Ultra High Energy Gamma Ray Detectors from China Challenge Astronomy's Century Mystery

On a day in July 1054 AD, Yang Weide, the celestial supervisor of the Northern Song Dynasty (an official in charge of observing astronomy), was observing the celestial phenomena in the night sky as usual. Just before dawn, he suddenly noticed an abnormal phenomenon: an extremely bright light appeared near the eastern star Tianguan. When the sun appears, this star will still be visible to the naked eye. Yang Weide immediately recorded such celestial phenomena. After several days of observation, he determined that this star should be a "guest star", which is a celestial body that has never appeared before. Two months later, Yang Weide wrote the following text: "I saw the appearance of the guest star, and its star had a slight radiance, yellow."



Afterwards, this bright star received continuous attention from astronomers around the world, and modern astronomical observations finally confirmed its identity: Yang Weide recorded a supernova explosion 6500 light-years away, and the product of this star's death process was the Crab Nebula - the first supernova remnant with clear observational records. Since Yang Weide discovered this celestial body nearly a thousand years ago, humans have conducted detailed observations of the Crab Nebula from multiple different wavelengths. The Crab Nebula is also used as a "standard candle" to measure the radiation intensity of other celestial bodies. Now, China's High Altitude Cosmic Ray Observatory (LHAASO) has given us a new understanding of this famous celestial body through unprecedented energy levels.

Capture "secondary particles"

To understand how LHAASO works, we need to start with its observed target cosmic ray.
The discovery of cosmic rays can be traced back to hot air balloon flight a century ago. At that time, in order to test whether the mysterious background ionizing radiation in the laboratory came from rocks, Austrian physicist Victor Hess conducted 10 hot air balloon flights between 1911 and 1912. According to the assumption, if the signal does indeed come from radioactive elements in the rock, the further away from the surface, the lower the atmospheric conductivity read by Hess in the instrument. But as the hot air balloon continued to rise, Hess found that although the reading initially decreased, it then rapidly increased with altitude. At this moment, he realized that there must be radiation sources from outside the Earth.
The center of the image is Victor Hess (Image source: public domain)



The sun was the first to be suspected, but on April 7th of the following year, a solar eclipse ended this possibility. If these radiation come from the sun, then during a solar eclipse, the radiation reaching Earth will inevitably decrease due to the obstruction of the moon. But what Hess saw was a completely different situation - there was no significant change in the readings during the solar eclipse. Therefore, the true source of ionizing radiation is deeper into the universe. Thus, Hess found high-energy particles from the universe in the atmosphere: cosmic rays.

The discovery of cosmic rays not only won Hess a Nobel Prize, but also opened a new window for human understanding of the universe. These charged particles (mainly including protons and other atomic nuclei) travel through the universe at speeds close to the speed of light. Among them, ultra-high energy cosmic rays with energies exceeding 1015 electron volts (i.e. 1 beat electron volt, 1 PeV) have received special attention. For astronomers, the source of these ultra-high energy cosmic rays can be said to be a century long mystery. You should know that on Earth, even the strongest colliders produce less than one percent of the energy of these particles. What kind of intense celestial events in the universe accelerate charged particles to have PeV level energy?



Physicists refer to celestial bodies that can drive particles to achieve such high energy as "PeVatron" cosmic ray accelerators. However, finding these "accelerators" is by no means an easy task. In the seemingly empty interstellar space, magnetic fields are everywhere. When charged cosmic ray particles pass through it, the magnetic field naturally deflects the direction of the cosmic ray's flight. Therefore, from the observed cosmic rays, we have no way of knowing their true starting point.
Fortunately, there is still a turning point in the situation. A portion of cosmic rays near their origin will interact with interstellar gas to produce gamma photons. Unlike cosmic rays, the direction of motion of gamma rays is not affected by magnetic fields - they still move in a straight line when crossing a magnetic field. Therefore, detecting these "secondary particles" with energy equivalent to about 1/10 of cosmic rays becomes a tool for us to trace the source of cosmic rays on Earth. The next challenge is to search for "ultra-high energy" gamma photons with energies above 0.1 PeV.

LHAASO appears on stage

On Mount Haizi in Daocheng County, Sichuan Province, at an altitude of 4410 meters, a giant "eye" leading to the high-energy region of the universe begins to reveal the secrets of those ultra-high energy particles in the universe. As the most sensitive ultra-high energy gamma ray detector in the world, LHAASO can accurately measure the energy spectrum of gamma rays from below 1 TeV (1012 electron volts) to over 1 PeV. Among them, the detection ability for ultra-high energy gamma photons is difficult for mainstream detection devices to achieve.



The LHAASO consists of three detection systems, namely a one square kilometer ground shower particle array (KM2A) containing 5195 electromagnetic particle detectors and 1188 muon detectors, a 78000 square meter water Cherenkov detector, and 18 wide-angle Cherenkov telescopes. These systems are arranged in a staggered manner, forming a composite array that enables LHAASO to observe gamma photons arriving on Earth from all angles.
Aerial photo of observation base (Image source: LHAASO collaboration group)

LHAASO began scientific observation in 2019. It is worth mentioning that LHAASO will not be fully completed until late July this year. For example, the full array of 5195 electromagnetic particle detectors was only fully completed a week ago, and only 1/2 to 3/4 of the total number of detectors put into observation before. However, even the LHAASO that has not yet reached the total number has brought us great surprises.

In May of this year, a paper in the journal Nature reported on an important discovery made through LHAASO. During the observation process in 2020, a half scale KM2A discovered 12 "ultra-high energy" gamma ray sources in the Milky Way, with the highest energy gamma photon reaching a record breaking 1.4 PeV. The celestial bodies represented by these signal sources are potential electron beam cosmic ray accelerators. This discovery marks the beginning of ultra-high energy gamma astronomy. Just two months later, LHAASO's achievements were once again published in the journal Science.

Challenge the "Standard Model"

In a paper two months ago, LHAASO observed 12 ultra-high energy gamma ray sources, including a signal from the Crab Nebula. As mentioned at the beginning of the article, the Crab Nebula can be said to be one of the most thoroughly studied celestial bodies by humans. After the observation of the supernova explosion in 1054, a newborn pulsar ejected positive and negative electron winds outward during its rapid rotation; These electron winds interact with the surrounding nebula material, allowing particles to quickly possess extremely high energy.

Crab Nebula (Image source: NASA, ESA, NRAO, AUI, NSF, and G. Dubner)

Astronomers have studied the spectra of radio, infrared, optical, ultraviolet, X-ray, and gamma rays in various different wavelengths. However, there is still a missing link in the image of the Crab Nebula, which is the high-energy gamma rays. In previous observations of the Crab Nebula, the highest energy spectrum was 0.3 PeV. The emergence of LHAASO allows us to observe the Crab Nebula from a new perspective.

Since the start of observation of the Crab Nebula in 2019, LHAASO has captured nearly a hundred gamma photon signals with energy levels exceeding 0.1 PeV. Among them, two ultra-high energy signals at the PeV level have received special attention: in January last year, LHAASO captured a signal at 0.88 PeV, which is one of the 12 ultra-high energy signals in the May Nature paper; Afterwards, LHAASO went further and detected gamma photons with energies as high as 1.1 PeV in January of this year. This not only provides more solid evidence for the highest energy photons from the Crab Nebula, but also challenges the "standard model" in this field to prove that the Crab Nebula is a potential beat electron volt cosmic ray accelerator.

Prior to this, astronomers had accurately measured the spectra of the Crab Nebula on 22 scales and obtained a relatively simple structure. For this structure, physicists can explain it using a simple electron acceleration model, which is the "standard model" in the field of high-energy celestial bodies. However, the emergence of 1.1 PeV photons poses a challenge to the limits of this theory.

Cao Zhen, the first author of the latest "Science" paper and a researcher at the Institute of High Energy Physics, Chinese Academy of Sciences, introduced that "if this photon does indeed originate from the impact of ultra-high energy electrons, then the energy of this electron would be as high as 2.3 PeV. In the environment of the Crab Nebula, this requires the acceleration process to have unimaginable high efficiency - even approaching the theoretical limits of classical electrodynamics and magnetohydrodynamics." If future observations find higher energy gamma photons, the "standard model" may face serious crises.

"In addition, the spectra observed by LHAASO in the ultra-high energy band have shown significant deviations from the 'standard model'," Cao Zhen said. At lower energy levels, the observed data of the Crab Nebula accurately conform to the theoretical model. However, the observation results of ultra-high energy seem to differ from the model. Of course, Cao Zhen also stated that the current number of ultra-high energy signals obtained by LHAASO is still too small. "If a large number of observation results in the future confirm this deviation, then the 'standard model' needs to be corrected. This may solve the first challenge before, but it may also provide evidence for the origin of cosmic rays and bring greater breakthroughs."
Next, LHAASO, which is about to enter its entirety, will continue to observe ultra-high energy gamma photons from celestial bodies such as the Crab Nebula for many years. According to the design goal, LHAASO should be able to detect at least 1-2 high-energy gamma photons from the Crab Nebula each year. Therefore, the research team hopes to confirm existing findings and uncover more about high-energy particles and mysteries through observations in the coming years.