NASA - National Aeronautics and Space Administration

GLAST is certain to make contributions in a number of areas, such as the study of active galactic nuclei, gamma-ray bursts, and neutron stars. But for GLAST science team members, perhaps the most tantalizing possibility is the prospect of finding something entirely new and unexpected. "This is a relatively unexplored field, so the potential for major discoveries is very high," says LAT science team member David Thompson of NASA's Goddard.

The history of astronomy shows that whenever instruments open a new window to the Universe, or make an order of magnitude improvement in capability, major discoveries almost always follow, often including the discovery of new classes of objects. GLAST represents such a giant leap over all previous gamma-ray satellites that it is not unrealistic to expect revolutionary findings.

A case in point can be seen when GLAST is compared to the EGRET instrument on NASA’s Compton Gamma-ray Observatory. EGRET logged 271 gamma-ray sources. But 172 of them - nearly two-thirds - remain unidentified, because EGRET could not pinpoint their location with sufficient precision to enable astronomers to associate them with known objects. "Since GLAST's Large Area Telescope (LAT) can localize many objects to better than 1 arcminute, astronomers will be able to identify most of those sources," says GLAST Project Scientist Steve Ritz of NASA Goddard.

Many of the unidentified point sources are probably in other galaxies - blazars. But a considerable number of the unidentified sources lie along the galactic plane, meaning that in all likelihood, they belong to our Milky Way Galaxy. Many of these sources are probably pulsars, whereas others could be supernova remnants, binary systems containing a black hole or neutron star, stars with powerful winds, or nebulae sculpted by pulsar winds. Some of the unidentified extragalactic sources may turn out to be clusters of galaxies, starburst galaxies, or ultraluminous infrared galaxies (known as ULIRGs). LAT observations should be able to pin down the nature of most of these objects. But as Thompson says, "There are surprises out there waiting to be found. Frankly, we're hoping to be surprised when it turns out to be none of the above."

And because of the LAT's major upgrade in sensitivity over EGRET, GLAST should see thousands of new point sources. "It might see 3,000 point sources, or it might see 9,000; we just don’t know. But we’re looking forward to the answer," says GLAST Project Scientist Steve Ritz of NASA Goddard.

Image left: The EGRET instrument on NASA's Compton Gamma-ray Observatory mapped 271 sources. Nearly two-thirds of these gamma-ray sources remain unidentified. Credit: Sonoma State University.

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GLAST Burst Monitor (GBM) Principal Investigator Charles "Chip" Meegan of NASA's Marshall Space Flight Center in Huntsville, Alabama, echoes this assessment: "Although we can anticipate some of the things that the GBM and the LAT will see, the most exciting results will be the surprises that always come from seeing what was previously invisible."

"It will be interesting to see how wrong this all is after one or two years of taking data," adds GLAST Deputy Project Scientist Julie McEnery of NASA Goddard. "I hope we are wrong on a lot of things, since it implies that the sky has surprises."

LAT science team member Alice Harding of NASA Goddard concludes, "We're going to find things that we don't understand. Then we will have to figure out how to deal with it. It will be fun to study the sources we know about, but even more fun to solve the mysteries of the sources we can’t explain."
by Robert Naeye