Studying the Milky Way Pulsar Population with Cosmic-Ray Leptons

Abstract

Recent measurements of cosmic-ray electron and positron spectra at energies from a GeV to 5 TeV, as well as radio, X-ray and a wide range of gamma-ray observations of pulsar-wind nebulae, indicate that pulsars are significant sources of high-energy cosmic-ray electrons and positrons. Here we calculate the local cosmic-ray $e^\pm$ energy spectra from pulsars taking into account models for (a) the distribution of the pulsars spin-down properties; (b) the cosmic-ray source spectra; and (c) the physics of cosmic-ray propagation. We then use the measured cosmic-ray fluxes from AMS-02, CALET and DAMPE to constrain the space of pulsar and cosmic-ray-propagation models and in particular, local cosmic-ray diffusion and energy losses, the pulsars' energy-loss time-dependence, and the injected $e^{\pm}$ spectra. We find that the lower estimates for the local $e^{\pm}$ energy losses are inconsistent with the data. We also find that pulsar braking indexes of 2.5 or less for sources with ages more than 10 kyr are strongly disfavored. Moreover, the cosmic-ray data are consistent with a wide range of assumptions on the $e^{\pm}$ injection spectral properties and on the distribution of initial spin-down powers. Above a TeV in energy, we find that pulsars can easily explain the observed change in the $e^{+} + e^{-}$ spectral slope. These conclusions are valid as long as pulsars contribute $\gtrsim10\%$ of the observed cosmic-ray $e^\pm$ at energies $\gtrsim100$ GeV.