Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei (original) (raw)

The positron fraction observed by PAMELA and other experiments up to ~ 100 GeV is analyzed in terms of models of cosmic-ray propagation. It is shown that generically we expect the positron fraction to reach ~ 0.6 at energies of several TeV, and its energy dependence bears an intimate but subtle connection with that of the boron to carbon ratio in cosmic rays. The observed positron fraction can be fitted in a model that assumes a significant fraction of the boron below ~ 10 GeV is generated through spallation of cosmic-ray nuclei in a cocoonlike region surrounding the sources, and the positrons of energy higher than a few GeV are almost exclusively generated through cosmic-ray interactions in the general interstellar medium. Such a model is consistent with the bounds on cosmic-ray anisotropies and other observations. Comment: Poster presented at XVI International Symposium on Very High Energy Cosmic Ray Interactions (ISVHECRI 2010), Batavia, IL, USA (28 June - 2 July 2010). 5 pages, ...

Recent data on Galactic cosmic-ray (CR) leptons and hadrons gave rise to two exciting problems: on the lepton side, the origin of the rise of the CR positron fraction e+/(e- + e+) at ∼10–300 GeV of energy; on the hadron side, the nature of the spectral hardening observed in CR protons and nuclei at ∼TeV energies. The lepton anomaly indicates the existence of a nearby e source. It has been proposed that high-energy positrons can be produced inside nearby supernova remnants (SNRs) via interactions of CR hadrons with the ambient medium. A distinctive prediction of this mechanism is a high-energy rise of the boron-to-carbon ratio, which has not been observed. It also requires old SNRs at work (with ineffective magnetic field amplification and slow shock speed) that cannot account for the CR hadronic spectra observed up to the knee energies (∼5 PeV). We propose a new picture where, in addition to such a nearby CR accelerator, the high-energy spectrum of CR hadrons is provided by the large-scale population of SNRs, younger on average, which can efficiently accelerate CRs up to the knee. Under this scenario, the spectral hardening of CR hadrons can be naturally interpreted as the transition between the two components. As we will show, our two-component model breaks the connection between the positron fraction and the boron-to- carbon ratio, which is now predicted to decrease with energy in accordance with the data. Forthcoming data from AMS will be crucial for testing this model.

Several cosmic ray experiments have measured the positron fraction up to few hundred GeV. Their data have revealed an excess of positrons above 10 GeV that is not consistent with the secondary production of these particles in the interstellar medium. A primary source like dark matter or astrophysical sources (e.g pulsars and their nebulae) were considered to account for such an excess. In this paper we analyse the possibility of a primary positron production due to pulsars. Under the assumption of equal initial spectra at the source for positrons, electrons, and gamma-rays we study the propagation of particle spectra using a diffusion model in the Galaxy. We focused our analysis on the Vela and Crab pulsars and their associated nebulae, which are well observed in gamma-rays. Comparison with experimental data is reported. The propagated positron and electron spectra generated from these sources result in a positron ratio, which is largely inconsistent with the excess observed by PAME...