Gamma-ray glow bathes Milky Way
(This story appeared on page 292 of the
Nov. 8, 1997, Science News.)
By Sid Perkins
Science News
A mysterious halo of gamma rays not associated with any known celestial
objects extends thousands of light-years from the core of the Milky Way
and may surround the entire galaxy, astronomers report.
"These gamma rays are providing the first evidence that some sort of
high-energy process is occurring at large distances from the galactic
core," says physicist David D. Dixon of the University of California,
Riverside.
Analyzing data from the Earth-orbiting Compton Gamma Ray Observatory,
Dixon and his collaborators have mapped the strength and extent of the
gamma-ray halo. He presented the findings this week at a meeting of the
High-Energy Astrophysics Division of the American Astronomical Society in
Estes Park, Colo.
Another assessment of satellite data, slated for publication in the
ASTROPHYSICAL JOURNAL early next year, also reveals the presence of an
extended halo, says David L. Bertsch of NASA's Goddard Space Flight Center
in Greenbelt, Md. Bertsch is an author of the forthcoming study.
The gamma rays forming the halo have energies up to a billion times that
of visible light. One possible explanation of the gamma rays' origin,
Dixon says, is that they are created when electrons traveling near the
speed of light collide with lower-energy, infrared photons. Cosmic rays
are a source of such electrons, an an aura of dim infrared photons around
the Milky Way like those found recently around similar spiral galaxies
could provide "seed photons."
The gamma-ray distribution may also provide indirect evidence of dark
matter -- the universe's missing mass, whose existence scientists have
inferred but not yet demonstrated -- Dixon says. According to one theory,
some massive, dark-matter particles occasionally collide with each other
and either generate gamma rays or produce particles that decay into
gamma rays.
"The most likely explanation [for the extended halo] is that cosmic rays
are boosting lower-energy photons to gamma-ray strength," asserts
Bertsch.
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Copyright 1997 by Science Service.
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