Tycho's Remnant Provides Shocking Evidence for Cosmic Rays (Forwarded)

Joseph Blumberg
Rutgers University
(Phone: 732-932-8412 ext. 652)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-6535)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)


September 22, 2005

CXC RELEASE: 05-07

Tycho's Remnant Provides Shocking Evidence for Cosmic Rays

Astronomers have found compelling evidence that a supernova shock wave
has produced a large amount of cosmic rays, particles of mysterious
origin that constantly bombard the Earth. This discovery, made with
NASA's Chandra X-ray Observatory, supports theoretical arguments that
shock waves from stellar explosions may be a primary source of cosmic rays.

This finding is important for understanding the origin of cosmic rays,
which are atomic nuclei that strike the Earth's atmosphere with very
high energies. Scientists believe that some are produced by flares on
the Sun, and others by similar events on other stars, or pulsars or
black hole accretion disks. But, one of the prime suspects has been
supernova shock waves. Now, a team of astronomers has used Chandra
observations of Tycho's supernova remnant to strengthen the case for
this explanation.

"With only a single object involved we can't state with confidence that
supernova shock waves are the primary source of cosmic rays," said John
P. Hughes of Rutgers University in Piscataway, New Jersey, and coauthor
of a report to be published in an upcoming issue of The Astrophysical
Journal. "What we have done is present solid evidence that the shock
wave in at least one supernova remnant has accelerated nuclei to cosmic
ray energies."

In the year 1572, the Danish astronomer Tycho Brahe observed and studied
the sudden appearance of a bright "new star" in the constellation
Cassiopeia. Now known as Tycho's supernova remnant, the event created a
sensation in Tycho's time because it exploded the myth that stars never
change.

Four centuries later, the Chandra results on Tycho's remnant show that
some modern ideas of the aftermath of supernova explosions may have to
be revised. The report by Hughes and colleagues demonstrates that the
shock wave produced by the explosive disruption of the star behaves in a
way that cannot be explained by the standard theory.

The supernova debris is observed to expand at a speed of about six
million miles per hour. This rapid expansion has created two X-ray
emitting shock waves -- one moving outward into the interstellar gas,
and another moving inward into the stellar debris. These shock waves,
analogous to the sonic boom produced by supersonic motion of an
airplanes, produce sudden, large changes in pressure, and temperature
behind the wave.

According to the standard theory, the outward-moving shock should be
about two light-years ahead of the stellar debris (that's half the
distance from our sun to the nearest star). What Chandra found instead
is that the stellar debris has kept pace with the outer shock and is
only about half a light-year behind.

"The most likely explanation for this behavior is that a large fraction
of the energy of the outward-moving shock wave is going into the
acceleration of atomic nuclei to speeds approaching the speed of light,"
said Jessica Warren, also of Rutgers University, and the lead author of
the report in the Astrophysical Journal.

Previous observations with radio and X-ray telescopes had established
that the shock wave in Tycho's remnant was accelerating electrons to
high energies. However, since high-speed atomic nuclei produce very weak
radio and X-ray emission also, it was not known whether the shock wave
was accelerating nuclei as well. The Chandra observations provide the
strongest evidence yet that nuclei are indeed accelerated, and that the
energy contained in high-speed nuclei is about 100 times that in the
electrons.

Hughes also pointed out that the Chandra result for Tycho's remnant
significantly changes astronomers' view of the evolution of supernova
remnants. A large component of cosmic ray nuclei alters the dynamics of
the shock wave, and may require changing the way that astronomers
estimate the explosive energy of a supernova from the properties of its
remnant.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for the agency's Science Mission Directorate. The
Smithsonian Astrophysical Observatory controls science and flight
operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/press/05_...ss_092205.html
and
http://chandra.nasa.gov