Ultrahigh energy cosmic rays   at the frontiers



As the flux of these particles decreases exponentially, the data is scarce at very high energy (we receive about one particle per kilometer square at 1020 eV). Ultrahigh energy cosmic ray experiences have thus to cover a large portion of ground. The latest experience which has been operating since 2005 is the Pierre Auger Observatory. It covers 3000 square km in the Pampa in Argentina and detects particles by Cerenkov effects induced in the spread out water tanks. Auger Observatory also uses a fluorescence technique to detect particles. This hybrid detection method enables us to a have much better precision on various parameters of the arriving particles (arrival direction, nuclear composition, energy etc.).


The study of the propagation of high energy cosmic rays is highly related to the understading of the distribution of magnetic fields in the Universe. Our lack of knowledge in this field is considerable, mainly due to the lack of observations and because the magnetic field generation in the Universe is not well understood. Cosmic rays could bear the signatures of these fields and a study of their spectrum could reveal some intersting extragalactic magnetic fields characteristics.


There are also many other issues related to the propagation of cosmic rays. According to some models, it could or not be possible to do cosmic ray astronomy, to validate or exclude some types of sources (continuously emitting sources or bursting sources). The correlation between Active Galactic nuclei and ultrahigh energy cosmic ray arrival directions recently seen by Auger Observatory could be interpreted in some cases as a delusion. This experiment might be seeing the last scattering surface of cosmic rays on magnetized objects, instead of their sources.


Secondary particles also attract much attention of cosmic ray physicists. They are mainly neutrinos and high energy photons (X or gamma rays) which are produced by the interaction of cosmic rays with other particles or photons (CMB photons for example). As these particles propagate rectilinearly contrarily to cosmic rays that are deflected by magnetic fields, they can provide a whole world of information concerning the sources.

The most promising mechanisms for cosmic ray acceleration are Fermi processes. Intensive work is being performed to study Fermi acceleration around ultrarelativistic and non relativistic shocks, including turbulence effects and poundering the influence of the streaming of particles on the magnetic fields.


Ultrahigh energy cosmic rays still remain a great puzzle to the scientific community. Their acceleration mechanism, as well as their progenitors and the magnetized medium they travel through before reaching the Earth are not yet unveiled, despite decades of experimental and theoretical research.

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Our papers related to this subject can be found here.

Read our ARAA review paper on the Astrophysics of ultrahigh energy cosmic rays here.

 

last updated: 25/10/13 - © Kumiko Kotera