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Bill Steigerwald 

NASA Goddard Space Flight Center		February 26, 2003
(Phone: 301/286-5017)
William.A.Steigerwald{at}nasa.gov

Dr. Ryoji Enomoto
enomoto{at}icrr.u-tokyo.ac.jp
tel: +81-4-7136-5116
fax: +81-4-7136-3133
cell: +81-90-5552-0251

Release: 03-19

AUSTRALIAN-BASED "CANGAROO" TELESCOPE FINDS HALO OF GAMMA RAYS

A nearby jewel of a galaxy famous for its furious rate of star 
formation and star explosions sits within a halo of ultra-hot gamma 
rays, a team of Japanese scientists has found.

The observation of galaxy NGC 253 marks the first time that scientists 
have detected gamma rays of such high energy from a galaxy similar in 
size to our own.  These gamma rays, a form of light, are likely 
created by cosmic rays, subatomic particles moving close to light 
speed.  The observation, the team says, provides compelling evidence 
that the primary source of cosmic rays in the Universe, a 
long-standing mystery, are star explosions.

A team led by Dr. Chie Itoh of Ibaraki University in Japan discusses 
these findings in the February 20, 2003, issue of Astrophysical 
Journal Letters.  The observation took place in the Australian outback 
with an innovative telescope called CANGAROO-II and relied on data 
archived at NASA Goddard Space Flight Center in Greenbelt, Md. The 
observation also shows how ground-based telescopes such as CANGAROO-II 
can support NASA's Gamma-ray Large Area Space Telescope (GLAST), 
planned for a 2006 launch.

Gamma rays are the most energetic form of light, capping the 
electromagnetic spectrum that runs from low-energy radio waves through 
infrared and optical light, and past ultraviolet and X rays. Gamma 
rays are millions of times more energetic that what the Hubble Space 
Telescope can detect.

Gamma rays are usually observed from space with satellites because the 
Earth's atmosphere blocks most gamma rays from reaching the Earth's 
surface.  The highest-energy gamma rays, however, do penetrate into 
the atmosphere.  These bullet-like photons (light particles) collide 
with molecules in the atmosphere to create a shower of secondary 
particles.

CANGAROO-II detects this particle shower and reconstructs the flow of 
gamma rays that created the shower. The highest-energy gamma rays are 
rare, and very large telescopes such as CANGAROO-II are needed to 
analyze them.

The CANGAROO team (comprising scientists from Australia and Japan) 
turned CANGAROO-II towards NGC 253, a starburst galaxy approximately 
eight million light years from Earth.  Because of its high rate of 
star explosions and, thus, theorized cosmic-ray activity, this galaxy 
was suspected to emit higher-energy gamma rays -- at what scientists 
call the TeV level. (This stands for tera-electron-volt, a trillion 
electron volts. Optical light particles carry an energy of only about 
one electron volt.)

Itoh and her group analyzed these TeV gamma rays along with a complete 
spectrum of emission, from radio through X rays, under a collaboration 
with Dr. Takeshi Tsuru of Kyoto University, a co-author on the report. 
Some of this lower-energy spectrum of emission is also produced by 
cosmic-ray interaction.

This complete multiwavelength analysis provides a total picture of 
cosmic-ray activity in NGC 253 and strongly bolsters the star 
explosion / cosmic-ray origin theory, said co-author Dr. Tatsuo 
Yoshida of Ibaraki University.

"The paper by C. Itoh et al. presents exciting new data to help us 
understand the cosmic ray distribution in galaxies," said Dr. Stanley 
D. Hunter, an astrophysicist at NASA Goddard. "The indication of very 
high energy electrons in an extended, galactic halo of NGC 253  may 
give us to a better understanding of a heretofore unobserved component 
of our own Galaxy, the Milky Way. The existence of a similar halo in 
the Milky Way may lead to an explanation of the excess diffuse 
gamma-ray emission toward the Galactic center observed with the EGRET 
instrument on the Compton Gamma Ray Observatory."

CANGAROO-II's detection of TeV gamma rays from NGC 253 also bodes well 
for the future of TeV astronomy, said co-author Dr. Ryoji Enomoto of 
Tokyo University.  Previously there were only 11 confirmed sources of 
TeV gamma rays; galaxy NGC 253 is now the 12th source, and it's not a 
particularly exotic source. This means that the Universe is likely 
rich in TeV gamma rays -- detectable with the current generation of 
improved TeV ground-based gamma-ray observatories such as CANGAROO.

"With CANGAROO-II, it's like we have propped open the window to the 
Universe by a few more inches, expanding our view," said co-author Dr. 
Shohei Yanagita of Ibaraki University.

CANGAROO-II represents a second-generation of TeV gamma-ray detectors 
now in operation. CANGAROO-II's increased sensitivity over CANGAROO, 
for example, is expected to uncover a multitude of new TeV sources 
and, at long last, fill out the multiwavelength analysis of the 
Universe. CANGAROO-II will extend the reach and complement 
observations from GLAST, which will observe gamma rays of a slightly 
slower energy compared to CANGAROO-II.  GLAST, from its space-based 
perch, will notify ground-based observatories such as CANGAROO-II of 
newly discovered cosmic sources of gamma rays.

CANGAROO stands for Collaboration of Australia and Nippon (Japan) for 
a GAmma Ray Observatory in the Outback.  More information about this 
observatory, including images, is available at 
http://icrhp9.icrr.u-tokyo.ac.jp/index.html.  A copy of the journal 
article is available at http://xxx.lanl.gov/astro-ph/0301147.

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