How does "Laso" reveal the super-powerful side of the Milky Way?

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The universe, this vast space, has inspired human beings' endless curiosity and desire for exploration since ancient times. Two thousand years ago, Qu Yuan explored the mysteries of the universe in "Heavenly Questions", "Where is the sky? How are the twelve? Where do the sun and the moon belong? Where are the stars arranged?" Today, Chinese scientists use the world's leading High Altitude Cosmic Ray Observatory (LHAASO, referred to as LHAASO) at an altitude of 4,410 meters in Daocheng to capture ultra-high energy particles from deep space, striving to solve the century-old scientific problem of the origin of cosmic rays.

The magical journey of cosmic rays

With the continuous advancement of science and technology, we are able to explore the mysteries of the universe through various means. Among them, cosmic rays, as high-energy particles from outer space, carry important scientific information on the origin of the universe, the evolution of celestial bodies, and many other aspects, and have become an important way for humans to explore the universe. Cosmic rays are mainly composed of protons, helium nuclei, iron nuclei, positrons and electrons, photons, neutrinos and other components. They are the only material samples that humans can currently obtain from the depths of the universe. "Cosmic rays carry important scientific information on the origin of the universe, the evolution of celestial bodies, and solar activity. Studying cosmic rays and their origins is an important way for humans to explore the universe." Cao Zhen, an academician of the Chinese Academy of Sciences, chief scientist of Lasso, and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, introduced.

A notable feature of cosmic rays is their extremely high energy. In order to study these high-energy particles, humans have built huge accelerators, such as the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN). However, even such accelerators have difficulty reaching the level of the highest energy particles in cosmic rays. This makes cosmic rays an ideal object for studying extreme physical phenomena and testing basic physical laws.

Why can such high-energy particles be accelerated in the universe? How are they accelerated? To answer these questions, it is necessary to accurately measure the properties of cosmic rays and trace their sources.

Lasso: A powerful tool for capturing cosmic rays

After entering the atmosphere, cosmic ray particles interact with atomic nuclei in the atmosphere to produce secondary particles. These particles continue to interact with atomic nuclei, one becomes ten, ten becomes a hundred, and repeat many times. Eventually, the number of secondary particles can reach up to tens of billions, like a particle "shower" in the air, spreading over an area of ​​several square kilometers in one billionth of a second. This process is called air shower. Scientists build ground-based detectors to detect secondary particles, which are the traces left by these cosmic rays in the atmosphere or after reaching the ground, and infer their properties and origins.

The high-altitude Lazo Cosmic Ray Observatory located in Haizi Mountain, Daocheng, Sichuan is one of the highest, largest and most sensitive ultra-high energy gamma-ray detection devices in the world.

Aerial photo of the cable/Source: Institute of High Energy Physics, Chinese Academy of Sciences

The reason why the Lasso Observatory was located in a high-altitude area is that the air in high-altitude areas is thin and the atmosphere absorbs less cosmic rays, which is conducive to capturing more cosmic ray information and provides a unique advantage for the study of cosmic rays. Lasso innovatively adopts a composite observation mode, consisting of three arrays: a 1-square-kilometer ground-based shower particle detector array (KM2A) composed of 5,216 electromagnetic particle detectors and 1,188 muon detectors, a 78,000-square-meter water Cherenkov detector array (WCDA) composed of 3,120 detection units, and a wide-angle Cherenkov telescope array composed of 18 telescopes. These detectors are used to receive "particle rain" from outer space and can accurately measure information such as the direction, energy, and composition of cosmic rays, providing scientists with data resources and in-depth research on key issues such as the origin, acceleration mechanism, and propagation process of cosmic rays. Cao Zhen introduced: "With the increase in observation time, Lasso may also detect the acceleration source of higher-energy cosmic rays, which is expected to solve the mystery of the origin of cosmic rays in the Milky Way."

A "super accelerator" deep in the galaxy

Due to Lazo's unprecedented ultra-high energy gamma-ray detection sensitivity and wide field of view, revolutionary changes have been brought about in gamma astronomy research. During the initial operation alone, 12 ultra-high energy gamma light sources, or ultra-high energy cosmic accelerator candidates, were discovered in the Milky Way. The highest energy photons ever observed by humans were also recorded, reaching 1.4PeV (eV "electron volt" is the unit of energy in particle physics, PeV is read as "petaelectron volt", 1PeV = 1×1015 eV), opening the era of "ultra-high energy gamma astronomy".

Schematic diagram of the 12 stable gamma-ray sources discovered in the experiment

From the Institute of High Energy Physics, Chinese Academy of Sciences

In the following two years, Lasso continued to observe a large number of gamma photons from stars in Cygnus, and discovered a giant ultra-high-energy gamma-ray bubble structure in Cygnus, from which he found a candidate celestial body for the origin of cosmic rays with energies higher than 100 trillion electron volts. It is reported that the giant ultra-high-energy gamma-ray bubble structure discovered by Lasso this time is at least 5,000 light-years away from us and has a scale larger than 10 million solar systems. There are multiple photons with energies exceeding 100 trillion electron volts in the bubble structure, with the highest reaching 200 trillion electron volts. The energy of the cosmic ray particles that emit these photons is thousands of times that of artificial accelerated particles. These new discoveries indicate that there is a "super cosmic ray accelerator" inside the bubble, which continuously emits high-energy cosmic rays with energies exceeding 10 trillion electron volts.

Schematic diagram of the ultra-high energy gamma-ray bubble produced by the super cosmic ray accelerator

From the Institute of High Energy Physics, Chinese Academy of Sciences

Based on observations, Lasso also inferred that the super cosmic ray accelerator in the bubble made the cosmic ray density in the surrounding interstellar space significantly higher than the average level of cosmic rays in the Milky Way. The spatial range affected is even far beyond the scale of the bubble observed so far, providing a possible explanation for the excess of diffuse gamma-ray radiation in the Milky Way detected by Lasso before.

Although humans discovered the existence of cosmic rays more than 100 years ago, their origin remains a mystery. "The vast universe contains infinite mysteries, and we are always on the journey of exploring these mysteries." As Academician Cao Zhen said, with the continuous advancement of science and technology and the continuous improvement of observatory construction, humans' understanding of cosmic rays will continue to deepen. We have reason to believe that in the near future, humans will be able to uncover more mysteries of cosmic rays and make greater contributions to exploring the nature and evolution of the universe.

(Sharing by Cao Zhen, academician of the Chinese Academy of Sciences, chief scientist of LHAASO, director of Tianfu Cosmic Ray Research Center, and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, at the 2025 Tian Lecture on Science Popularization New Year's Eve, compiled by Guangming Online reporter Tian Xinyu)