New Class of Gamma Ray Objects Discovered in Milky Way

Dolores Beasley
Headquarters, Washington, DC
(Phone: 202/358-1753)

Nancy Neal
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-0039)

RELEASE: 00-42

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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