Searching for axion dark matter gegenschein of the Vela supernova remnant with FAST

Abstract

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 $\Delta \nu /\nu \sim {\sigma }_d/c\sim 10^{-3}$ due to the velocity dispersion of dark matter, ${\sigma }_d$. In this pilot study, we perform the first search for the expected axion gegenschein image of Vela supernova remnant (SNR) with 26.4 hours 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 $g_{a\gamma \gamma }\lesssim 2\times 10^{-10}{\mathrm{GeV}}^{-1}$ in the mass ranges of $8.7\mu \mathrm{eV}\le m_a\le 9.44\mu \mathrm{eV}$ and $10.85\mu \mathrm{eV}\le m_a\le 12.01\mu \mathrm{eV}$. These results provide a stronger constraint on $g_{a\gamma \gamma }$ in this axion mass range than the current limits obtained by the direct search of axion decay signal from galaxy clusters which uses FAST observations, but is a factor of $\sim 3$ times weaker than the current CAST limit.Based on our observation strategy, data processing methods, and results, the expected sensitivity will reach $\sim 10^{-11}{\mathrm{GeV}}^{-1}$ with $\sim 2000$ hours of observation in the future.

Comments: 20 pages, 23 figures