On the Origin of Ultra High Energy Cosmic Rays

Charles D. Dermer yesterday sent his contribution to the 30^th International Cosmic Ray Conference celebrated in México. Here is the beginning of his introduction.

Introduction

A high-significance steepening in the UHECR spectrum at energy E∼= 10^19.6 eV was reported earlier this year by the HiRes collaboration[1], and here at the 2007 Mérida ICRC based on observations taken with the Auger Observatory [2]. This result confirms the prediction of the Greisen-Zatsepin-Kuzmin (GZK) cutoff [3, 4] in the UHECR spectrum due to photohadronic interactions of UHECRs with photons of the cosmic microwave background radiation(CMBR),and favors astrophysical bottom-up vs. particle physics top-down scenarios for the UHECRs,provided that sources are found within the GZK radius. At the same time, Auger data shows[5] evidence for mixed composition with substantial ion content in UHECRs with energies as high as a few × 10¹⁹ eV, based on studies of the depth of shower maxima. With hybrid fluorescence detectors and shower counters,Auger provides the strongest evidence yet for metals in the UHECRs,possibly with mean atomic mass < A > ~ 8–26, significantly different from pure proton and pure Fe composition. This result depends on the accuracy of the
nuclear interaction physics used to model showers, but points to the importance of nuclei in the UHECRs,and the meaning of this for GZK physics [6,7,8].

To read the paper you can go to:

Charles D. Dermer.

Summary

Auger has already contributed three major discoveries to cosmic-ray physics:

1. the GZK cutoff, also found with HiRes;

2. Mixed ionic composition in the UHECRs up to a few × 10¹⁹ eV; and

3. statistical demonstration that the clustering of 27 UHECRs with energies >∼ 6 × 10 ¹⁹ eV follow the matter distribution as traced by nearby ( < ∼ 75 Mpc) AGNs.

An important formula for the study of UHECR is the Larmor radius one:


r_L = E/QB ∼= 1.1 (E/10¹⁹ eV)/ [(Z/10)B(μG)] kpc

r_L ~=600 (E/6 × 10¹⁹ eV)/ [(Z/10) B−11] Mpc

where B = 10⁻¹¹B_-11 G is the mean magnetic field in which the ion propagates.

One can read here the size of the objects involved in the shaping of the UHECR. They are in the kpc range for the size of galaxies, and in the Mpc range for the intergalactic distances; because galactic fields are μG, and intergalactic fields are nG.

Dermer also finds in his article that Auger detected particles that can be made consistent with observations coming from Gamma Ray Bursts (GRB), and other known astrophysical objects.

His conclusion is:

"With the γ -ray, cosmic ray, and neutrino observations, it is likely that the problem of UHECR origin will soon be solved."