Searching for GeV Gamma-Ray Polarization and Axion-Like Particles with AMS-02

TL;DR

This paper evaluates AMS-02 and AMS-100's ability to detect GeV gamma-ray polarization and ALP-induced effects, finding limited sensitivity for Galactic sources but promising prospects for extragalactic sources like NGC1275.

Contribution

It provides the first detailed estimates of gamma-ray polarization detectability and ALP search potential with AMS-02 and AMS-100, highlighting their capabilities and limitations.

Findings

AMS-02 can detect polarization in the brightest Galactic sources like Vela and Geminga.

AMS-100 can probe new ALP parameter space using extragalactic sources such as NGC1275.

Polarization measurements could test ALP-induced energy-dependent features in gamma-ray spectra.

Abstract

We study the detectability of GeV-band gamma-ray polarization with the AMS-02 experiment and its proposed successor AMS-100, from Galactic and extragalactic sources. Characterizing gamma-ray polarization in this energy range could shed light on gamma-ray emission mechanisms in the sources; physics beyond the Standard Model, such as the presence of axion-like particles (ALPs), could also induce a distinctive energy-dependent polarization signal due to propagation effects in magnetic fields. We present estimates for the minimum detectable polarization from bright sources and the forecast reach for axion-like particles (ALPs). We show that AMS-02 will have sensitivity to gamma-ray polarization only for the brightest steady-state Galactic sources, such as the Vela and Geminga pulsars; it is not expected to be capable of detecting polarization in Galactic or extragalactic sources that have been previously proposed as good targets for ALP searches with gamma-ray intensity measurements. However, AMS-100 observing the extragalactic source NGC1275 would be expected to probe new parameter space even for unfavorable B-field models, with prospects to measure the energy-dependence of such a signal. For Galactic sources, polarization measurements could provide a unique test of scenarios where ALPs induce energy-dependent features in the photon intensity. However, in the absence of a bright transient source (such as a Galactic supernova), the parameter space that would be probed by this approach with ten years of AMS-100 data is already nominally excluded by other experiments, although this conflict may be avoided in specific ALP models.