BAYESIAN ANALYSIS OF COSMIC RAY PROPAGATION: EVIDENCE AGAINST HOMOGENEOUS DIFFUSION

G Jóhannesson; R Ruiz de Austri; A C Vincent; I V Moskalenko; E Orlando; T A Porter; A W Strong; R Trotta; F Feroz; P Graff; M P Hobson

doi:10.3847/0004-637x/824/1/16

BAYESIAN ANALYSIS OF COSMIC RAY PROPAGATION: EVIDENCE AGAINST HOMOGENEOUS DIFFUSION

Astrophys J. 2016 Jun 10;824(1):16. doi: 10.3847/0004-637x/824/1/16. Epub 2016 Jun 3.

Authors

G Jóhannesson¹, R Ruiz de Austri², A C Vincent³, I V Moskalenko^{4

5}, E Orlando^{4

5}, T A Porter^{4

5}, A W Strong⁶, R Trotta^{7

8}, F Feroz⁹, P Graff^{10

11}, M P Hobson⁹

Affiliations

¹ Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland.
² Instituto de Física Corpuscular, IFIC-UV/CSIC, Valencia, Spain.
³ Institute for Particle Physics Phenomenology (IPPP), Department of Physics, Durham University, Durham DH1 3LE, UK.
⁴ Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA.
⁵ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA.
⁶ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany.
⁷ Astrophysics Group, Imperial Centre for Inference and Cosmology, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK.
⁸ Data Science Institute, William Penney Laboratory, Imperial College London, SW7 2AZ London, UK.
⁹ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK.
¹⁰ The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
¹¹ Department of Physics, University of Maryland, College Park, MD 20742, USA.

Abstract

We present the results of the most complete scan of the parameter space for cosmic ray (CR) injection and propagation. We perform a Bayesian search of the main GALPROP parameters, using the MultiNest nested sampling algorithm, augmented by the BAMBI neural network machine-learning package. This is the first study to separate out low-mass isotopes (p, $\bar{p}$ , and He) from the usual light elements (Be, B, C, N, and O). We find that the propagation parameters that best-fit p, $\bar{p}$ , and He data are significantly different from those that fit light elements, including the B/C and ¹⁰Be/⁹Be secondary-to-primary ratios normally used to calibrate propagation parameters. This suggests that each set of species is probing a very different interstellar medium, and that the standard approach of calibrating propagation parameters using B/C can lead to incorrect results. We present posterior distributions and best-fit parameters for propagation of both sets of nuclei, as well as for the injection abundances of elements from H to Si. The input GALDEF files with these new parameters will be included in an upcoming public GALPROP update.

Keywords: Galaxy: general; ISM: general; astroparticle physics; cosmic rays; diffusion; methods: statistical.

Grants and funding

NNX13AC47G/NASA/NASA/United States