A weak upper limit for axion-related continuum polarization signal at 630 nm
Axions could be a component of the dark matter and are of great interest for particle physics. If present, their interaction with magnetic fields in the solar photosphere should produce a polarization signal of real photons, whose amplitude depends quadratically on the magnetic field strength. I search for possible traces of axion-induced low-energy polarization signal in the visible wavelength range near 630 nm. I use an observation of a sunspot and the nearby active region to study the dependence of the continuum polarization signal on magnetic field strength. I correct the continuum polarization signal for the polarization introduced by the telescope due to oblique reflections. I reproduce the corrected polarization spectra using a spectral synthesis of solar model atmospheres that include the Zeeman effect. These synthetic spectra serve as a reference for the amount of polarization from a known source. I find no trace of an enhanced polarization signal in continuum polarization above a polarization level of around 0.05 % of I_c. I provide a weak upper limit for the existence and interaction of axions with solar magnetic fields by proposing that any model of solar axion generation and interaction should not lead to polarization effects above 0.05 % of the continuum intensity at a wavelength of 630 nm, i.e., not more than 5 out of 10000 photons should show polarization due to axion-related processes. Using estimates of photon and axion flux rates, I derive an upper limit of $g_{a\gamma} < 700*10^{-10}$ GeV$^{-1}$ for the axion-photon coupling constant $g_{a\gamma}$.