Almost once a day, somewhere in the sky a sudden burst of gamma rays appears. Called, imaginatively, gamma ray busters, these events happen in distant early galaxies in the universe. There are a range of theories as to their possible origins – large stars collapsing to black holes without a supernova, merging galaxies bringing together their central black holes. To learn more about them, astronomers have gamma ray survey telescopes looking all over the sky for them, and better telescopes, or telescopes that work in other wavelengths ready to swing round to see their other afterglows.
One such survey telescope is the gamma ray burst monitor on the Fermi Gamma Ray Space Telescope. However, it was another instrument on this device – the Large Area Telescope that spotted an interesting GRB at the end of August. The LAT only looks at a sixth of the sky and looks at it in higher energy gamma rays than normal. As a result, it doesn’t normally see GRBs, but has caught one or two – ten in total.
LAT recorded the energy of the gamma rays and found one to be 33 GeV (that is 30,000,000,000 eV compared to the 10s-100s eV in UV light and 1000s-1000,000s in X-rays). This was the highest energy gamma ray to be seen coming from a GRB. The high energies are believed to be the result of Doppler shifts associated with high relative approach velocities of the sources rather than extra high energetic things happening compared to the average GRB.