GeV observations of the extended pulsar wind nebulae constrain the pulsar interpretations of the cosmic-ray positron excess

TL;DR

This study uses GeV observations from Fermi-LAT to constrain the role of nearby pulsars in explaining the cosmic-ray positron excess, finding that under typical diffusion models, these pulsars are unlikely the main sources.

Contribution

It provides the first direct GeV constraints on positron densities in TeV nebulae, challenging pulsar interpretations of the positron excess under standard diffusion assumptions.

Findings

No extended GeV emission detected from the nebulae.

Data disfavors pulsars as dominant sources under Kolmogorov diffusion.

Energy-independent diffusion models cannot exclude pulsars as main sources.

Abstract

It has long been suggested that nearby pulsars within $\sim 1 \,{\rm kpc}$ are the leading candidate of the 10-500 GeV cosmic-ray positron excess measured by PAMELA and other experiments. The recent measurement of surface brightness profile of TeV nebulae surrounding Geminga and PSR~B0656+14 by the High-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. Here we argue that GeV observations provide more direct constraints on the positron density in the TeV nebulae in the energy range of 10-500 GeV and hence on the origin of the observed positron excess. Motivated by this, we search for GeV emission from the TeV nebulae with the \textsl{Fermi} Large Area Telescope (LAT). No spatially-extended GeV emission is detected from these two TeV nebulae in the framework of two-zone diffusion spatial templates, suggesting a relatively low density of GeV electrons/positrons in the TeV nebulae. A joint modelling of the data from HAWC and \textsl{Fermi}-LAT disfavors Geminga and PSR~B0656+14 as the dominant source of the positron excess at $\sim 50-500$ GeV for the usual Kolmogorov-type diffusion, while for an energy-independent diffusion, a dominant part of the positron excess contributed by them cannot be ruled out by the current data.