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Durham University

Department of Physics

Staff profile

Publication details for Prof Sir Arnold Wolfendale

Wibig, T & Wolfendale, AW (2005). At what particle energy do extragalactic cosmic rays start topredominate? Journal Of Physics G-nuclear And Particle Physics 31(3): 255-264.
  • Publication type: Journal Article
  • ISSN/ISBN: 0954-3899

Author(s) from Durham


We have previously argued (e.g. Szabelski et al 2002 Astropart. Phys.
17 125) that the well-known 'ankle' in the cosmic ray energy spectrum,
at log E (eV) similar to 8.7-19.0, marks the transition from mainly
galactic sources at lower energies to mainly extragalactic above.
Recently, however, there have been claims for lower transitional
energies, specifically from log E (eV) similar to 17.0 (Thompson et al
2004 Proc. Catania Cosmic Ray Conf.) via 17.2-17.8 (Berezinsky et al
2004 Astropart. Phys. 21617) to 18.0 (Hillas 2004 Proc.Leeds Cosmic Ray
Conf.). In our model the ankle arises naturally from the sum of simple
power law-spectra with slopes differing by Deltagamma similar to 1.8;
from differential slope gamma = -3.8 for galactic particles (near log E
= 19) to gamma similar to -2.0 for extragalactic sources. In the other
models, on the other hand, the ankle is intrinsic to the extragalactic
component alone, and arises from the shape of the rate of energy loss
versus energy for the (assumed) protons interacting with the cosmic
microwave background (CMB). Our detailed analysis of the world's data
on the ultra-high energy spectrum shows that taken together, or
separately, the resulting mean sharpness of the ankle (second
derivative of the log (intensity x E-3) with respect to log E) is
consistent with our 'mixed' model. For explanation in terms of
extragalactic particles alone, however, the ankle will be at the wrong
energy-for reasonable production models and of insufficient magnitude
if, as seems likely, there is still a significant fraction of heavy
nuclei at the ankle energy.