Systematic uncertainties on the cosmic-ray transport parameters

Is it possible to reconcile B/C data with δ = 1/3 or δ = 1/2?

¹ Laboratoire de Physique Nucléaire et des Hautes Énergies (LPNHE), Universités Paris VI et Paris VII, CNRS/IN2P3, Tour 33, Jussieu, Paris, 75005, France e-mail: dmaurin@lpnhe.in2p3.fr
² Dept. of Physics and Astronomy, University of Leicester, Leicester, LE17RH, UK
³ Institut d'Astrophysique de Paris (IAP), UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁴ Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut Polytechnique de Grenoble, 53 avenue des Martyrs, 38026 Grenoble, France
⁵ The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 7 January 2010
Accepted: 14 March 2010

Abstract

Context. The B/C ratio is used in cosmic-ray physics to constrain the transport parameters. However, from the same set of data, the various published values show a puzzling large scatter of these parameters.

Aims. We investigate the impact of using different inputs (gas density and hydrogen fraction in the Galactic disc, source spectral shape, low-energy dependence of the diffusion coefficient, and nuclear fragmentation cross-sections) on the best-fit values of the transport parameters. We quantify the systematics produced when varying these inputs, and compare them to statistical uncertainties. We discuss the consequences for the slope of the diffusion coefficient δ.

Methods. The analysis relies on the propagation code USINE interfaced with the Minuit minimisation routines.

Results. We find the typical systematic uncertainties to be greater than the statistical ones. The several published values of δ (from 0.3 to 0.8) can be recovered when varying the low-energy shape of the diffusion coefficient and the convective wind strength. Models including a convective wind are characterised by δ ≳ 0.6, which cannot be reconciled with the expected theoretical values (1/3 and 1/2). However, from a statistical point of view ( analysis), models with both reacceleration and convection – hence large δ – are favoured. The next favoured models in line yield δ, which can be accommodated with 1/3 and 1/2, but require a strong upturn of the diffusion coefficient at low energy (and no convection).

Conclusions. To date, using the best statistical tools, the transport parameter determination is still plagued by many unknowns at low energy (~GeV/n). To disentangle all these configurations, measurements of the B/C ratio at TeV/n energies and/or combination with other secondary-to-primary ratios is necessary.