It detects one energy source above 10 GeV every four months

Jan 11, 2012 14:28 GMT  ·  By

Experts operating the NASA Fermi Gamma-ray Space Telescope have just released the first census of energy sources exceeding 10 billion electron volts (GeV). This is the first full survey of such extreme sources ever developed.

The reason why this is such an accomplishment is that gamma-rays at such high energies can be detected only on rare occasion. The Large Area Telescope (LAT) aboard Fermi scans the night sky once every 3 hours, and even it can only detect 10GeV+ gamma-rays once every four months or so.

This is therefore a largely unexplored electromagnetic range, and astrophysicists decided to make that a thing of the past. This new realm of energy has been Fermi's target for a very long time, and the constant effort apparently paid off.

For comparison, keep in mind that optical wavelength (visible light) has an energy level of between 2 and 3 electron volts (eV). What the LAT is searching for is gamma-rays with energy levels of between 20 million and 300 billion eV (GeV).

“Before Fermi, we knew of only four discrete sources above 10 GeV, all of them pulsars. 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,” NASA Goddard Space Flight Center (GSFC) astrophysicist David Thompson explains.

The new census revealed 496 such sources, of which more than half are active galactic nuclei, the super-active cores of distant galaxies that get their power from supermassive black holes. As matter falls into these dark behemoths, they latter produce huge radiation jets that travel at relativistic speeds.

Interestingly, only about 50 of these sources are located inside the Milky Way. Experts agree that many more may exist in our galaxy, but they somehow managed to remain hidden thus far. Most commonly, gamma-rays in the Milky Way originate in supernova remnants, binary systems and pulsars.

Pulsars are a particular type of fast-spinning neutron stars, whose radiation jets are oriented exactly towards Earth. As such, they appear just like a cosmic lighthouse from our perspective, hence the name.

What is also very interesting is the fact that more than 33 percent of all high-energy sources in the sky are unidentified. That is to say, they have no counterparts in other portions of the electromagnetic spectrum, so astrophysicists have no idea what they represent.

“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,” explains Fermi science team member David Paneque, who is based at the Max Planck Institute for Physics (MPI-P), in Munich.

“As Fermi's exposure constantly improves our view of hard sources, ground-based telescopes are becoming more sensitive to lower-energy gamma rays, allowing us to bridge these two energy regimes,” adds Pascal Fortin, an expert at the Ecole Polytechnique's Laboratoire Leprince-Ringuet, in Palaiseau, France.