The propagation of cosmic rays inside our galaxy plays a fundamental role in shaping their injection spectra into those observed at Earth. One of the best tools to investigate this issue is the ratio ...of fluxes for secondary and primary species. The boron-to-carbon (B/C) ratio, in particular, is a sensitive probe to investigate propagation mechanisms. This paper presents new measurements of the absolute fluxes of boron and carbon nuclei as well as the B/C ratio from the PAMELA space experiment. The results span the range 0.44-129 GeV/n in kinetic energy for data taken in the period 2006 July to 2008 March.
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, , and He) from the usual light elements (Be, B, C, N, and O). We find that the propagation parameters that best-fit , 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.
We report the properties of sodium (Na) and aluminum (Al) cosmic rays in the rigidity range 2.15 GV to 3.0 TV based on 0.46 million sodium and 0.51 million aluminum nuclei collected by the Alpha ...Magnetic Spectrometer experiment on the International Space Station. We found that Na and Al, together with nitrogen (N), belong to a distinct cosmic ray group. In this group, we observe that, similar to the N flux, both the Na flux and Al flux are well described by the sums of a primary cosmic ray component (proportional to the silicon flux) and a secondary cosmic ray component (proportional to the fluorine flux). The fraction of the primary component increases with rigidity for the N, Na, and Al fluxes and becomes dominant at the highest rigidities. The Na / Si and Al / Si abundance ratios at the source, 0.036 ± 0.003 for Na / Si and 0.103 ± 0.004 for Al / Si, are determined independent of cosmic ray propagation.
The escape of cosmic rays from the Galaxy is expected to shape their spectrum inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the absence of a physical description, it is ...usually modeled as a free escape from a given boundary, typically located at a few kiloparsec distance from the Galactic disc. We show that the assumption of free escape leads to the conclusion that the cosmic ray current propagating in the circumgalactic medium is responsible for a nonresonant cosmic ray induced instability that in turn leads to the generation of a magnetic field of strength ∼2×10^{-8} G on a scale ∼10 kpc around our Galaxy. The self-generated diffusion produces large gradients in the particle pressure that induce a displacement of the intergalactic medium with velocity ∼10-100 km/s. Cosmic rays are then carried away by advection. If the overdensity of the intergalactic gas in a region of size ∼10 kpc around our Galaxy is ≳100 with respect to the cosmological baryon density Ω_{b}ρ_{cr}, then the flux of high energy neutrinos due to pion production becomes comparable with the flux of astrophysical neutrinos recently measured by IceCube.
Context.
A large-scale magnetic field permeates our Galaxy and is involved in a variety of astrophysical processes, such as star formation and cosmic ray propagation. Dust polarization has been ...proven to be one of the most powerful observables for studying the field properties in the interstellar medium (ISM). However, it does not provide a direct measurement of its strength. Different methods have been developed that employ both polarization and spectroscopic data in order to infer the field strength. The most widely applied method was developed by Davis (1951, Phys. Rev., 81, 890) and Chandrasekhar & Fermi (1953, ApJ, 118, 1137), hereafter DCF. The DCF method relies on the assumption that isotropic turbulent motions initiate the propagation of Alfvén waves. Observations, however, indicate that turbulence in the ISM is anisotropic and that non-Alfvénic (compressible) modes may be important.
Aims.
Our goal is to develop a new method for estimating the field strength in the ISM that includes the compressible modes and does not contradict the anisotropic properties of turbulence.
Methods.
We adopt the following assumptions: (1) gas is perfectly attached to the field lines; (2) field line perturbations propagate in the form of small-amplitude magnetohydrodynamic (MHD) waves; and (3) turbulent kinetic energy is equal to the fluctuating magnetic energy. We use simple energetics arguments that take the compressible modes into account to estimate the strength of the magnetic field.
Results.
We derive the following equation:
B
0
= √2πρδv/√δθ, where
ρ
is the gas density,
δv
is the rms velocity as derived from the spread of emission lines, and
δθ
is the dispersion of polarization angles. We produce synthetic observations from 3D MHD simulations, and we assess the accuracy of our method by comparing the true field strength with the estimates derived from our equation. We find a mean relative deviation of 17%. The accuracy of our method does not depend on the turbulence properties of the simulated model. In contrast, the DCF method, even when combined with the Hildebrand et al. (2009, ApJ, 696, 567) and Houde et al. (2009, ApJ, 706, 1504) method, systematically overestimates the field strength.
Conclusions.
Compressible modes can significantly affect the accuracy of methods that are based solely on Alfvénic modes. The formula that we propose includes compressible modes; however, it is applicable only in regions with no self-gravity. Density inhomogeneities may bias our estimates to lower values.
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