Reflected Polarization of Close Binaries: A New Probe of Axion Birefringence ✦
A method is proposed to search for birefringence induced by ultralight axion dark matter using polarimetry of close binary systems. In these systems, linear polarization of reflected or scattered light is modulated by orbital motion, creating a phase pattern. The axion oscillation of the polarization angle manifests as sidebands around the orbital harmonics. For a single bright binary, assuming white noise, a sensitivity to the axion-photon coupling of about 10^-12 GeV^-1 is achievable at an axion mass of 10^-20 eV. An array of suitable systems in an optimistic scenario could improve this to 10^-13 GeV^-1. The method complements existing astrophysical searches, offering a high-frequency optical probe of axion birefringence.
Context
The nature of dark matter remains one of physics' greatest mysteries. Among the candidates, ultralight axions stand out—particles that can weakly interact with photons, causing a rotation of the polarization plane (birefringence). Detecting this effect requires sources with predictable intrinsic polarization. Close binary systems, where light from one star scatters in the companion's atmosphere, produce polarization whose phase is tightly synchronized with the orbital motion. This makes them ideal “polarimetric clocks” for axion signal searches.
Methods
The researchers developed an analytical model in which the binary's intrinsic polarization is expanded into a Fourier series over the harmonics of the orbital frequency Ω, governed by Kepler's third law. The axion field, described by modified Maxwell's equations with the interaction Lagrangian a F ~F, induces an additional rotation of the polarization plane, oscillating at a frequency equal to the particle mass μ. This rotation is superimposed on the orbital harmonics, creating sideband frequencies nΩ ± μ. Signal analysis is performed using the maximum likelihood method, assuming white noise from the measurement apparatus and uncorrelated stellar variability. For a single system, the significance of the Earth and source terms is assessed, while for an array, a common Earth term is extracted by averaging the independent source phases.
Results
For a benchmark corresponding to the system μ¹ Sco (period ~4 days, polarization amplitude P_rms ≈ 300 ppm, single-measurement precision σ_p ≈ 10 ppm, cadence 10 min, observations 30 days), the statistical sensitivity to the axion-photon coupling constant g_{aγ} was 2.4×10⁻¹² GeV⁻¹ at μ = 10⁻²⁰ eV. Combining N = 14 suitable systems (selected by temperature > 10⁴ K and period < 5 days) and improving precision to 1 ppm, the projected sensitivity reaches 1.3×10⁻¹³ GeV⁻¹. These values are comparable to the best current limits from cosmic microwave background and pulsar arrays, but in a new optical band and with high time resolution (hours to days), which is especially valuable for axion masses near 10⁻²⁰ eV. The method also proved robust to asynchronous variability, which averages out when observing many targets.
Implications
The proposed approach fills the niche of high-cadence optical polarimetry in astrophysical axion searches. It naturally complements existing methods: cosmic microwave background analysis, pulsar polarization timing, and protoplanetary disk observations, expanding the explored parameter space into the poorly studied mass range of 10⁻²¹–10⁻¹⁸ eV. Moreover, the work stimulates the development of precise scattering models in hot star atmospheres, tracing back to the work of Chandrasekhar.
Future development
Further development involves conducting dedicated surveys of close binaries using high-precision polarimeters on large telescopes. Improvements in numerical models of radiative transfer and stellar evolution will enable more accurate predictions of polarization templates, while machine learning algorithms will help isolate weak signals against complex variability. Special attention will need to be paid to non-stationary effects: pulsations, mass transfer, and circumstellar material, which could mimic an axion signal in individual systems.
Impact
The method will impact dark matter physics, the astrophysics of close binary systems, high-precision polarimetry, and cosmology, offering a new independent tool to test extensions of the Standard Model, in particular axion models of Peccei–Quinn.
Next steps
The immediate next steps include detailed polarimetric mapping of candidates from catalogs like DEBCat and developing statistical methods to suppress systematic errors (angle calibration, instrumental polarization).
Key open problems
The work directly touches on deep unsolved problems: the nature of dark matter, the strong CP problem for which the axion was first proposed, and the hypothetical existence of ultralight scalar fields predicted by string theory.
🎯 The polarization of reflected light in binary systems is so tiny that reliable measurement became possible only in recent years with instruments like HIPPI-2. For example, in Spica it is only 200 ppm — akin to a speck of dust landing on a brightly lit sheet of paper, subtly altering its reflective properties.
Key numbers
- benchmark polarization degree (Spica, μ¹ Sco): 2×10⁻⁴ – 3×10⁻⁴
- orbital period of typical system: ≈ 4 days
- axion mass for peak sensitivity: 10⁻²⁰ eV
- expected sensitivity (single system): 2.4×10⁻¹² GeV⁻¹
- projected sensitivity (array of 14 systems): 1.3×10⁻¹³ GeV⁻¹