What Can We Really Learn from Positron Flux ’anomalies’

We present a critical analysis of the observational constraints on, and of the theoretical modeling of, aspects of CR generation and propagation in the Galaxy, which are relevant for the interpretation of recent positron and anti-proton measurements. We give simple, analytic, model independent expressions for the secondary anti-proton flux, and an upper limit for the positron flux, obtained by neglecting positron radiative losses, e/(e + e) < 0.2 ± 0.1 up to ∼ 200GeV. These expressions are completely determined by the rigidity dependent X=grammage, which is measured from stable CR secondaries up to ∼ 200GeV, and by nuclear cross sections measured in the lab. Anti-proton and positron measurements, available up to ∼ 100GeV, are consistent with these estimates, implying that there is no need for new anti-proton/positron sources. The radiative loss suppression fs,e+ of positrons depends on their propagation in the Galaxy, which is not understood theoretically. A rough, model independent, estimate of fs,e+ ∼ 1/3 can be made at a single energy, ε ∼ 20GeV, from unstable secondary decay and is found to be consistent with positron measurements, including the positron fraction measured by PAMELA. Specific detailed models that agree with compositional CR data, are shown to agree with our simple expressions for the positron and anti-proton flux up to a factor of ∼ 2. The claims that the positron fraction measured by PAMELA require new primary positron sources are based on assumptions that are not supported by observations. If PAMELA results are correct, they suggest that fs,e+(ε) is slightly increasing with energy, which provides an interesting constraint on CR propagation models. We argue that measurements of the absolute positron flux, or better yet, the positron to anti-proton ratio, are more useful for challenging secondary models than the positron fraction. Subject headings: Astrophysics High Energy Astrophysical Phenomena, High Energy Physics Phenomenology