  Axions and axion-like-particles (ALPs) are characterised by their two-photon
coupling, which entails so-called photon-ALP oscillations as photons propagate
through a magnetic field. These oscillations lead to distinctive signatures in
the energy spectrum of high-energy photons from astrophysical sources, allowing
one to probe the existence of ALPs. In particular, photon-ALP oscillations will
induce energy dependent oscillatory features, or "ALP wiggles", in the photon
spectra. We propose to use the discrete power spectrum to search for ALP
wiggles and present a model-independent statistical test. By using PKS 2155-304
as an example, we show that the method has the potential to significantly
improve the experimental sensitivities for ALP wiggles, and that the ALP
wiggles may be detected using the Cherenkov Telescope Array (CTA) for
optimistic values of the photon-ALP coupling constant and the magnetic field.
Moreover, we discuss how these sensitivities depend on the modelling of the
magnetic field. We find that the use of realistic magnetic field models, due to
their larger cosmic variance, substantially enhances detection prospects
compared to the use of simplified models.
