New Class of Gamma Ray Objects Discovered In Milky Way
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Goddard Space Flight Center, Greenbelt, MD
March 22, 2000
The exotic world of gamma-ray astronomy has taken yet another surprising turn with the revelation that half of the previously unidentified 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 as to its nature, which includes such scenarios as massive gusts of colliding stellar winds and pulsing gamma rays from spinning neutron stars, appears in the current issue of Nature.
"These are objects we've never seen before," said Dr. Neil Gehrels, an astrophysicist at NASA Goddard Space Flight Center and lead author on the Nature article. "We can't make out what they are yet, but we know they're different and, boy, there's a lot of them."
Gehrels' co-authors for the Nature report are Drs. Daryl Macomb, David Bertsch, David Thompson and Robert Hartman, all at NASA Goddard. He said the new class of objects 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.
Gamma rays, although invisible to the human eye, are the most powerful form of radiation, far more energetic than visible light, ultraviolet radiation and X rays. Gamma rays are generated in several ways: Gas at a temperature of 10 billion degrees, far hotter than what the sun could produce, glows in gamma rays; also, energetic particles smashing into other particles or spiraling through magnetic fields release gamma rays.
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 the Compton Gamma-Ray Observatory (CGRO). Scientists have struggled for 20 years to associate the unidentified sources with known objects in other wavelengths to no avail. The new class reported today represents one of the first breakthroughs in their understanding.
Gehrels said that 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.
What objects could possibly be emitting gamma rays in the Gould Belt? Possibilities are black holes with particle jets, the massive stars themselves, and clusters of oddball pulsars, among other theories.
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.
For the massive star scenario, 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.
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), Stephen Holt (NASA Goddard) and David Bertsch showed that the Geminga pulsar is 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.
"We have come face-to-face once again with the knowledge that the universe is unknown to us, but has patterns that lead us to understanding," said Dr. Alan Bunner, Director of NASA's Structure and Evolution of the Universe science theme. "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 30 times more sensitive than the EGRET instrument.
CGRO, launched in 1991 aboard the space shuttle Atlantis, has been a workhorse far exceeding its five-year projected mission. CGRO comprises four main instruments, each observing a different band of gamma-ray radiation. EGRET detects the highest energy range among the CGRO instruments, gamma rays produced by particle collisions.
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