Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter
  Conversion in Neutron Star Magnetospheres
  Axion dark matter (DM) may convert to radio-frequency electromagnetic
radiation in the strong magnetic fields around neutron stars. The radio
signature of such a process would be an ultra-narrow spectral peak at a
frequency determined by the mass of the axion particle. We analyze data we
collected from the Robert C. Byrd Green Bank Telescope in the L-band and the
Effelsberg 100-m Telescope in the L-Band and S-band from a number of sources
expected to produce bright signals of axion-photon conversion, including the
Galactic Center of the Milky Way and the nearby isolated neutron stars RX
J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able
to set some of the strongest constraints to-date on the existence of axion DM
in the highly-motivated mass range between ~5-11 $\mu$eV.
