Origin of the spectral features observed in the cosmic-ray spectrum

¹ INAF-Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
² Dipartimento di Fisica, Università di Torino, via P. Giuria 1, 10125 Torino, Italy
³ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy
⁴ Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France

^⋆ Corresponding author; sarah.recchia@inaf.it

Received: 18 December 2023
Accepted: 10 October 2024

Abstract

Context. Recent measurements reveal the presence of several features in the cosmic-ray (CR) spectrum. In particular, the proton and helium spectra exhibit a spectral hardening at ≈300 GV and a spectral steepening at ≈15 TV, followed by the well-known knee-like feature at ≈3 PV. The spectra of heavier nuclei also harden at ≈300 GV, while no claim can be currently made about the presence of the ≈15 TV softening, due to low statistics. In addition, the B/C ratio also exhibits a hardening at ≈100 GeV/n and seems to be rather shallow at ≈1 TeV/n

Aims. We propose a possible explanation of the observed spectral features in the framework of a composite diffusion scenario and considering different classes of sources.

Methods. The proposed scenario is based on two assumptions. First, in the Galactic disk, where magnetic field lines are mainly oriented along the Galactic plane, particle scattering is assumed to be very inefficient. Therefore, the transport of CRs from the disk to the halo is set by the magnetic field line random walk induced by large-scale turbulence. Second, we propose that the spectral steepening at ≈15 TV is related to the typical maximum rigidity reached in the acceleration of CRs by the majority of supernova remnants, while we assume that only a fraction of sources, contributing to ≈10 − 20% of the CR population, can accelerate particles up to ∼PV rigidities.

Results. Within this framework we show that it is possible to reproduce the proton and helium spectra from GV to multi-PV; the p/He ratio; the spectra of CRs from lithium to iron; the p̄ flux and the p̄/p ratio; and the abundance ratios B/C, B/O, C/O, Be/C, Be/O, and Be/B. We also discuss the ¹⁰Be/⁹Be ratio in view of the recent AMS02 preliminary measurements.