Pulsar wind nebulae contribution to positron excess in AMS-02 data

Abstract

In this project, we investigate several existing paradigms seeking to explain the origin of positron energy flux excess as shown in the latest cosmic ray (CR) data from alpha magnetic spectrometer (AMS-02) currently installed on the International Space Station. We build a model of positron flux taking into account the contribution from pulsar wind nebulae (PWNe) production and secondary shower. As observed excess of positron flux only occurs in high-energy range $E\ge 10$ GeV, we omit in our model the effect of solar modulation as it only becomes significant at low energy range. When fitting the PWNe model to the AMS-02 positron energy flux spectrum, we obtain a decent fit with reasonable PWNe-related and secondary-related parameters. In addition, we investigate the possibility of positron flux contribution from dark matter (DM) annihilation for different channels; however, we fail to observe any significant DM component necessary to dramatically improve the fit. Motivated by this, in the end we include an investigation into the upper limit of DM particle cross section on a mass spectrum; namely, for different DM mass values, what is the upper limit of particle cross section that prohibits the $\chi ^2$ value of the fit from becoming unreasonably high.