebula to lose energy quickly, because when electrons wander on curved paths, they release so-called synchrotron radiation, which allows them to cool down. At some point the energy they lose will exceed the energy they gain from the accelerator. “But the pulsar is about the size of our largest collider,” says Cao. “There has to be some incredible mechanism in the Crab Nebula to maximize acceleration against energy loss.”
So far, the 2,3-PeV electron scenario “is permitted by classical electrodynamics and ideal magnetohydrodynamics, but very, very close to the theoretical limit,” says Ahronian. The acceleration efficiency is close to 100 percent. Given that the rotation of the pulsar is the only source of energy and the acceleration process is so complex, “it is really surprising that nature’s accelerator works with such efficiency as if it were an ideally designed machine,” he says, ” except that nobody really designed it. “
When a very energetic particle hits the Earth’s atmosphere, it triggers a cascade of secondary particles, 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 that are otherwise too difficult to track.
LHAASO is one of the largest and most sensitive instruments of its kind. It covers a total area of 1.3 square kilometers and consists of three detector arrays. The largest is the Square Kilometer Array with around 6,000 above-ground meters and more than 1,100 underground muon detectors for capturing cosmic rays and gamma rays. The second array, the Water Cherenkov Detector Array, uses huge ponds of water and light-activated scintillators to search for high-energy gamma rays. Finally, Experiment 18 uses Cherenkov wide field of view telescopes to detect blue radiation called Cherenkov light, which is emitted during air showers.
When Cao first suggested building LHAASO in 2009, people told him that he might not be able to see anything. “There was a widespread opinion that there was a ‘cutoff’ in the energy spectrum of our galaxy at around 100 TeV, which seemed like a theoretical upper limit,” he recalls. “But I didn’t buy it. As an experimenter, my mission is to experiment, and LHAASO would strive for precisely the unknown regime beyond 100 TeV. ”Construction of the observatory began in 2017. Operations began two years later when LHAASO was not even halfway through. Using data from the first few months, Cao and his team reported over a dozen gamma-ray sources at the PeV level across the galaxy, nearly doubling the total number of these sources discovered to date. “Our results clearly showed that there is no such cutoff at 100 TeV,” he says. “Instead, as in the case of the Crab Nebula, the energy spectrum extends further and further forward and beyond 1 PeV.”
The results were not easy, especially since China was a laggard in gamma-ray astronomy. Cao remembers well when he learned to build China’s first gamma-ray detectors in 1986 in a peach garden in the suburbs of Beijing. Across the Pacific Ocean at the time, the late astrophysicist and Nobel Prize winner James Cronin was preparing to detect PeV gamma rays in the deserts of Utah through a project called CASA-MIA (the Chicago Air Shower Array-Michigan Muon Array). At that time, CASA-MIA was the largest and most ambitious experiment to investigate gamma rays at energies above 100 TeV. Unfortunately, it has not discovered any during its five-year observation period. “CASA-MIA was very sensitive back then, but it wasn’t enough to get the job done,” says Ong, who was part of the CASA-MIA team. Nobody tried this technique again until LHAASO. The new observatory is everything CASA-MIA was plus a bigger and better surface array, much better muon detectors, a more sophisticated layout, and a higher height. “And that’s why it worked,” says Ong. “For me personally it is very gratifying to see that someone has taken up what we have worked hard on for 10 years and done a really great job with it.”
Statistics on acceleration to the PeV level in the Crab Nebula are so far limited to two photons, admits Cao. However, since LHAASO is expected to detect at least one or two such events each year, the team hopes to be able to confirm its results in a few years.
To answer the ultimate questions about cosmic accelerators and cosmic rays, LHAASO needs to work with other detectors. Although the experiment is powerful enough to dominate its energy band for years to come, it suffers from relatively poor angular resolution and sky coverage, and lacks the ability to detect it immediately. It will be with the coming Cherenkov Telescope Array (CTA), a global effort to use more than 100 telescopes in the northern and southern hemispheres to detect high-energy gamma rays in and out of our galaxy. Unlike LHAASO, CTA will use atmospheric Cherenkov telescopes and will greatly complement this observatory. “LHAASO and CTA have to work together for about a decade to really determine the origin of cosmic rays,” says Ong, co-spokesman for CTA. LHAASO is ready to collaborate with other experiments from around the world, says Cao. 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 monitoring of some of the sources that LHAASO reported in its previous report nature Paper.
LHAASO will complete the final construction work by the end of this month. “The work has only just begun, although it is already very impressive,” says Ahronian. The experiment reflects the rapid rise of China, an ancient astronomical powerhouse, in modern astrophysics, he says. Due to its well-educated young people and its economic strength, as well as the willingness of the government to invest in basic research, the country is in a good position to conduct world-leading astrophysical research. “LHAASO is just a project that shows how today’s China can conduct science in a timely and extremely cost-efficient manner,” says Ahronian.