Spectral signatures of photon-particle oscillations from celestial objects

We give detailed predictions for the spectral signatures arising from photon-particle oscillations in astrophysical objects. The calculations include quantum electrodynamic effects and effects due to active relativistic plasma. We show that, by studying the spectra of compact sources, it may be possible to directly detect (pseudo-)scalar particles, such as the axion, with much greater sensitivity, by up to three orders of magnitude, than is currently achievable by other methods. In particular, if such particles exist with masses m_a < 10^-2 eV and coupling constant to the electromagnetic field, g > 10^-13 GeV^-1, then their oscillation signatures are likely to be lurking in the spectra of magnetars, pulsars, and quasars.