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

TL;DR

The paper investigates the detectability of GeV gamma-ray polarization arising from photon-ALP mixing in Galactic and extragalactic magnetic fields, using AMS-02 and the proposed AMS-100 to forecast sensitivity across ALP parameter space. It develops a photon-ALP mixing framework, models the Galactic and extragalactic magnetic fields, and simulates polarization signals for representative sources (pulsars, SNRs, and NGC1275), employing both minimum detectable polarization estimates and Monte Carlo propagation. The results show that AMS-02 can measure polarization only for the brightest Galactic sources, while AMS-100 can probe new ALP parameter space—especially via NGC1275—and test energy-dependent polarization, albeit with dependence on magnetic-field modeling. The findings indicate that AMS-100 could access $g_{a\gamma\gamma}$ down to about $7\times10^{-13}$ GeV$^{-1}$ for $m_a\lesssim 2\times10^{-9}$ eV, highlighting GeV-band gamma-ray polarimetry as a complementary probe to X-ray searches and intensity-based ALP constraints, with practical implications for future mission design and multi-messenger ALP searches.

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.

Figures (15)

• Figure 1: The strength of the Galactic magnetic field, based on the model in Ref. Jansson2012, as a function of position in the Galactic plane. The selected pulsars and supernova remnants are indicated as crosses and triangles, respectively. The position of the Sun is marked as a red point. Geminga and Vela are not indicated because of their proximity to the Sun (which also makes them poor targets for an ALP search).
• Figure 2: Example of expected photon intensity and polarization from the pulsar PSR J2021+3651, with ALP parameters $g_{a\gamma\gamma}=3.543\times 10^{-10}$ GeV$^{-1}$, $m_a = 4.41 \times 10^{-9}$ eV. (a) The intensity of photons at 3 GeV versus the distance along the line of sight from the pulsar. (b) The intensity of photons versus the photon energy. (c) The polarization degree of photons at 3 GeV versus the distance along the line of sight from the pulsar. (d) The polarization degree of photons versus the photon energy. (e) The polarization angle of photons at 3 GeV versus the distance along the line of sight from the pulsar. (f) The polarization angle of photons versus the photon energy, at the Earth's location. The red curve assumes an initially unpolarized source; the blue and violet curves assume initially 100% polarized source with different initial angles (see text for details). The transversal magnetic field $\mathrm{B_\perp}$ in units of $\mu$G is shown as the dashed green line in (a), (c), and (e).
• Figure 3: Kinematics of electron-positron pair production. The $x$ and $y$ axes are aligned with the directions of the electric field and magnetic field carried by the light source, respectively. The 3-momenta of the incoming photon, the emitted electron and positron are denoted as $\vec{k}_{\gamma}$, $\vec{p}_{e^{-}}$, and $\vec{p}_{e^{+}}$. The $x-$ or electric field direction is also called the polarization direction of the source. The electron and positron form a plane (green) which is perpendicular to the $x-y$ plane (blue). The azimuthal angle $\phi$ is defined as the angle between the electron-positron plane and the direction of the electric field.
• Figure 4: The expected event distribution as a function of azimuthal angle $\phi$ with fully polarized (red solid line) and unpolarized (blue dashed line) photons. A sinusoid shape is expected for polarized photons while a flat distribution is expected for unpolarized photons.
• Figure 5: The event distribution as a function of azimuthal angle $\phi$ with initially unpolarized photons from W44 in the unbinned analysis, in one example realization. We assume ten years of exposure with AMS-100. The orange line is the fitted function and the blue dots are results from MC simulation. The ALP parameters are $g_{a\gamma \gamma} = 3.543\times 10^{-10}$ GeV$^{-1}$, $m_a = 4.41\times 10^{-9}$ eV.