Hunting for Extragalactic Axion-like Dark Matter in a Decade-long Blazar Optical Polarimetry

TL;DR

This study targets axion-like dark matter (ALPs) via cosmic birefringence: the ALP-photon coupling can induce periodic rotations of the polarization angle (PA) of light from distant sources. Using ten years of optical PA measurements of the blazar 1ES 1959+650, the authors search for ALP-induced PA oscillations across $m_a \sim [1.4\times10^{-23}, 5.2\times10^{-20}]$ eV by (i) a Standard Deviation method and (ii) a Lomb-Scargle periodogram-based Monte Carlo analysis. No significant periodic signal is found, enabling 95% confidence level upper limits on the coupling: $g_{a\gamma} < (5.8\times10^{-14} - 1.8\times10^{-10})$ GeV^-1. These constraints, derived from an extragalactic AGN, are competitive with Galactic pulsar timing and surpass VLBA jet polarimetry in this ALP mass window, illustrating that long-term blazar polarimetry is a valuable, complementary probe of ultra-light dark matter in extragalactic contexts.

Abstract

Axions or axion-like particles (ALPs) are well-motivated dark matter (DM) candidates whose coupling to photons induces periodic oscillations in the polarization angle of astrophysical light. This work reports the first search for such a signature using ten years of optical polarimetric monitoring of the blazar 1ES 1959+650. No statistically significant periodicity is detected using a Lomb-Scargle periodogram and Monte Carlo analysis. Assuming a central DM density in the host galaxy, this null result places tight upper limits on the ALP-photon coupling constant at $g_{aγ}<(5.8 \times 10^{-14}-1.8\times 10^{-10})\,\mathrm{GeV}^{-1}$ across a broad ALP mass range of $m_a \sim (1.4\times10^{-23}-5.2\times10^{-20})\,\mathrm{eV}$. Our constraints surpass those from Very Long Baseline Array polarimetry of active galactic jets and are competitive with those from long-term Galactic pulsar timing of PSR J0437-4715 over the same ALP mass window. These results establish long-term blazar polarimetry as a competitive and complementary approach for probing axion-like DM on extragalactic scales.

Figures (3)

• Figure 1: Long-term linear polarization measurements of the blazar 1ES 1959+650 over a decade. Individual PA measurements are shown as blue dots, with vertical error bars indicating the $1\sigma$ uncertainties. The dashed line indicates the mean PA value of $146.61^{\circ}$, with the gray shaded band representing the $\pm 1\sigma$ uncertainty of $\pm12.30^{\circ}$.
• Figure 2: LS periodogram of the PA time series for the blazar 1ES 1959+650. The blue solid line shows the power spectrum ($P_{\text{LS}}(\nu)$), and the red dashed line marks the FAP threshold of 0.27%. No significant peaks exceed this threshold, which is consistent with the absence of a periodic signal at the 99.73% CL ($3\sigma$).
• Figure 3: Constraints on the ALP-photon coupling constant $g_{a\gamma}$ as a function of ALP mass $m_a$. The gold shaded region shows the 95% CL upper limit derived from ten years of optical polarization monitoring of the blazar 1ES 1959+650. Other astrophysical limits (95% CL) are shown for comparison, including VLBA polarimetry of jets from active galaxies (green dashed line) Ivanov2019, FAST observations of FRB 20220912A (blue solid line) wang2025, SPT-3G CMB data (light pink region) Ferguson_2022, Chandra observations of the quasar H1821+643 (olive dashed line) Sisk_Reyn_2021 and of M87 (gray dashed line) Marsh_2017, EHT observations of Sgr A* (royal blue region) Yuan2021, and polarimetry of PSR J0437-4715 (light blue region) Caputo_2019.