Can the production cross-section uncertainties explain the cosmic fluorine anomaly?

Abstract

The stable secondary-to-primary flux ratios of cosmic rays (CRs), represented by the boron-to-carbon ratio (B/C), are the main probes of the galactic CR propagation. However, the fluorine-to-silicon ratio (F/Si) predicted by the CR diffusion coefficient inferred from B/C is significantly higher than the latest measurement of AMS-02. This anomaly is commonly attributed to the uncertainties of the F production cross sections. In this work, we give a careful test to this interpretation. We consider four different cross-section parametric models. Each model is constrained by the latest cross-section data. We perform combined fits to the B/C, F/Si, and cross-section data with the same propagation framework. Two of the cross-section models have good overall goodness of fit with ${\ensuremath{\chi}}^{2}/{n}_{\mathrm{d}.\mathrm{o}.\mathrm{f}.}\ensuremath{\sim}1$. However, the goodness of fit of the cross-section part is poor with ${\ensuremath{\chi}}_{\mathrm{cs}}^{2}/{n}_{\mathrm{cs}}\ensuremath{\gtrsim}2$ for these models. The best-fitted B production cross sections are systematically larger than the measurements, while the fitted cross sections for F production are systematically lower than the measurements. This indicates that the F anomaly can hardly be interpreted by neither the random errors of the cross-section measurements nor the differences between the existing cross-section models. We find that by developing a hypothetical parametrization this anomaly could be explained, while future measurements will test this possibility. We then propose that the spatially dependent diffusion model could help to explain B/C and F/Si consistently. In this model, the average diffusion coefficient of the Ne-Si group is expected to be larger than that of the C-O group.