Anisotropic diffusion of Galactic cosmic ray protons and their steady-state azimuthal distribution

Effenberger, Frederic; Fichtner, H.; Scherer, K.; Büsching, I.

doi:10.1051/0004-6361/201220203

Cited by 57 publications

(59 citation statements)

References 64 publications

(83 reference statements)

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“…In addition, anisotropic diffusion is also seen to be essential for CR-driven magnetic dynamo action in galaxies (Hanasz et al 2009). Including a moderate anisotropy for diffusion has given more consistent results with recent estimates of the local interstellar proton spectrum compared to normal isotropic diffusion (Effenberger et al 2012). As this model has only one spatial dimension, we only implement a one-halo model.…”

Section: Discussionsupporting

confidence: 68%

Modelling the cosmic ray electron propagation in M 51

Mulcahy

Fletcher

Beck

et al. 2016

A&A

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Context. Cosmic ray electrons (CREs) are a crucial part of the interstellar medium and are observed via synchrotron emission. While much modelling has been carried out on the CRE distribution and propagation of the Milky Way, little has been done on normal external star-forming galaxies. Recent spectral data from a new generation of radio telescopes enable us to find more robust estimations of the CRE propagation. Aims. To model the synchrotron spectral index of M 51 using the diffusion energy-loss equation and to compare the model results with the observed spectral index determined from recent low-frequency observations with LOFAR. Methods. We solve the time-dependent diffusion energy-loss equation for CREs in M 51. This is the first time that this model for CRE propagation has been solved for a realistic distribution of CRE sources, which we derive from the observed star formation rate, in an external galaxy. The radial variation of the synchrotron spectral index and scale-length produced by the model are compared to recent LOFAR and older VLA observational data and also to new observations of M 51 at 325 MHz obtained with the GMRT. Results. We find that propagation of CREs by diffusion alone is sufficient to reproduce the observed spectral index distribution in M 51. An isotropic diffusion coefficient with a value of 6.6 ± 0.2 × 10 28 cm 2 s −1 is found to fit best and is similar to what is seen in the Milky Way. We estimate an escape time of 11 Myr from the central galaxy to 88 Myr in the extended disk. It is found that an energy dependence of the diffusion coefficient is not important for CRE energies in the range 0.01 GeV-3 GeV. We are able to reproduce the dependence of the observed synchrotron scale-lengths on frequency, with l ∝ ν −1/4 in the outer disk and l ∝ ν −1/8 in the inner disk.

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Section: Discussionsupporting

confidence: 68%

Modelling the cosmic ray electron propagation in M 51

Mulcahy

Fletcher

Beck

et al. 2016

A&A

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“…In particular, for a given realization of galactic source spatial distribution, the dipole anisotropy would point toward the source with the largest contribution (Erlykin & Wolfendale 2006;Blasi & Amato 2012;Ptuskin 2012;Pohl & Eichler 2013;Sveshnikova et al 2013;Savchenko et al 2015), which may change with energy, in agreement with observations. On the other hand, the systematic overestimation of the anisotropy amplitude may be partially compensated by the fact that diffusion is expected to be anisotropic (see, e.g., Effenberger et al 2012), thus modifying the expected cosmic-ray density gradient shape as a function of the source direction with respect to the regular galactic magnetic field (Kumar & Eichler 2014). The misalignment between the cosmic-ray density gradient and the regular galactic magnetic field would prevent pointing to any specific source, and it would suppress the anisotropy amplitude to a value closer to what has been observed (Mertsch & Funk 2015).…”

Section: Introductionmentioning

confidence: 99%

Cosmic-Ray Small-Scale Anisotropies and Local Turbulent Magnetic Fields

López-Barquero

Farber

Desiati

et al. 2016

ApJ

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Cosmic-ray anisotropy has been observed in a wide energy range and at different angular scales by a variety of experiments over the past decade. However, no comprehensive or satisfactory explanation has been put forth to date. The arrival distribution of cosmicrays at Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field. On the contrary, the medium-and small-scale angular structure could be an effect of nondiffusive propagation of cosmicrays in perturbed magnetic fields. In particular, a possible explanation forthe observed small-scale anisotropy observed at theTeV energy scalemay be the effect of particle propagation in turbulent magnetized plasmas. We perform numerical integration of test particle trajectories in low-β compressible magnetohydrodynamic turbulence to study how the cosmicrays' arrival direction distribution is perturbed when they stream along the local turbulent magnetic field. We utilize Liouville's theorem for obtaining the anisotropy at Earth and provide the theoretical framework for the application of the theorem in the specific case of cosmic-ray arrival distribution. In this work, we discuss the effects on the anisotropy arising from propagation in this inhomogeneous and turbulent interstellar magnetic field.

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“…These need to be taken into account whenever the predominant direction of spatial diffusion does not coincide with the directions of the numerical grid. Effects of such an anisotropic diffusion tensor have already been studied previously in the context of spiral-arm cosmic-ray source distributions (see [6]), where the authors showed a significant impact of such an anisotropic diffusion tensor. Apart from that [9] used spatially variable diffusion as an alternative explanation to the cosmic-ray gradient problem.…”

Section: Propagation Physicsmentioning

confidence: 82%

“…The impact of spiral-arm source distributions on the different cosmic ray observables has been studied recently by several groups including [3,6,10,13] and [26]. Results obtained for such a study for the highest resolution so far done with PICARD are shown in Fig.…”

Section: Pos(icrc2015)554mentioning

confidence: 99%

Galactic cosmic ray propagation models using Picard

Kissmann¹,

Reimer²,

Strong

2016

Proceedings of the 34th International Cosmic Ray Conference — PoS(ICRC2015)

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We present results obtained from our newly developed Galactic cosmic-ray transport code PI-CARD, that solves the cosmic-ray transport equation. This code allows for the computation of cosmic-ray spectra and the resulting gamma-ray emission. Relying on contemporary numerical solvers allows for efficient computation of models with deca-parsec resolution. PICARD can handle locally anisotropic spatial diffusion acknowledging a full diffusion tensor. We used this framework to investigate the transition from axisymmetric to spiral-arm cosmic-ray source distributions. Wherever possible we compare model predictions with constraining observables in cosmic-ray astrophysics.

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Anisotropic diffusion of Galactic cosmic ray protons and their steady-state azimuthal distribution

Cited by 57 publications

References 64 publications

Modelling the cosmic ray electron propagation in M 51

Modelling the cosmic ray electron propagation in M 51

Cosmic-Ray Small-Scale Anisotropies and Local Turbulent Magnetic Fields

Galactic cosmic ray propagation models using Picard

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