科研之友
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摘要
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.
- 出版日期2010-7
