One of the biggest challenges in cosmic-ray physics is to accurately pin down the absolute energy of a measured cosmic ray. This is traditionally done with an array of particle detectors deployed on a large grid, which then sample the energetic "air-shower" particles made in atmospheric interactions of the original cosmic particle. This is a tough challenge for particle detectors because the complex interaction physics at the highest energies has to be extrapolated from measurements at collider experiments, which operate at significantly lower energies - even at the Large Hadron Collider (LHC). To accurately set the absolute energy scale, scientists using the Pierre Auger Observatory thus rely on combining the particle detectors (for Auger, water tanks used to measure the Cherenkov light flash made in the water by relativistic charged particles) with the nitrogen fluorescence detection technique. This works very well, but requires tremendous effort, in particular to control the effects of scattering and absorption in the ever-changing atmosphere. Now, we have shown that radio detection of extensive air showers can be a very powerful means to cross-calibrate the absolute energy scale of different experiments.
Billions of subatomic particles can be created in cosmic-ray induced air showers. They travel at nearly the speed of light through the atmosphere, and eventually through the surface detector water tanks of the Pierre Auger Observatory, deployed over 3000 km2 of the Argentine pampas. Because of their relativistic speed, when propagating in the water they generate a flash of so-called Cherenkov light, the same phenomenon that causes the bluish glow in a water pool of radioactive material. The light flash is faint and requires specialized electronic light sensors (photomultiplier tubes) that can detect the intensity of the light and its time structure. For example the "risetime" is measured: this is the time that it takes for the light generated to go from 10% to 50% of the total in the overall Cherenkov flash. This time is very short, of the order of a few hundred nanoseconds, thus the need for the specialized photomultiplier sensors.
The Auger Engineering Radio Array (AERA) measures radio properties of cosmic-ray induced air showers within the Pierre Auger Observatory. The analysis of the radio data requires a very precise time-synchronisation between the individual radio detector stations. The GPS-clocks built into the AERA stations only provide a resolution of about 10 ns, whereas 1-2 ns is needed.
For this purpose, we conceived and installed a reference transmitter ("beacon") whose signals are recorded within the AERA data stream. Based upon these measurements the timing of each station is adjusted. But how can we independently check if this technique really achieves the precision we need?
Celebrating 15 years of achievements and signature ceremony of a new International Agreement for the next 10 years
The Pierre Auger Observatory is the world’s leading science project for the exploration of cosmic rays. More than 500 scientists from 16 countries have been working together since 1998 in the Province of Mendoza, Argentina, to elucidate the origin and properties of the most energetic particles in the Universe, coming to us from the far reaches of the cosmos. The Pierre Auger Observatory measures gigantic showers of relativistic particles that are the result of collisions between the very rare, highest-energy cosmic rays and atomic nuclei of the atmosphere. Properties of such air showers are used to infer the energy, direction, and mass of the cosmic particles.