Pierre Auger Observatory
Public Event Explorer
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Here goes a list of the terms used on this web page. More generic information on the Pierre Auger Observatory and its detectors can be found on the Southern site or the main site of Auger.

  • Station (sometimes called tank): one of the 1600 surface detectors of the Pierre Auger Observatory. They form all together the ground array of the observatory.
  • Zenith Angle (Theta): angle from the local vertical. 0 degrees is therefore vertical, while 90 degrees is horizontal. The current data available is cut at 60 degrees of zenith angle.
  • Azimuth Angle (Phi): angle on the ground with respect to the east. 0 is east, 90 north, 180 west and 270 south.
  • Energy (EeV): the energy of the cosmic ray is measured in EeV, Exa electron volts. 1 EeV is about 0.16 Joules.
  • VEM: Vertical Equivalent Muon. Signals in the surface detectors of the Pierre Auger Observatory are expressed in units of VEM. 1 VEM is the average signal obtained when a vertical and central muon goes through the detector. Muons are the most numerous charged particles at ground level.
  • LDF: Lateral Distribution Function. At the core of the cosmic ray showers, particles are much more numerous than at some distance. They however spread at up to a few kilometers. The signal in a detector as a function of the distance to the shower axis present a regular decrease similar to a power law. The absolute normalization of the LDF gives the total energy of the incoming cosmic ray.
  • Curvature: a cosmic ray shower propagates in the atmosphere almost at the speed of light and the time of arrival of particles at various detectors can be used to determine the direction of arrival of the cosmic ray (simple triangulation). The shower front is however not a plane but has some curvature, an indication on how far away did the shower originate. One can measure the deviation of the shower front to a plane, and extract the curvature.
  • Easting/Northing: the coordinates at ground of the shower core are given in Easting/Northing, UTM coordinates.
  • Reduced Chi2: all the parameters (direction, LDF) are obtained by minimizing a fitted function, and the quality of the fit is given by its reduced Chi2. Note that some additional terms enter the minimized function (it is in fact some sort of Likelyhood), moving the average reduced Chi2 away from 1.
  • T5: various triggers level are operated in the Pierre Auger Observatory. The T1, first level trigger, occurs locally at each surface detector whenever some signal is seen. If the signal is large or spread (indication of a cosmic ray shower), it is promoted to T2. When a group of neighbouring stations report a T2 in a short period, a potential cosmic ray shower has been detected, and a T3 is issued. If a closer look to the data confirm it is a cosmic ray shower, it is called a T4. T5 ensures the data is furthermore of high quality and the event can be safely reconstructed (for example, events happenning on the borders or just outside of the array are rejected by the T5).
  • PMT: Photo Multiplier Tube. Each surface station is equipped with 3 very sensitive light sensors called PMT.
  • AGN: Active Galactic Nuclei are thought to be powered by supermassive black holes that are devouring large amounts of matter. They have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy. While most galaxies have black holes at their center, only a fraction of all galaxies have an AGN. The Pierre Auger Observatory has observed a correlation between the highest energy cosmic rays of its data set and the distribution of nearby AGN.