A new look at the cosmic ray positron fraction

TL;DR

This paper investigates the rising positron fraction in cosmic rays, exploring dark matter and pulsar sources using latest AMS-02 data, and highlights significant uncertainties in cosmic ray propagation models.

Contribution

It provides a comprehensive analysis of dark matter and pulsar hypotheses for the positron excess, incorporating updated measurements and propagation uncertainties.

Findings

Favored dark matter mass > 500 GeV

Identified five pulsars fitting the positron data

Propagation uncertainties significantly affect results

Abstract

The positron fraction in cosmic rays was found to be steadily increasing in function of energy, above $\sim$10 GeV. This behaviour contradicts standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during the propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy. Alternatively, it could be produced by nearby astrophysical sources, such as pulsars. Both hypotheses are probed in this work in light of the latest AMS-02 positron fraction measurements. The transport of primary and secondary positrons in the Galaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used to model the positron flux generated by dark matter species. We provide mass and annihilating cross section that best fit AMS-02 data for each single annihilating channel as well as for combinations of channels. We find that the mass of the favoured dark matter candidates is always larger than 500 GeV. The description of the positron fraction from astrophysical sources is based on the pulsar observations included in the ATNF catalogue. The region of the distance-to-age plane that best fits the positron fraction for a single source is determined and a list of five pulsars from the ATNF catalogue is given. Those results are obtained with the cosmic ray transport parameters that best fit the B/C ratio. Uncertainties in the propagation parameters turn out to be very significant.