  Oscillations from high energy photons into light pseudoscalar particles in an
external magnetic field are expected to occur in some extensions of the
standard model. It is usually assumed that those axionlike particles (ALPs)
could produce a drop in the energy spectra of gamma ray sources and possibly
decrease the opacity of the Universe for TeV gamma rays. We show here that
these assumptions are in fact based on an average behavior that cannot happen
in real observations of single sources. We propose a new method to search for
photon-ALP oscillations, taking advantage of the fact that a single observation
would deviate from the average expectation. Our method is based on the search
for irregularities in the energy spectra of gamma ray sources. We predict
features that are unlikely to be produced by known astrophysical processes and
a new signature of ALPs that is easily falsifiable.
