Galactic electrons and positrons at the Earth: new estimate of the primary and secondary fluxes

Abstract

Context. The so-called excess of cosmic ray (CR) positrons observed by the PAMELA satellite up to 100 GeV has led to many interpretation attempts, from standard astrophysics to a possible exotic contribution from dark matter annihilation or decay. The Fermi data subsequently obtained about CR electrons and positrons in the range 0.02-1 TeV, and HESS data above 1 TeV have provided additional information about the leptonic content of local Galactic CRs. Aims. We analyse predictions of the CR lepton fluxes at the Earth of both secondary and primary origins, evaluate the theoretical uncertainties, and determine their level of consistency with respect to the available data. Methods. For propagation, we use a relativistic treatment of the energy losses for which we provide useful parameterizations. We compute the secondary components by improving on the method that we derived earlier for positrons. For primaries, we estimate the contributions from astrophysical sources (supernova remnants and pulsars) by considering all known local objects within 2 kpc and a smooth distribution beyond. Results. We find that the electron flux in the energy range 5-30 GeV is well reproduced by a smooth distant distribution of sources with index γ ∼ 2.3−2.4, while local sources dominate the flux at higher energy. For positrons, local pulsars have an important effect above 5-10 GeV. Uncertainties affecting the source modeling and propagation are degenerate and each translates into about one order of magnitude error in terms of local flux. The spectral shape at high energy is weakly correlated with the spectral indices of local sources, but more strongly with the hierarchy in their distance, age and power. Despite the large theoretical errors that we describe, our global and self-consistent analysis can explain all available data without over-tuning the parameters, and therefore without the need to consider any exotic physics. Conclusions. Though a standard paradigm of Galactic CRs is well established, our results show that we can hardly talk about any standard model of CR leptons, because of the very large theoretical uncertainties. Our analysis provides details about the impact of these uncertainties, thereby sketching a roadmap for future improvements.

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.…”

The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

“…in Delahaye et al (2010); Grasso et al (2011);di Bernardo et al (2011). Its relation to the high-energy positron excess discoved by the PAMELA experiment (Adriani et al 2009) is also discussed in these papers.…”