New Probe's First Gamma Ray Sky Map Unveiled

Victoria Jaggard
National Geographic News
August 26, 2008
A new map based on early results from the spacecraft formerly known as GLAST is revealing the probe's potential for unraveling some of the most perplexing problems in astrophysics.

The Gamma-ray Large Area Space Telescope was today officially dubbed the Fermi Gamma-ray Space Telescope in honor of Nobel prize-winning physicist Enrico Fermi. The probe was launched into low-Earth orbit on June 11 to scan the heavens for gamma rays, the most energetic wavelengths in the electromagnetic spectrum.

This high-energy radiation comes from a variety of distant and poorly understood cosmic sources, including neutron stars, supermassive black holes, and powerful events known as gamma ray bursts.

The new telescope "will explore the most extreme environments in the universe," Jon Morse, NASA's astrophysics division director, said at a press briefing this afternoon.

Using the craft's first 95 hours of active observation, mission scientists produced a map of gamma ray sources as seen from Earth that shows the same level of detail as previous maps that took more than a year to create.

The new map, which one mission scientist likened to the night sky during a Fourth of July celebration, is "a promise of things to come," noted project scientist Steve Ritz at NASA's Goddard Space Flight Center in Maryland.

It showcases, for example, Fermi's unprecedented field of view, which can image the entire sky in just three hours. This wide scope allows scientists to see gamma ray sources as they flare up and then signal other observatories to follow their progress.

"We have an all-sky alert capability," said Peter Michelson, principal investigator for the probe's large area telescope, who is based at Stanford University in California.

"When one of these things goes off, we see it coming, we see it peak, and we see it go away. That will provide us tremendous insights into the physics of these objects."

Galaxy's Busy Core

The short wavelengths of gamma rays are blocked by Earth's atmosphere, and are so high-energy that they pass right through the lenses in traditional telescopes, making the rays difficult to track.

But based on Einstein's famed equation E=mc2, scientists know that the energy in gamma rays can become matter under the right conditions.

The Large Area Telescope aboard Fermi uses 16 detector towers fitted with layers of the metal tungsten and strips of silicon to record gamma rays.

When a gamma ray hits a layer of tungsten, it sometimes becomes a pair of subatomic particles. The silicon strips then track the paths of these particles, revealing where the gamma ray came from.

Understanding gamma ray sources could be the key to unlocking mysteries such as the nature of dark matter, how black holes can accelerate matter to near the speed of light, and how solar flares generate dangerous high-energy particles.

The research can also answer fundamental questions about the composition and dynamics of our own Milky Way galaxy.

"The center of the galaxy is an incredibly busy place, a crowded place," NASA's Ritz said. "We know that there is a supermassive black hole at the core of our galaxy and quite a lot of high-energy processes going on."

In the new map created using Fermi data, the bright band across the center reveals very high-energy gamma rays coming from the core of the Milky Way.

"This radiation is due mostly to gamma rays generated by the collision of high-energy cosmic rays with dust and gases" swirling around the supermassive black hole, Michelson said.

The map, he noted, also shows the Vela pulsar, "a rotating neutron star and the brightest source in this sky."

Powerful Explosions

In addition to the full-sky map, the first data from Fermi's burst monitor include records of "a gamma ray burst about once a day … the highest rate of detection of any satellite," said Chip Meegan, principal investigator for the instrument, based at the Marshall Space Flight Center.

The bursts, which arrive from random positions all over the sky, are evidence that "all 14 of our detectors are working just beautifully," Meegan said.

Gamma ray bursts are of special interest to astronomers because they are among the brightest events ever observed. (See a related image captured in March of a gamma ray burst that was visible to the naked eye.)

The intense flashes of gamma rays can release within seconds the same amount of energy that the sun will put out over its entire ten-billion-year lifetime—but no one is sure what causes them.

The going theory is that the bursts are tied to the explosive deaths of massive stars, but exactly what types of stars and how the explosions are triggered remains a mystery.

Astronomers hope to make strides in solving gamma ray bursts and other mysteries over Fermi's planned five years in operation.

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