September 20, 2004
Anatomy of a big stereo hybrid event from the Auger Observatory
by James Cronin

The Pierre Auger Observatory is now operating with more than 420 tanks and two complete fluorescence telescopes. On average 2.5 events with energy ≥ 10¹⁹ eV are recorded each day. On June 26^th 2004 a big event - one of the highest energy cosmic rays ever recorded with energy ~ 5 x 10¹⁹ eV was observed by the surface detector and both fluorescence telescopes.

The view of the event on the surface is shown in Figure 1.

The axis of the shower was aimed at Coihueco and the light received was totally dominated by Cherenkov light. The observed light profile at Coihueco is shown in Figure 2. The light observed Is so dominated by direct Cherenkov light that the profile, while useful for geometric reconstruction is not suitable for energy determination.

The light received at Los Leones is nearly pure and after a small correction for scattered Cherenkov light and atmospheric absorption the number of charged particles can be determined using the fluorescence efficiency determined in laboratory experiments. Integration of this curve using a dE/dx of 2.2 MeV/gm/cm² gives an electromagnetic energy of 4.9 x 10¹⁹ eV. Correcting for the energy carried by muons and neutrinos the total energy is estimated to be 5.4 x 10¹⁹ eV. The number of charged particles as a function of atmospheric depth is shown in Figure 3.

Related Links:
KICP Members: James W. Cronin
Scientific projects: Pierre Auger Observatory (AUGER)