Extraterrestrial Axion Search with the Breakthrough Listen Galactic
  Center Survey
  Axion dark matter (DM) may efficiently convert to photons in the
magnetospheres of neutron stars (NSs), producing nearly monochromatic radio
emission. This process is resonantly triggered when the plasma frequency
induced by the underlying charge distribution approximately matches the axion
mass. We search for evidence of this process using archival Green Bank
Telescope data collected in a survey of the Galactic Center in the C-Band by
the Breakthrough Listen project. While Breakthrough Listen aims to find
signatures of extraterrestrial life in the radio band, we show that their
high-frequency resolution spectral data of the Galactic Center region is ideal
for searching for axion-photon transitions generated by the population of NSs
in the inner pc of the Galaxy. We use data-driven models to capture the
distributions and properties of NSs in the inner Galaxy and compute the
expected radio flux from each NS using state-of-the-art ray tracing
simulations. We find no evidence for axion DM and set leading constraints on
the axion-photon coupling, excluding values down to the level $g_{a \gamma
\gamma} \sim 10^{-11}$ GeV$^{-1}$ for DM axions for masses between 15 and 35
$\mu$eV.
