Transition from galactic to extragalactic cosmic rays

Abstract

a b s t r a c tThe study of the transition between galactic and extragalactic cosmic rays can shed more light on the end of the galactic cosmic rays spectrum and the beginning of the extragalactic one. Three models of transition are discussed: ankle, dip and mixed composition models. All these models describe the transition as an intersection of a steep galactic component with a flat extragalactic one. Severe bounds on these models are provided by the Standard Model of galactic cosmic rays according to which the maximum acceleration energy for Iron nuclei is of the order of E max Fe % 1 Â 10 17 eV. In the ankle model the transition is assumed at the ankle, a flat feature in the all particle spectrum which observationally starts at energy E a $ ð3 À 4Þ Â 10 18 eV. This model needs a new high energy galactic component with maximum energy about two orders of magnitude above that of the Standard Model. The origin of such component is discussed. As observations are concerned there are two signatures of the transition: change of energy spectra and mass composition. In all models a heavy galactic component is changed at the transition to a lighter or proton component. As a result the ankle model predicts a galactic Iron component at E < 5 Â 10 18 eV, while both HiRes and Auger data show that at ð2 À 5Þ Â 10 18 eV primaries are protons, or at least light nuclei. In the dip model the transition occurs at the second knee observed at energy ð4 À 7Þ Â 10 17 eV and is characterized by a sharp change of mass composition from galactic Iron to extragalactic protons. The ankle in this model appears automatically as a part of the e þ e À pair-production dip. The mixed composition model describes transition at E $ 3 Â 10 18 eV with mass composition changing from the galactic Iron to extragalactic mixed composition of different nuclei. In most mixed composition models the spectrum is proton-dominated and it better fits HiRes than Auger data. The latter show a steadily heavier mass composition with increasing energy, and we discuss the models which explain it.

Propagation of ultrahigh energy cosmic rays in extragalactic magnetic fields: a view from cosmological simulations

“…Therefore, energies between 10 17 and 10 19 eV are expected to mark the transition from galactic to extragalactic sources (e. g. Aloisio et al 2012). The sources of ultra high energy cosmic rays (UHECRs) have not yet been identified.…”

The Physics of UHECRs: Spectra, Composition and the Transition Galactic-Extragalactic

“…The extragalactic origin of UHECRs, at least at energies above the ankle E > 10 19 eV, is widely accepted [18]. The propagation of UHECR through the intergalactic medium is conditioned primarily by astrophysical photon backgrounds and, if any, by the presence of [17].…”

“…A careful investigation of the transition region in the context of the dip and ankle models was carried out (see [18] and references therein), assuming an exponential cutoff in the galactic CR component. The authors concluded that the ankle model is basically ruled out by measurements of the depth of shower maximum, X max (E), and its RMS fluctuations.…”

Extragalactic cosmic rays and their signatures

“…In fact, the conclusion about ruling out the ankle as transition feature has been already made in [61] and more recently in [62] from analysis of elongation curve X max (E). One can see from right-upper and right-lower panels of Fig.…”