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.

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