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- Maurin, D., et al.
(författare)
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Systematic uncertainties on the cosmic-ray transport parameters Is it possible to reconcile B/C data with delta=1/3 or delta 1/2?
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 516, s. A67-
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Tidskriftsartikel (refereegranskat)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 delta. 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 delta (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 delta greater than or similar to 0.6, which cannot be reconciled with the expected theoretical values (1/3 and 1/2). However, from a statistical point of view (chi(2) analysis), models with both reacceleration and convection - hence large delta - are favoured. The next favoured models in line yield delta, 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 (similar to 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.
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| 2. |
- Putze, Antje, et al.
(författare)
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A Markov Chain Monte Carlo technique to sample transport and source parameters of Galactic cosmic rays II. Results for the diffusion model combining B/C and radioactive nuclei
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 516, s. A66-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Ongoing measurements of the cosmic radiation ( nuclear, electronic, and gamma-ray) are providing additional insight into cosmic-ray physics. A comprehensive picture of these data relies on an accurate determination of the transport and source parameters of propagation models. Aims. A Markov Chain Monte Carlo method is used to obtain these parameters in a diffusion model. By measuring the B/C ratio and radioactive cosmic-ray clocks, we calculate their probability density functions, placing special emphasis on the halo size L of the Galaxy and the local underdense bubble of size r(h). We also derive the mean, best-fit model parameters and 68% confidence level for the various parameters, and the envelopes of other quantities. Methods. The analysis relies on the USINE code for propagation and on a Markov Chain Monte Carlo technique previously developed by ourselves for the parameter determination. Results. The B/C analysis leads to a most probable diffusion slope delta = 0.86(-0.04)(+0.04) for diffusion, convection, and reacceleration, or delta = 0.234(-0.005)(+ 0.006) for diffusion and reacceleration. As found in previous studies, the B/C best-fit model favours the first configuration, hence pointing to a high value for delta. These results do not depend on L, and we provide simple functions to rescale the value of the transport parameters to any L. A combined fit on B/C and the isotopic ratios (Be-10/Be-9, Al-26/Al-27, Cl-36/Cl) leads to L = 8(-7)(+8) kpc and r(h) = 120(-20)(+20) pc for the best-fit model. This value for r(h) is consistent with direct measurements of the local interstallar medium. For the model with diffusion and reacceleration, L = 4(-1)(+1) kpc and r(h) = 3(-3)(+70) pc (consistent with zero). We vary delta, because its value is still disputed. For the model with Galactic winds, we find that between delta = 0.2 and 0.9, L varies from O(0) to O(2) if r(h) is forced to be 0, but it otherwise varies from O(0) to O(1) (with r(h) similar to 100 pc for all delta greater than or similar to 0.3). The results from the elemental ratios Be/B, Al/Mg, and Cl/Ar do not allow independent checks of this picture because these data are not precise enough. Conclusions. We showed the potential and usefulness of the Markov Chain Monte Carlo technique in the analysis of cosmic-ray measurements in diffusion models. The size of the diffusive halo depends crucially on the value of the diffusion slope delta, and also on the presence/absence of the local underdensity damping effect on radioactive nuclei. More precise data from ongoing experiments are expected to clarify this issue.
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