Cosmic-ray transport from AMS-02 boron to carbon ratio data: Benchmark models and interpretation

This article aims at establishing new benchmark scenarios for Galactic cosmic-ray propagation in the GV-TV rigidity range, based on fits to the AMS-02 B/C data and the \usine{} propagation code. We employ a new fitting procedure, cautiously taking into account systematic error correlations in different rigidity bins and considering Solar modulation potential and leading nuclear cross-section as nuisance parameters. We delineate specific low, intermediate, and high-rigidity ranges, that can be related to both features in the data and peculiar microphysics mechanisms resulting in spectral breaks. We single out a scenario which yields excellent fits to the data and includes all the presumably relevant complexity, the \BIG{} model. This model has two limiting regimes: (i) the \SLIM{} model, a minimal diffusion-only setup, and (ii) the \QUAINT{} model, a convection-reacceleration model where reacceleration is tuned by non-relativistic effects. All models lead to robust predictions in the high-energy regime ($\gtrsim10$~GV), i.e. independent of the propagation scenario: at $1\sigma$, the diffusion slope $\delta$ is $[0.43-0.53]$, whereas $K_{10}$, the diffusion coefficient at 10~GV, is $[0.26-0.36]$~kpc$^2$~Myr$^{-1}$; we confirm the robustness of the high-energy break, with a typical value $\sim 0.2$. We also find a hint for a similar (reversed) feature at low rigidity around the B/C peak ($\sim 4$~GV) which might be related to some effective damping scale in the magnetic turbulence.