Galactic secondary positron flux at the Earth

Abstract

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.

Cosmic ray anomalies from the MSSM?

“…However, this should have little impact on our analysis as the effects of convection and re-acceleration are largely unimportant to CRE spectra above 10 GeV, at least for commonly accepted sizes of these effects [47][48][49]. Our analysis is similar in this regard to that of [76], especially in the finding that ∂V c /∂z ≈ 0.…”

“…In this respect, the treatment of CRE spectra is particularly difficult due to the large uncertainty inherent in modeling their sources and propagation [47][48][49]. In this section we discuss how we take account of these uncertainties by performing a large scan over some of the astrophysical parameters used to model cosmic-ray propagation, thereby creating JHEP01(2011)064 a variety of astrophysical background spectra that we regard as internally consistent and plausible with respect to relevant astrophysical measurements.…”

“…We can simplify eq. (3.1) by specializing to stable nuclei (τ r , τ f → ∞ andṗ ≈ 0, as nuclei undergo negligible radiative energy losses) and dropping the terms involving V a and V c , which have a small effect on spectra above ≈ 20 GeV [47][48][49]. We are then left with only the spatial diffusion term.…”

“…Such losses are thus completely negligible for nuclei such as boron and carbon, which lose energy predominantly via interactions with matter, and so should be able to diffuse substantially throughout the galactic magnetic field from their point of creation. Conversely, high-energy ( > ∼ 10 GeV) e ± lose energy very efficiently through synchrotron radiation and IC scattering and so must come from a region within the local ∼ kpc to be detected near our solar system [47][48][49]. In such a picture the "δ" appropriate to fitting B/C effectively describes an average over the entire diffusion zone while the δ parameter appropriate for modeling CRE flux spectra near earth is that which describes diffusion in the local galactic neighborhood, a scale at which diffusion is likely highly complicated.…”

High-energy neutrino signatures of dark matter

“…The unusual features of the WMAP [36] as well as the FGST [37] haze, and the PAMELA and FGST excesses may well be explained by astrophysical sources [28,29]. Alternatively, some of the excess may be explained by large uncertainties in GALPROP [38]. FGST and PAMELA can both be well explained by supernovae and a known population of Pulsars [28,29,39].…”