The Origin of Cosmic Ray Electrons and Positrons

Casadei, D.; Bindi, V.

doi:10.1086/422514

Cited by 32 publications

(41 citation statements)

References 48 publications

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“…We have not discussed ionization due to electrons because the electron spectrum is very uncertain (see Casadei & Bindi 2004), especially below 100 MeV, where it is sensitive to the models used to demodulate the CR electron flux at the Earth (e.g., Webber 1998) or used to disentangle the g-ray bremsstrahlung, inverse Compton, and unresolved point-source emission at MeV energies (e.g., Strong 2001). However, CR electrons, like CR protons, are confined to magnetic waves and should therefore follow the CR protons (Melrose & Wentzel 1970), and have the same density variations, although the critical densities for free streaming may be different.…”

Section: Results and Conclusionmentioning

confidence: 99%

Confinement-driven Spatial Variations in the Cosmic-Ray Flux

Padoan

Scalo

2005

ApJ

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Low-energy cosmic rays (CRs) are confined by self-generated MHD waves in the mostly neutral interstellar medium. We show that the CR transport equation can be expressed as a continuity equation for the CR number density involving an effective convection velocity. Assuming a balance between wave growth and ion-neutral damping, this equation gives a steady state condition up to a critical density for free streaming. This 1/2 n ∝ n cr i relation naturally accounts for the heretofore unexplained difference in CR ionization rates derived for dense diffuse clouds (McCall et al.) and dark clouds, and predicts large spatial variations in the CR heating rate and pressure.

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Section: Results and Conclusionmentioning

confidence: 99%

Confinement-driven Spatial Variations in the Cosmic-Ray Flux

Padoan

Scalo

2005

ApJ

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“…Because the permitted range of spectral indices that provide good fits is significant, we have applied restrictions by taking the central value and variance found by Casadei & Bindi (2004), i.e., γ = 3.44 ± 0.03. Thus, an index range defined by γ ± 3σ allows to encompass most of the available data below 100 GeV.…”

Section: The Positron Fractionmentioning

confidence: 99%

Galactic secondary positron flux at the Earth

Delahaye

Lineros²,

Donato³

et al. 2009

A&A

218

308

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Context. Secondary positrons are produced by spallation of cosmic rays within the interstellar gas. Measurements have been typically expressed in terms of the positron fraction, which exhibits an increase above 10 GeV. Many scenarios have been proposed to explain this feature, among them some additional primary positrons originating from dark matter annihilation in the Galaxy. Aims. The PAMELA satellite has provided high quality data that has enabled high accuracy statistical analyses to be made, showing that the increase in the positron fraction extends up to about 100 GeV. It is therefore of paramount importance to constrain theoretically the expected secondary positron flux to interpret the observations in an accurate way. Methods. We focus on calculating the secondary positron flux by using and comparing different up-to-date nuclear cross-sections and by considering an independent model of cosmic ray propagation. We carefully study the origins of the theoretical uncertainties in the positron flux. Results. We find the secondary positron flux to be reproduced well by the available observations, and to have theoretical uncertainties that we quantify to be as large as about one order of magnitude. We also discuss the positron fraction issue and find that our predictions may be consistent with the data taken before PAMELA. For PAMELA data, we find that an excess is probably present after considering uncertainties in the positron flux, although its amplitude depends strongly on the assumptions made in relation to the electron flux. By fitting the current electron data, we show that when considering a soft electron spectrum, the amplitude of the excess might be far lower than usually claimed. Conclusions. We provide fresh insights that may help to explain the positron data with or without new physical model ingredients. PAMELA observations and the forthcoming AMS-02 mission will allow stronger constraints to be aplaced on the cosmic-ray transport parameters, and are likely to reduce drastically the theoretical uncertainties.

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“…Solar modulation also suppresses the flux of low-energy CR electrons, that already shows considerable fluctuations at energies of 10-100 GeV (see e.g. Casadei & Bindi 2004). Since the cross section for ionization of molecular hydrogen by collisions with protons and electrons has a maximum at ∼10 keV and ∼50 eV, respectively (see Sect.…”

Section: Introductionmentioning

confidence: 98%

Cosmic-ray ionization of molecular clouds

Padovani¹,

Galli²,

Glassgold³

2009

A&A

359

475

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Context. Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal, and dynamical evolution. Aims. The purpose of this work is to explore the possibility that a low-energy component of cosmic rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds. Methods. We collected the most recent experimental and theoretical data on the cross sections for the production of H + 2 and He + by electron and proton impact and discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium. Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we computed the propagated spectra in molecular clouds in the continuous slowing-down approximation taking all the relevant energy loss processes into account. Results. The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter agrees with the observational data only if the flux of either cosmic-ray electrons or of protons increases at low energies. The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions. However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained. Conclusions. Available data combined with simple propagation models support the existence of a low-energy component (below ∼100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes.

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The Origin of Cosmic Ray Electrons and Positrons

Cited by 32 publications

References 48 publications

Confinement-driven Spatial Variations in the Cosmic-Ray Flux

Confinement-driven Spatial Variations in the Cosmic-Ray Flux

Galactic secondary positron flux at the Earth

Cosmic-ray ionization of molecular clouds

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