The Separation of Secondary Positrons Produced in the Galaxy from the High Energy Positrons that are Observed Recent Space Experiments on PAMELA and AMS2

TL;DR

This paper investigates the origin of high-energy positrons observed by space experiments, suggesting a significant galactic contribution and highlighting the need to reassess background assumptions for accurate interpretation.

Contribution

It provides a new analysis of galactic positron production limits and compares the observed excess to background spectra, proposing a revised understanding of positron origins.

Findings

Galactic production accounts for 70-100% of observed positrons at 10 GeV.

Excess positron spectrum follows an E-2.75 power law.

Background positron spectrum also follows an E-2.75 power law.

Abstract

The large intensity of greater than 10 GeV positrons which apparently come from sources outside the Earth-Sun system observed recently by many spacecraft (PAMELA, FERMI, AMS2) is still a mystery with broad implications. In our attempts to solve this mystery we have first tried to define reasonable limits to the positrons produced in our own galaxy by nuclear interactions of cosmic rays. This is best done by using the secondary B/C ratio produced by these same cosmic rays in order to define the amount of matter traversed by galactic cosmic ray nuclei. Using new values of the B/C ratio together with earlier calculations of positron production by Moskalenko and Strong, 1998, we find that at 10 GeV this galactic production is from 70% to almost 100% of the positrons observed by the above experiments. At 100 GeV these fractions are still from 20 to 33% of the positrons observed. The resulting excess positron spectrum above this normal galactic background is found to have an exponent -2.75, possibly flattening at lower energies. If these positrons are coming to us from a uniform source distribution in and beyond the disk of the galaxy with a source spectrum that is E-2.0, a spectral steepening caused by synchrotron and inverse Compton losses, which are largest near the galactic plane, will produce a spectrum E-2.75 at the Earth. This is similar to the excess spectrum we find. The excess positron spectrum E-2.75 that we obtain is also very similar to the positron spectrum produced by galactic protons in the Earths atmosphere and in the spacecraft itself which also has a spectrum E-2.75. This results in a background for all the above experiments. The assumption is that this type of background is removed by the stringent criteria that are imposed on each event. This calculation needs to be reconsidered before the implications of these important positron results can be fully evaluated.