Megan's Blog

In the News this month... Fermi sheds light on high energy cosmic rays

There has been much speculation over the cause of an excess of cosmic-ray electrons and positrons recently detected by the ATIC and PAMELA experiments. Suggested sources of this excess not only include Galactic pulsars and supernova remnants, but also more exotic explanations such as dark matter annihilations. Now, new results from the Large Area Telescope, or LAT, on board the Fermi satellite have added new information to the puzzle.

Models of cosmic ray electrons and positrons interacting with the interstellar medium predict a featureless distribution in the number of particles with energies between 10 and a few hundred giga electron volts. However, last year the European satellite PAMELA detected surprisingly large quantities of high-energy positrons, while the balloon-borne ATIC experiment found a significant peak in the total electron plus positron count at high energies. Like ATIC, the LAT on Fermi is sensitive to the total electron plus positron flux. The new results from Fermi, published in the journal Physical Review Letters on the 4th of May, do show a larger number of particles with energies of around 500 giga electron volts, but the excess is no where near as large as that measured by the ATIC experiment. The new results are, however, consistent with the excess of positrons seen by PAMELA.

While these Fermi observations are the most precise yet at these energies, they are still not enough to either confirm or rule out a particular origin for these high energy particles. The LAT team are planning further observations to reduce the uncertainties and hopefully determine whether the particles are caused by dark matter annihilations or known local sources of electrons such as pulsars and supernova remnants.