Radio Line Properties of Axion Dark Matter Conversion in Neutron Stars
  Axions are well-motivated candidates for dark matter. Recently, much interest
has focused on the detection of photons produced by the resonant conversion of
axion dark matter in neutron star magnetospheres. Various groups have begun to
obtain radio data to search for the signal, however, more work is needed to
obtain a robust theory prediction for the corresponding radio lines. In this
work we derive detailed properties for the signal, obtaining both the line
shape and time-dependence. The principal physical effects are from refraction
in the plasma as well as from gravitation which together lead to substantial
lensing which varies over the pulse period. The time-dependence from the
co-rotation of the plasma with the pulsar distorts the frequencies leading to a
Doppler broadened signal whose width varies in time. For our predictions, we
trace curvilinear rays to the line of sight using the full set of equations
from Hamiltonian optics for a dispersive medium in curved spacetime. Thus, for
the first time, we describe the detailed shape of the line signal as well as
its time dependence, which is more pronounced compared to earlier results. Our
prediction of the features of the signal will be essential for this kind of
dark matter search.
