GZK cutoff

Cosmic ray GZK cutoff The interaction with the cosmic microwave background is predicted to produce a strong cutoff in the observed spectrum at 5 x 10 eV called the GZK cutoff. This cutoff is not seen. 

Cosmic ray GZK cutoff. The interaction with the cosmic microwave background should produce a strong cutoff... 

There is no significant anisotropy of cosmic rays at any energy, not even at the highest energies, beyond the GZK cutoff. Only at those highest energies is there a persistent hint that events at quite different energies occasionally cluster into pairs and triplets in the sky in the arrival direction, and this is hard to understand in almost any model. 

The cosmic rays between the ankle and the expected GZK cutoff are readily explained by many possible sources, almost all outside our Galaxy. 

Some, but not all of these proposals can also explain particles beyond the GZK cutoff, discussed in a separate section. 

Since we do not observe the expected GZK cutoff, we need to look for particles which defy the interaction with the microwave background or for a source distribution which reduces the time for interaction with the microwave background substantially. 

Here we emphasize those concepts which can explain the events beyond the GZK cutoff these proposals do not necessarily also explain those particles below the expected cutoff. 

A source distribution which is closely patterned after the actual galaxy distribution in the nearby universe greatly enhances the expected flux of events near the GZK cutoff and may allow an interpretation of the observed events given a properly biased version of the galaxy distribution and a source population like a special class of galaxies. 

However, the data are also compatible with a spectrum which suggests a rise beyond the expected GZK cutoff, and this strongly hints at a totally different and new event class. This supports an interpretation as the result of the... 

Obviously, if we could understand the sky distribution of events at these extreme energies, then the option of using pulsars would become very attractive. For particles with a single charge as protons, the anisotropies in arrival directions would be severe, but for iron nuclei this constraint would be much alleviated. Therefore, the option of considering pulsars entails an interpretation as iron particles, inconsistent at the transition from galactic to extragalactic cosmic rays at about 3 EeV, but conceivable at a possible transition from one source population to another at the expected GZK cutoff. 

In conclusion, the origin of the events beyond the expected GZK cutoff remains a unsolved problem in modern high-energy physics. 

Accurate spectra, sky distribution, and information on the nature of particles in the three energy ranges from the knee to the ankle, through the energy range from 3 EeV to 50 EeV, the expected GZK cutoff, and beyond. 

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