Contribution of pulsars to cosmic-ray positrons in light of recent observation of inverse-Compton halos

Abstract

The hypothesis that pulsar wind nebulae (PWNe) can significantly contribute to the excess of the positron (${e}^{+}$) cosmic-ray flux has been consolidated after the observation of a $\ensuremath{\gamma}$-ray emission at TeV energies of a few degree size around Geminga and Monogem PWNe, and at GeV energies for Geminga at a much larger extension. The $\ensuremath{\gamma}$-ray halos around these PWNe are interpreted as due to electrons (${e}^{\ensuremath{-}}$) and ${e}^{+}$ accelerated and escaped by their PWNe, and inverse Compton scattering low-energy photons of the interstellar radiation fields. The extension of these halos suggests that the diffusion around these PWNe is suppressed by 2 orders of magnitude with respect to the average in the Galaxy. We implement a two-zone diffusion model for the propagation of ${e}^{+}$ accelerated by the Galactic population of PWNe. We consider pulsars from source catalogs and build up simulations of the PWN Galactic population. In both scenarios, we find that within a two-zone diffusion model, the total contribution from PWNe and secondary ${e}^{+}$ is at the level of AMS-02 data, for an efficiency of conversion of the pulsar spin-down energy in ${e}^{\ifmmode\pm\else\textpm\fi{}}$ of $\ensuremath{\eta}\ensuremath{\sim}0.1$. For the simulated PWNe, a $1\ensuremath{\sigma}$ uncertainty band is determined, which is of at least 1 order of magnitude from 10 GeV up to few TeV. The hint for a decreasing ${e}^{+}$ flux at TeV energies is found, even if it is strongly connected to the chosen value of the radius of the low diffusion bubble around each source.

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