The exotic world of gamma-ray astronomy has taken yet another surprising turn with the revelation that half the previously unidentified high-energy gamma ray sources in our own galaxy, the Milky Way, actually comprise a new class of mysterious objects.
The discovery of this new class and speculation regarding its qualities appear in the March 22 issue of Nature. "These are objects we've never seen before," said Dr. Neil Gehrels, an astrophysicist at NASA's Goddard Space Flight Center, Greenbelt, MD, and lead author on the Nature article. "We can't make out what they are yet, but we know they're strange and, boy, there's a lot of them. These are very different than the famous gamma-ray burst sources, because the gamma rays shine continuously instead of coming in a flash, like the gamma-ray bursts."
The co-authors for the Nature article are Drs. Daryl Macomb, David Bertsch, David Thompson and Robert Hartman, all from Goddard.
Gamma rays, although invisible to the human eye, are in fact the most powerful form of light, far more energetic than visible light, ultraviolet radiation and X-rays. The gamma rays emitted by these mystery objects are a hundred million times more powerful than visible light.
The known gamma-ray universe contains 170 yet-unidentified gamma-ray sources, as listed in a 271-source catalog compiled by the Energetic Gamma Ray Telescope Experiment (EGRET) aboard NASA's Compton Gamma Ray Observatory (CGRO) spacecraft. Scientists have struggled for 20 years to associate the unidentified sources with known objects emitting other types of light. The new class reported today represents one of the first breakthroughs in their understanding.
Gehrels said that of the 170 unidentified sources in our galaxy, about half lie in a narrow band along the Milky Way plane. These may be well-known classes of objects that simply shine too faintly in other types of light to be identified. The other types of light may also be obscured by intervening "fog." Gamma rays easily pass through such material. 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.
What objects could be emitting gamma rays in the Gould Belt? Possibilities are black holes acting as particle accelerators, the massive stars themselves, and clusters of oddball pulsars, among other theories.
A black hole with jets of particles shooting away from it and toward 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 smaller black holes within our own galaxy.
For the massive-star scenario, stars 10 to 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.
Rapidly spinning, magnetic neutron stars known as pulsars are yet another candidate for the mystery gamma-ray sources. An earlier finding by Drs. Jules Halpern (Columbia University, New York, NY), Stephen Holt (Goddard) and David Bertsch showed that the Geminga pulsar is detectable only in X-rays and gamma rays. Several of the EGRET unidentified gamma-ray sources could be exotic high-energy pulsars like this one. Such a discovery would radically change scientists' understanding of pulsar and neutron star populations, as the current census is based largely on those pulsars only detected by radio telescopes.
"Once again we have come face-to-face with the knowledge that the universe is unknown to us, but has patterns that lead us to understanding," said Dr. Alan Bunner, Science Director of NASA's Structure and Evolution of the Universe program. "It's an exciting feeling." Bunner said that the unidentified gamma-ray sources will remain a tantalizing mystery until the 2005 launch of GLAST, the Gamma Ray Large Area Space Telescope. Instruments aboard GLAST will be 50 times more sensitive than the EGRET instrument.
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