Washington, Jan 11 : NASA's Fermi Gamma-ray Space Telescope, which has been surveying high-energy sky for more than three years, is extending its view into a largely unexplored electromagnetic range.

The Fermi team announced its first census of energy sources in this new realm.

Fermi's Large Area Telescope (LAT) scans the entire sky every three hours, continually deepening its portrait of the sky in gamma rays, the most energetic form of light.

While the energy of visible light falls between about 2 and 3 electron volts, the LAT detects gamma rays with energies ranging from 20 million to more than 300 billion electron volts (GeV).

At higher energies, gamma rays are rare. Above 10 GeV, even Fermi's LAT detects only one gamma ray every four months.

This all-sky Fermi view includes only sources with energies greater than 10 GeV. From some of these sources, Fermi's LAT detects only one gamma-ray photon every four months. Brighter colours indicate brighter gamma-ray sources.

"Before Fermi, we knew of only four discrete sources above 10 GeV, all of them pulsars," said David Thompson, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md.

"With the LAT, we've found hundreds, and we're showing for the first time just how diverse the sky is at these high energies," he added.

Any object producing gamma rays at these energies is undergoing extraordinary astrophysical processes. More than half of the 496 sources in the new census are active galaxies, where matter falling into a supermassive black hole powers jets that spray out particles at nearly the speed of light.

Only about 10 percent of the known sources lie within our own galaxy. They include rapidly rotating neutron stars called pulsars, the expanding debris from supernova explosions, and in a few cases, binary systems containing massive stars.

More than a third of the sources are completely unknown, having no identified counterpart detected in other parts of the spectrum.

With the new catalog, astronomers will be able to compare the behaviour of different sources across a wider span of gamma-ray energies for the first time.

"Our catalog will have a significant impact on ground-based facilities'' work by pointing them to the most likely places to find gamma-ray sources emitting above 100 GeV," said David Paneque at the Max Planck Institute for Physics in Munich. (ANI)