Probing the Pulsar Origin of the Anomalous Positron Fraction with AMS-02 and Atmospheric Cherenkov Telescopes

TL;DR

This paper investigates whether nearby pulsars like Geminga and Monogem can explain the observed rise in cosmic-ray positron fraction, proposing that atmospheric Cherenkov telescopes could detect anisotropies confirming this origin.

Contribution

It demonstrates that existing and future atmospheric Cherenkov telescopes can effectively test the pulsar origin hypothesis for the positron excess through anisotropy measurements.

Findings

Nearby pulsars can account for the positron excess.

Cherenkov telescopes can detect anisotropies in cosmic-ray arrival directions.

Current data already constrain pulsar contributions.

Abstract

Recent observations by PAMELA, Fermi-LAT, and AMS-02 have conclusively indicated a rise in the cosmic-ray positron fraction above 10 GeV, a feature which is impossible to mimic under the paradigm of secondary positron production with self-consistent Galactic cosmic-ray propagation models. A leading explanation for the rising positron fraction is an additional source of electron-positron pairs, for example one or more mature, energetic, and relatively nearby pulsars. We point out that any one of two well-known nearby pulsars, Geminga and Monogem, can satisfactorily provide enough positrons to reproduce AMS-02 observations. A smoking-gun signature of this scenario is an anisotropy in the arrival direction of the cosmic-ray electrons and positrons, which may be detectable by existing, or future, telescopes. The predicted anisotropy level is, at present, consistent with limits from Fermi-LAT and AMS-02. We argue that the large collecting area of Atmospheric Cherenkov Telescopes (ACTs) makes them optimal tools for detecting such an anisotropy. Specifically, we show that much of the proton and gamma-ray background, which affects measurements of the cosmic-ray electron-positron spectrum with ACTs, may be controlled in the search for anisotropies. We conclude that observations using archival ACT data could already constrain or substantiate the pulsar origin of the positron anomaly, while upcoming instruments (such as the Cherenkov Telescope Array) will provide strong constraints on the source of the rising positron fraction.