A scenario for the Galactic cosmic rays between the knee and the second-knee

MOLLERACH, SILVIA; ROULET, ESTEBAN

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS

IOP PUBLISHING LTD

Año: 2019 vol. 2019 p. 17 - 17

1475-7516

We perform a fit to measurements of the cosmic ray spectrum in theenergy range between 1015 eV and 1018 eV usingdata from the TALE, Tunka and Auger experiments. We also fit the data onthe depth of shower maximum, Xmax, from Tunka and Yakutsk orfrom Auger to constrain the cosmic ray composition. We consider aGalactic component that is a mixture of five representative nuclearspecies (H, He, N, Si and Fe), for which we adopt rigidity dependentbroken power-law spectra, and we allow for an extragalactic componentwhich becomes strongly suppressed for decreasing energies. The relativeabundances of the Galactic components at 1015 eV are taken tobe comparable to those determined by direct measurements at1013 eV. The main features of the spectrum and of thecomposition are reproduced in these scenarios. The spectral knee resultsfrom the break of the H spectrum at Eksimeq 3×1015 eV, although it is broadened by the comparablecontribution from He which has a break at about 6× 1015eV. The low-energy ankle at Elasimeq 2× 1016eV coincides with the strong suppression of the H and He Galacticcomponents and the increasing relative contribution of the heavier ones,but the observed hardening of the spectrum at this energy turns out toresult from the growing contribution of the extragalactic component. Thesecond-knee at Esksimeq 26 Eksimeq 8×1016 eV is associated with the steepening of the Galactic Fecomponent. The transition to the regime in which the total cosmic rayflux is dominated by the extragalactic component takes place at anenergy of about 1017 eV. The parameters of the fit depend onthe specific Xmax dataset that is considered, with theYakutsk and Tunka data leading to a suppression of the light componentsbeing steeper beyond the knee so as to allow a faster growth of theaverage mass around the low-energy ankle. The results also depend on thehadronic model that is used to interpret Xmax measurementsand we compare the parameters obtained with Sibyll 2.3, EPOS-LHC andQGSJET II-04. The impact of the possible existence of a maximum rigiditycutoff in the Galactic components is also discussed.