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

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 $g_{a\gamma }<(5.8\times {10}^{-14}-1.8\times {10}^{-10}){\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$ across a broad ALP mass range of $m_a\sim (1.4\times {10}^{-23}-5.2\times {10}^{-20})\mathrm{e}\mathrm{V}$. 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.