File:Fermi Gamma-ray Space Telescope 3 years of observations (energies larger than 1 GeV).jpg

Summary

After more than three years in space, NASA's <a href="https://en.wikipedia.org/wiki/Fermi_Gamma-ray_Space_Telescope" class="extiw" title="en:Fermi Gamma-ray Space Telescope">Fermi Gamma-ray Space Telescope</a> is extending its view of the high-energy sky into a range that to date has been largely unexplored territory. Now, the Fermi team has presented its first "head count" of sources in this new realm.

Fermi's <a href="https://en.wikipedia.org/wiki/Large_Area_Telescope" class="extiw" title="en:Large Area Telescope">Large Area Telescope</a> (LAT) scans the entire sky every three hours, continually deepening its portrait of the sky in <a href="https://en.wikipedia.org/wiki/gamma_rays" class="extiw" title="en:gamma rays">gamma rays</a>, the most extreme form of light. While the energy of visible light falls between about 2 and 3 <a href="https://en.wikipedia.org/wiki/electron_volts" class="extiw" title="en:electron volts">electron volts</a>, the LAT detects gamma rays with energies ranging from 20 million electron volts (MeV) to more than 300 billion (GeV).

But at higher energies, gamma rays are few and far between. Above 10 GeV, even Fermi's LAT detects only one gamma ray every four months from some sources. The LAT's predecessor, the EGRET instrument on NASA's Compton Gamma Ray Observatory, detected only 1,500 individual gamma rays in this range during its nine-year lifetime, while the LAT detected more than 150,000 in just three years.

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 <a href="https://en.wikipedia.org/wiki/galaxies" class="extiw" title="en:galaxies">galaxies</a>, where matter falling into a <a href="https://en.wikipedia.org/wiki/supermassive_black_hole" class="extiw" title="en:supermassive black hole">supermassive black hole</a> powers jets that spray out particles at nearly the <a href="https://en.wikipedia.org/wiki/speed_of_light" class="extiw" title="en:speed of light">speed of light</a>.

Fermi's view of the gamma-ray sky continually improves. This image of the entire sky includes three years of observations by Fermi's Large Area Telescope (LAT). It shows how the sky appears at energies greater than 1 billion electron volts (1 GeV). Brighter colors indicate brighter gamma-ray sources. A diffuse glow fills the sky and is brightest along the plane of our galaxy (middle). Discrete gamma-ray sources include pulsars and supernova remnants within our galaxy as well as distant galaxies powered by supermassive black holes.

Licensing

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