Cosmic rays have been discovered for more than a hundred years. But how are cosmic ray particles accelerated? What is its origin object? There is still no definitive answer. "The origin of cosmic rays and their acceleration mechanism" is one of the 11 "mysteries of the century" of scientific research in the new century identified by the National Science and Technology Council of the United States in 2004.
What is a cosmic ray?
In 1912, the Austrian physicist Victor F. Hess took a hot air balloon to an altitude of more than 5,000 meters above sea level and measured the change of radiation ionization rate with altitude. He found that above 1 km, with the increase of altitude, the radiation ionization rate increased significantly, and the ionization rate at 5 km above sea level was several times higher than that of the ground, thus determining that ionization was a highly penetrating ray from outside the Earth, which was later named "cosmic ray" or "cosmic ray". Hess was also awarded the Nobel Prize in Physics in 1936 for his discovery.
Cosmic rays are a general term for high-energy particles detected from space on Earth, of which protons account for about 90%, helium nuclei account for 9%, and 1% are other heavy nuclei, electrons, photons and other high-energy radiation. Cosmic rays have an extremely wide energy range, spanning 11 orders of magnitude. On October 15, 1991, the high-resolution cosmic particle detector of the University of Utah in the United States detected the highest energy cosmic ray particles currently known to mankind, and jokingly called them "oh-my-god particles". Its energy is ~300EeV (3×10 20eV, the text is italicized for the superscript, here is 10 to the 20th power), and when it collides with other atomic nuclei, this energy is more than 50 times higher than the highest collision energy that the human-made large hadron collider LHC can provide! [1]
Cosmic rays are the only samples of matter outside the solar system that can be detected on earth, which come from the depths of the universe and contain rich information such as the origin of the universe and the evolution of celestial bodies, and are important cosmic "messengers".
Where do cosmic rays come from?
Since most cosmic ray particles are charged and will be deflected by the magnetic field of space on the way to the Earth, it is difficult to trace the origin of celestial bodies by observing their direction of arrival. But high-energy particles can interact with magnetic fields and interstellar media to produce uncharged radiation, such as electromagnetic radiation, neutrinos, and we can indirectly trace their origins by observing these radiations. High-energy activity on the surfaces of the Sun and other stars, supernova explosions, pulsars, quasars, and active galactic nuclei could all be sources of cosmic rays. According to the production zone from near to far, the cosmic rays can be roughly divided into solar cosmic rays, galactic cosmic rays and extragalactic high-energy cosmic rays, and their corresponding energies are gradually increasing.
The Sun is our closest source of cosmic rays, which are streams of high-energy particles produced by solar activity, which are less energetic, usually no more than ~1010eV.
Energy greater than 1013 eV cosmic ray energy spectrum. The flow of cosmic rays decreases rapidly with the increase of energy , and this energy spectrum population can be represented by a power law spectrum , approximately a straight line at logarithmic coordinates. Steepening around 1015-1016eV is called the "Knee Zone", and flattening at 1018-1019 eV is called the "Ankle Zone" steepening again around 1017eV, called the second "Knee Zone". | Source: References[2]
Cosmic rays from different kinds of high-energy celestial bodies arrive on Earth and appear in different energy segments of the cosmic ray spectrum. It is generally believed that cosmic rays with energies below the "ankle zone" (energy 1018 to 1019 eV) may originate in the Milky Way, while cosmic rays above the "ankle zone" mainly come from extragalactic high-energy celestial sources.
Supernova remnant shock acceleration is widely considered to be the most important source of cosmic ray acceleration in the Milky Way.
Top: Cassiopeia supernova remains | Image source: https://chandra.harvard.edu
Bottom: Schematic diagram of a pulsar | Image source: https://en.wikipedia.org
In 1934, W. Bard Baade) and F. Zwicky was the first to propose that supernova explosions are major objects that produce cosmic rays. Supernova explosions will eject large amounts of precursor star material, forming supernova remnants. Observations have found that these supernova remnants usually have strong magnetic fields and carry huge kinetic energy, which is itself a natural accelerator. Supernova remnant shocks carry an average of 1,051 erg of kinetic energy, equivalent to the total energy released by the Sun over its lifetime (about 10 billion years), and converting 10% of this energy into high-energy particles can maintain the energy density of the currently observed cosmic rays. [3]
A. Hilas M. Hillas) system analyzed the maximum energy that various celestial accelerated particles can achieve, and believed that the maximum energy of particles being accelerated is mainly related to the size of the acceleration zone and the strength of the magnetic field, which is called the Hillas condition. Supernova remnants are likely to have the ability to accelerate cosmic rays to 1015 to 1018 eV. Over the past decade, scientists have found direct evidence that supernova remnant shocks can accelerate high-energy nuclei by analyzing the Fermi satellite's observations of several supernova remnants. [4] [5]
The characteristic size and magnetic field of various celestial bodies and the highest energy of the particles they are capable of accelerating | Source: References[6]
Cosmic ray acceleration mechanism
Regarding the physical mechanism of acceleration of high-energy charged particles, in 1949, Enrico Fermi first proposed the famous Fermi acceleration mechanism. He pointed out that charged particles can increase their energy by scattering back and forth between two magnetic clouds that are close to each other. This mechanism was later generalized as charged particles being scattered by a moving magnetic field and accelerated. The high-speed shock waves produced by supernova explosions can provide the required magnetic field environment for the acceleration of charged particles. Later, the Fermi acceleration mechanism was applied to the shock wave environment and developed into a quantitative theory of "diffuse shock acceleration" (DSA for short). The theory predicts that when there are enough accelerated particles to affect the structure of the shock, the numerical density of the high-energy particles downstream of the shock wave is inversely proportional to the square of the energy. However, this simple theoretical prediction does not explain many of the observational features of cosmic rays and supernova remnants. More sophisticated particle acceleration models are being developed to account for observations of supernova remnants and cosmic rays. [7]
The High Altitude Cosmic Ray Observatory (LHAASO) under construction on Haizi Mountain in Daocheng, Sichuan province, | Image source: Network
In terms of observation, the national major scientific infrastructure "LHAASO Observatory (LHAASO)", which is being built in Daocheng, Sichuan, will give higher precision measurements of high-energy cosmic rays and gamma rays, which will help us solve problems such as the origin of cosmic rays.
Resources
[1] Steve Nerlich. Oh-My-God Particles.Universe Today,2016-6-13
[2] Patrignani C, Particle Data Group. Review of Particle Physics[J]. Chinese physics C,2016,40(10):100001
Liu Siming. The theory of the origin of supernova remnants of the Galactic Cosmic Ray. Modern Physics Knowledge, 2019, (2): 3-8
[4] Ginger Pinholster (13 February 2013). "Evidence Shows that Cosmic Rays Come from Exploding Stars".
[5] Ackermann M, et al. Detection of characteristic Pion-decay signature in supernova remnants. Science ,2013,339:807
[6] Federico Fraschetti.On the acceleration of ultra-high-energy cosmic rays. PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES,2008,366(1884):4417-4428
[7] Stefano Gabici, Carmelo Evoli, Daniele Gaggero, Paolo Lipari, Philipp Mertsch, Elena Orlando, Andrew Strong, Andrea Vittino .“ The origin of Galactic cosmic rays: challenges to the standard paradigm”. International Journal of Modern Physics D, (2019) 1930022
About the Author
Peng Ke: Chang'an University, class of 2018 undergraduate. Internship at the Purple Mountain Observatory of the Chinese Academy of Sciences, instructor: Liu Siming.
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