Secondary cosmic-ray nuclei from supernova remnants and constraints on the propagation parameters

¹ INFN – Sezione di Perugia, 06122 Perugia, Italy
e-mail: nicola.tomassetti@pg.infn.it
² Physics Department, Torino University and INFN, 10125 Torino, Italy
e-mail: fiorenza.donato@unito.it

Received: 6 February 2012
Accepted: 23 March 2012

Abstract

Context. The secondary-to-primary boron-to-carbon (B/C) ratio is widely used to study the cosmic-ray (CR) propagation processes in the Galaxy. It is usually assumed that secondary nuclei such as Li-Be-B are generated entirely by collisions of heavier CR nuclei with the interstellar medium (ISM).

Aims. We study the CR propagation under a scenario where secondary nuclei can also be produced or accelerated by Galactic sources. We consider the processes of hadronic interactions inside supernova remnants (SNRs) and the re-acceleration of background CRs in strong shocks. We investigate their impact in the propagation parameter determination within present and future data.

Methods. Analytical calculations are performed in the frameworks of the diffusive shock acceleration theory and the diffusive halo model of CR transport. Statistical analyses are performed to determine the propagation parameters and their uncertainty bounds using existing data on the B/C ratio, as well as the simulated data expected from the AMS-02 experiment.

Results. The spectra of Li-Be-B nuclei emitted from SNRs are harder than those due to CR collisions with the ISM. The secondary-to-primary ratios flatten significantly at ~TeV/n energies, both from spallation and re-acceleration in the sources. The two mechanisms are complementary to each other and depend on the properties of the local ISM around the expanding remnants. The secondary production in SNRs is significant for dense background media, n₁ ≳ 1 cm^-3, while the amount of re-accelerated CRs is relevant to SNRs expanding into rarefied media, n₁ ≲ 0.1 cm^-3. Owing to these effects, the diffusion parameter δ may be underestimated by a factor of ~5–15%. Our estimations indicate that an experiment of the AMS-02 caliber can constrain the key propagation parameters, while breaking the source-transport degeneracy for a wide class of B/C-consistent models.

Conclusions. Given the precision of the data expected from ongoing experiments, the SNR production/acceleration of secondary nuclei should be considered, if any, to prevent a possible mis-determination of the CR transport parameters.