Illustration of the process of a cosmic ray particle colliding with an air core to produce a large number of secondary particles and the author of the lhaaso planning diagram of the high-altitude cosmic ray observatory
Lhaaso is the strongest ultra-high-energy gamma ray detection device at present and in the next 20 years, and some arrays have made breakthroughs in the past 1 year, which is expected to lead us to unravel the mystery of the century of cosmic rays in the Milky Way, and explore the vast universe in the ultra-high-energy gamma band, the highest energy electromagnetic wave window.
In the vast universe, the starry sky, in the empty interstellar space, there are many microscopic high-energy particles that are invisible to the naked eye flying at nearly the speed of light. On average, these particles can fly within the Milky Way for millions of years, and a very small number of them meet the earth unexpectedly, becoming mysterious "extraterrestrial visitors" on the earth.
In 1911, the Austrian physicist Hess flew to an altitude of 5 kilometers in a balloon and first discovered this "guest" from the universe, this "guest" was named "cosmic ray", and Hess also won the 1936 Nobel Prize in Physics for the discovery of cosmic lines. This discovery opened a new chapter in human exploration of the mysteries of the universe.
Recently, the national major scientific and technological infrastructure high-altitude cosmic ray observation station lasso (lhaaso) officially passed the performance process acceptance, which marks that the cable has been built and officially entered the scientific operation stage. Built at the end of the 4,000-meter high-altitude mountain, with the core scientific goal of exploring the origin of high-energy cosmic rays and related high-energy celestial evolution and dark matter research, it can accumulate 170 million ultra-high-energy cosmic ray cases and more than 2 billion very high-energy cosmic ray cases every day after the official start of scientific operation.
What is a cosmic ray?
Hess proved its existence through the ionization effect of cosmic rays in the air, and the first question that followed was what particles cosmic rays were, a question that has plagued humans for a long time.
At first, most people mistakenly believed that it was a high-frequency electromagnetic radiation from the universe that was much higher than X-rays, and the name "cosmic rays (i.e., cosmic rays)" was first proposed by the American experimental physicist Millikan in 1925.
In 1932, the American physicist Compton organized a large number of people to measure the strength of cosmic rays at different geographical latitudes on the earth, discovered the modulation effect of the earth's magnetic field on the strength of the cosmic rays, and determined that the original cosmic rays were charged particles, not photons.
Nowadays, human beings can use advanced particle identification technology to carry high-altitude balloons, satellites or space stations to the top of the atmosphere to directly determine the type of cosmic rays, knowing that cosmic rays are mainly composed of positively charged atomic nuclei, of which the highest content is protons (that is, hydrogen nuclei), as well as a variety of atomic nuclei in the periodic table, and also contains a small number of photons, electrons, neutrinos and antiparticles.
In the era before the birth of artificial particle accelerators, cosmic rays were the only source of high-energy particles and the only tool for human beings to study the laws of interaction between high-energy microscopic particles and matter. In the 1960s, the development of artificial accelerators and the emergence of particle colliders replaced the role of cosmic rays in particle physics, and the study of cosmic rays gradually shifted to particle astrophysics.
Where do cosmic rays come from?
To date, the highest energy of cosmic ray particles has been observed to reach 1,020 electron volts (ev), which is 10 million times the energy of particles can be accelerated by the largest particle accelerator of humanity, the CeRN Large Hadron Collider. What celestial body did such a high-energy cosmic ray originate from? How are they accelerated to extremely high energies? These problems have long pushed human beings to explore the mysteries of the universe and nature, and the most basic and core problem is the problem of origin, known as the "mystery of the century".
Cosmic rays are charged particles, which will be affected by the magnetic field in space during propagation and then deflect the direction of motion and lose the source position information, so the origin of the cosmic ray cannot be found through cosmic ray particle detection. The energy spectrum of cosmic rays from 1011ev to 1020ev is roughly in the form of a power law, which is characterized by non-thermal acceleration origin. There are two distinct features in the middle: the energy spectrum softens near 1015 ev, showing a "knee" shaped structure; near 1018 ev, the energy spectrum hardens, showing an "ankle" shaped structure, which contains important information about the origin of cosmic rays. Based on estimates of the upper limit of cosmic ray acceleration due to the scale and magnetic field strength of celestial bodies in the Milky Way, it is generally believed that the cosmic rays with energy in the "knee" region and below originate from celestial sources within the Milky Way, while cosmic rays above the energy in the "ankle" region originate outside the Milky Way.
The measurement characteristics of cosmic rays indicate that they originated from non-thermal radiation processes and are very energetic. Based on the understanding of the sun, humans believe that ordinary stars cannot accelerate particles to such high energies. Therefore, the birthplace of cosmic rays must undergo extremely drastic changes or have extreme physical conditions. According to gamma-ray astronomical observations, the current candidate objects mainly include supernovae and their remnant nebulae, pulsars and their cloud clouds, young massive star clusters, binary star systems, gamma ray bursts, active galactic nuclei, etc. The common feature of these candidate sources is the presence of strong shock waves.
How do I find cosmic rays?
High-energy gamma astronomy, high-energy neutrino astronomy, and extremely high-energy cosmic ray astronomy are the three important pillars for finding the origin of cosmic rays. The detection of high-energy neutrinos and very high-energy cosmic ray celestial sources can provide "one-shot" evidence for the exploration of cosmic ray origins.
In addition, gamma rays are important probes for tracing the acceleration of charged particles of their parents, and these gamma-ray sources provide important candidates for the search for objects of cosmic ray origin. There are two possible origins of gamma radiation: one is the generation of high-energy electrons and low-energy photons inverse Compton scattering process, that is, the origin of leptons; the other is the low-energy hadron cosmic ray and surrounding matter through the hadron-hadron interaction of secondary neutral π meson decay, that is, hadron origin.
Hadron cosmic rays occupy an absolute dominant share in cosmic rays, and the study of the origin of cosmic rays is to find the origin of hadron cosmic rays. Therefore, the focus of finding the origin of cosmic rays through gamma-ray observation is to determine the radiation mechanism of gamma rays, exclude the origin of leptons and find evidence of hadron origin, but this is also the difficulty, because most of the sources are in the gev-tev (1g=109, 1t=1012) energy region, it is difficult to distinguish between these two radiation mechanisms, and most of the gamma-ray sources currently tend to lepton sources.
One point of differentiation between lepton radiation and hadron radiation is in the ultra-high energy zone. The cooling time scale of high-energy electrons in the interstellar magnetic field and radiation field becomes shorter with the increase of energy, there is a klein-nishina high-energy pressure effect above 100tev, and there is no such problem as gamma rays above 100 tev radiated by hadron sources, so ultra-high energy gamma rays are currently the hope of confirming the origin of cosmic rays through gamma rays, and can directly solve the problem of the origin of galactic cosmic rays with energy up to the order of pev (1p=1015), lhaaso is designed for this goal.
What can Russo do?
Lhaaso, as a major national scientific and technological infrastructure with cosmic ray observation and research as the core in recent years, has a detection area of 1.36 square kilometers, which is 20 times that of the international similar device Tibet Yangbajing asγ experiment and 60 times that of the American hawc experiment. The sensitivity of lhaaso in the ultra-high energy zone is more than 10 times that of similar international devices, and it is also much higher than that of the next generation of large-scale Cherenkov telescope arrays, and it is expected to maintain an international leading position in the ultra-high energy zone for a long time in the future. In addition, LHAASO is the world's most sensitive large field of view very high-energy gamma ray detection device.
Based on 1/2 array of 11-month data, Lhaaso made the first breakthrough and published it in Nature on May 17, 2021, that is, discovered 12 highly significant stable ultra-high-energy gamma ray sources whose photon spectrum extended all the way to 1 pev without obvious truncation, thus confirming the first pev particle cosmic accelerator in the Milky Way and revealing that pev accelerators may be ubiquitous in the milky way. These discoveries opened the era of ultra-high-energy gamma astronomical observation, showing that young massive star clusters, supernova remnants, pulsar storm clouds, etc. are the best candidate objects for accelerating ultra-high-energy cosmic rays in the Milky Way, pointing the way to solve the mystery of the century of cosmic rays.
The results also include the highest energy photon recorded so far for human observation, with an energy of 1.42±0.13 pev, a large number of violent activities of star life and death inside the region, with a complex strong shock wave environment, which is an ideal space for cosmic ray acceleration. If lhaaso conducts further observations in the future, it is possible to provide strong evidence for the origin of hadron radiation and will be a breakthrough to solve the "mystery of the century".
On July 9, 2021, Science released lhaaso's second major scientific achievement, measuring the highest energy-end energy spectrum of the standard candle-lit crab nebula in high-energy astronomy, which not only confirmed observations from other experiments in this range for decades, but also expanded the measurement range of standard candlelight from 0.3 pev to 1.1 pev.
LHAASO expects to record 1-2 pev photons from the Crab Nebula each year, and will be able to explore more mysteries about the acceleration of pev particles in the next few years.
lhaaso is the strongest ultra-high energy gamma ray detection device at present and in the next 20 years, some of the arrays have made breakthroughs in the past 1 year, and its full array has officially started operation in July 2021, which is expected to lead us to unveil the "mystery of the century" of the origin of cosmic rays in the Milky Way, and explore the vast universe in the ultra-high energy gamma band, the highest energy electromagnetic wave window.
(Chen Songzhan, researcher of Institute of High Energy Physics, Chinese Academy of Sciences)
Source: Science and Technology Daily