Spectral signatures of axionlike dark matter
  We derive spectral line shapes of the expected signal for a haloscope
experiment searching for axionlike dark matter. The knowledge of these line
shapes is needed to optimize an experimental design and data analysis
procedure. We extend the previously known results for the axion-photon and
axion-gluon couplings to the case of gradient (axion-fermion) coupling. A
unique feature of the gradient interaction is its dependence not only on
magnitudes but also on directions of velocities of galactic halo particles,
which leads to the directional sensitivity of the corresponding haloscope. We
also discuss the daily and annual modulations of the gradient signal caused by
the Earth's rotational and orbital motions. In the case of detection, these
periodic modulations will be an important confirmation that the signal is
sourced by axionlike particles in the halo of our Galaxy.
