Bayesian Analysis of Cosmic Ray Propagation: Evidence Against Homogeneous Diffusion

Abstract

We present the results of the most complete ever 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 such study to separate out low-mass isotopes Be secondary-to-primary ratios normally used to calibrate propagation parameters. This suggests 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.

Power requirements for cosmic ray propagation models involving diffusive reacceleration; estimates and implications for the damping of interstellar turbulence

“…This model has typical parameter values which reproduce B/C in reacceleration models (see also Trotta et al (2011);Jóhannesson et al (2016)). …”

Interstellar gas in 3D, implications for CR propagation and gamma-ray emission.

“…Because changing the gas models requires re-tuning of the propagation parameters the method described in [3] is followed. The propagation model parameters determined for the 2D gas model case are typical as found in other studies [22]. Using the 3D gas models produces significant changes in the propagation parameters because the total gas mass in these models is larger: the diffusion coefficient and the Alfven velocity both being about a factor of 2 lower with the new gas model.…”

“…Two models for the gas are employed: the standard 2D azimuthally symmetric distributions in GALPROP that have been used successfully many times to explain CR data [22], and where the gas models are replaced with the ones described in last section. The CR source distribution is in both cases 2D azimuthally symmetric following the radial distribution of pulsars [23].…”

Inside out: unveiling local interstellar spectra of cosmic ray species

“…As pointed out in [13], there could be a significant difference between the propagation parameters derived from the light isotopes (p,p, He), and nuclei (boron to silicon). Our study does not show any evident discrepancies between the light isotopes (p,p, He), and the B/C ratio (nuclei).…”

“…The key idea behind GALPROP is that all CR-related data, including direct measurements, γ-rays, synchrotron radiation, etc., are subject to the same Galactic physics and must be modeled simultaneously. The GALPROP model for CR propagation is being continuously developed in order to provide a framework for studies of CR propagation in the Galaxy and interpretation of relevant observations [4][5][6][7][8][9][10][11][12][13][14][15][16]. The latest version and supplementary datasets are available through a WebRun interface at the dedicated website.…”