Cosmic Ray Mystery May Be Solved
Cosmic Ray Mystery May Be Solved
Donald Savage Headquarters, Washington, DC November 21, 1995 (Phone: 202/358-1547)
Jim Sahli Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-0697)
RELEASE: 95-208
COSMIC RAY MYSTERY MAY BE SOLVED
Physicists from Japan and the United States have discovered a possible solution to the puzzle of the origin of high energy cosmic rays that bombard Earth from all directions in space.
Using data from the Japanese/U.S. X-ray astronomical satellite ASCA, physicists have found what they term "the first strong observational evidence" for the production of these particles in the shock wave of a supernova remnant, the expanding fireball produced by the explosion of a star.
"We are very pleased to contribute to the solution of an 83-year old mystery," said Dr. Koyama, of the Department of Physics at Kyoto University, Kyoto, Japan.
Cosmic rays were discovered in 1912 by the Austrian physicist Victor Hess, who subsequently received the Nobel Prize in Physics for that work. They are subatomic particles, mostly electrons and protons, that travel near the speed of light. Ever since their discovery, scientists have debated where cosmic rays come from and how ordinary subatomic particles can be accelerated to such high speeds. Supernova remnants have long been thought to provide the high energy cosmic rays, but the evidence has been lacking until now.
The international team of investigators used the satellite to determine that cosmic rays are generated at a high rate in the remains of the Supernova of 1006 AD -- which appeared to medieval viewers to be as bright as the Moon -- and that they are accelerated to high velocities by a process first suggested by the nuclear physicist Enrico Fermi in 1949.
The satellite contains telescopes for simultaneously taking images and spectra of X-rays from celestial sources, allowing astronomers to distinguish different types of X-ray emission from nearby regions of the same object.
The tell-tale clue to the discovery was the detection of two oppositely-located regions in the rapidly expanding supernova remnant, the debris from the stellar explosion. The two regions glow intensely in what is called synchrotron radiation, which is produced when electrons move at nearly the speed of light through a magnetic field in space. The remainder of the supernova remnant, in contrast, produces ordinary "thermal" X-ray emission, meaning radiation from hot gases such as oxygen, neon, and gaseous forms of magnesium, silicon, sulfur, and iron.
The cosmic rays are accelerated in the two regions that glow with synchrotron radiation, the physicists concluded. Specifically, charged particles are accelerated to nearly the speed of light and energies of 100 trillion electron volts as they bounce off turbulent regions inside the shock front from the supernova explosion. This amount of energy is over 50 times higher than can be produced in the most powerful particle accelerator on Earth. Like a ping pong ball bouncing between a table and a paddle while the paddle is brought ever closer to the table, an electron, proton or an atomic nucleus bounces back and forth within the supernova remnant, continually gaining speed, until it attains a high energy. This process was first proposed as a theory by Fermi in 1949.
"Since we found cosmic ray acceleration under way in the remnant of Supernova 1006, this process probably occurs in other young supernova remnants," according to Dr. Robert Petre, of NASA's Goddard Space Flight Center's Laboratory for High Energy Astrophysics, Greenbelt, MD. Astronomers estimate that there is a supernova explosion in the Milky Way galaxy, which contains the Earth, about once every 30 years. Supernova 1006 is classified by astronomers as the explosion of a white dwarf star, known as a Type IA supernova. Other types of supernovae, involving the collapse of massive stars in the Milky Way, and in galaxies beyond, may also produce cosmic rays.
The discovery observations were made with solid-state X-ray cameras on the ASCA satellite, which was launched from Kagoshima Space Center, Japan, aboard a Japanese M-3S-II rocket on Feb. 20, 1993. Major contributions to the scientific instrumentation were provided by Goddard's Laboratory for High Energy Astrophysics and by the Center for Space Research at the Massachusetts Institute of Technology.
"The capability to obtain spatially resolved X-ray spectra -- that is to determine the different spectra at various locations in an image -- is a tremendous advance in space technology," said Dr. Stephen Holt, Director of Space Sciences at Goddard.
Approximately 25 cosmic rays bombard one square inch every second in space just outside the Earth's atmosphere. The atmosphere shields the surface of the Earth from these "primary" cosmic rays. However, collisions of the primary cosmic rays with atoms in the upper atmosphere produce slower moving "secondary" cosmic rays, some of which reach ground level and even may penetrate to depths of many feet below the ground.
This file was last modified on Monday, 13-Sep-1999 15:37:28 EDT
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