2025ApJ...988..104Y

Axions are one of the leading dark matter candidates. If we are embedded in a Milky Way dark matter halo comprised of axions, their stimulated decay would enable us to observe a counterimage (“axion gegenschein”) with a frequency equal to half the axion mass in the opposite direction of a bright radio source. This spectral line emission will be broadened to Δν/ν ∼ σ_d/c ∼ 10^–3 due to the velocity dispersion of dark matter, σ_d. In this pilot study, we perform the first search for the expected axion gegenschein image of Vela supernova remnant with 26.4 hr of effective ON-OFF data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) L-band (1.0-1.5 GHz) 19 beam receiver. Our null detection limits the axion-photon coupling strength to be g_aγγ ≲ 2 × 10^–10 GeV^–1 in the mass ranges of 8.7 μeV ≤ m_a ≤ 9.44 μeV and 10.85 μeV ≤ m_a ≤ 12.01 μeV. These results provide a stronger constraint on g_aγγ in this axion mass range than the current limits obtained by the direct search of an axion decay signal from a dwarf galaxy that uses FAST observations, but are a factor of ∼3 times weaker than the current CERN Axion Solar Telescope limit. Based on our observation strategy, data processing methods, and results, the expected sensitivity will reach ∼10^–11 GeV^–1 with ∼2000 hr of observation in the future.