spaceSpace and Physics

Billionth Gamma-Ray Observed By NASA Fermi Satellite Instrument


Jonathan O'Callaghan

Senior Staff Writer

Artist's impression of Fermi. NASA

Everyone loves a good cosmic milestone, right? Well this one’s particularly impressive, as an instrument on NASA’s Fermi telescope has just observed its one billionth gamma-ray.

The telescope has two primary instruments. One is the Gamma-ray Burst Monitor (GBM), which looks for flashes of gamma-rays from things like gamma-ray bursts and solar flares. The other, which hit this milestone on April 12, is the Large Area Telescope (LAT).


Fermi orbits Earth at an altitude of about 530 kilometers (330 miles), completing an orbit every 95 minutes. It was launched into space by a Delta II rocket on June 11, 2008.

The LAT works a bit more like a particle accelerator, using thin metal shields to detect individual gamma-rays from deep in space. Among its many discoveries, it has found more than 200 pulsars, which are the rapidly rotating cores of collapsed stars, and found that small stellar explosions called novae release gamma-rays.

It is able to see about 20 percent of the sky at any given time, and every two orbits of Earth it can create a gamma-ray map of the entire sky. It must sift out gamma-rays for the huge amount of other particles it sees in the form of cosmic rays.

“Since Fermi’s launch in 2008, the LAT has made a number of important discoveries of gamma-ray emissions from exotic sources in our galaxy and beyond,” said Robert Cameron, head of the LAT Instrument Science Operations Center (ISOC) at the SLAC National Accelerator Laboratory in California, in a statement.


“Understanding pulsars tells us about the evolution of stars because they are one possible end point in a star’s life. The LAT data have led us to totally revise our understanding of how pulsars emit gamma-rays.”

The LAT has also been used to study microquasars, which are star-sized versions of superheated regions around the supermassive black holes at the center of galaxies. It’s explored gamma-rays in our own atmosphere too, produced by thunderstorms and interactions with solar flares.

And it’s also been used to search for dark matter particles in the center of our galaxy and others. Some theories suggest that when dark matter particles collide and destroy each other, they may produce gamma-rays, although we haven’t found direct evidence of this yet. It could also be used to look for gamma-rays associated with ripples in space-time known as gravitational waves.

Next year Fermi will reach its goals of operating for 10 years in space. It’s not clear if it will be funded beyond this, but it’s had a pretty good run so far.


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