Compton's Lingering Mystery
The Unidentified CGRO Sources
Compton has identified hundreds of blazars, gamma-ray bursts and other
high-energy phenomena. But, near the end of the mission in Spring
2000, Compton scientists announced their discovery of a new class of
mysterious objects.
The known gamma-ray
universe contains 170 yet-unidentified high-energy gamma-ray
sources, as listed in a 271-source catalog compiled by
EGRET. Scientists have struggled for 20 years to associate the
unidentified sources with known objects in other wavelengths to no
avail.
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Left - EGRET Map of all Gamma-ray sources;
Right - Map of the unidentified sources
(IMAGE CREDIT: NASA/Honeywell Max Q Digital Group, Angela Cheyunski)
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Of the 120 unidentified sources in our galaxy (50 unknown are beyond
the galaxy), about half lie in a narrow band along the Milky Way
plane. These may be well-known classes of objects merely too faint to
distinguish in other wavelengths. The other half of the unidentified
galactic sources are closer to Earth and make up the new class. These
lie just off the Milky Way plane and seemingly follow the Gould Belt,
a ribbon of nearby massive stars and gas clouds that winds through the
Milky Way plane.
As NASA astrophysicist Neil Gehrels said, "These are objects we've
never seen before. We can't make out what they are yet, but we know
they're different and, boy, there's a lot of them." Gehrels speculated
that the mystery gamma-rays might be from a number of possible
sources, all of which are exotic variations on familiar objects:
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black holes with jets of particles shooting away from the black hole
and towards us might be visible as gamma-rays. Scientists have
observed this phenomenon with EGRET in supermassive black holes, which
lurk in the centers of distant galaxies, but never in stellar-size
black holes within our galaxy.
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Massive stars, 10-20 times as massive as the sun, could generate
stellar winds that throw high velocity particles into the surrounding
space. The particles would slam into gas atoms surrounding the star to
produce gamma-rays.
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The Geminga pulsar is one
neutron star
detectable only in X rays and gamma-rays. A fair fraction of the EGRET
unidentified gamma-ray sources could be such exotic high-energy
pulsars. Such a discovery would radically change our understanding of
pulsars and neutron star populations, as the current census is based
largely on only those pulsars detected by
radio telescopes.
Scientists who have used CGRO now look forward to the Gamma-Ray Large
Area Space Telescope (GLAST), scheduled for a 2005 launch. GLAST
will have an instrument 30-50 times more sensitive than the EGRET and
will be able to solve this mystery.
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| The magnetic field of a rotating neutron star
accelerates particles. This acceleration
produces beams of gamma-rays which stream away from
the neutron star.
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When the beam is pointed at earth, we see a bright
source of gamma-rays. |
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(IMAGE CREDIT: NASA/Honeywell Max Q Digital Group, Dana Berry)
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Additional Links
 CGRO Web Site (http://cossc.gsfc.nasa.gov/)
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