LIMITS TO THE RADIATIVE DECAY OF THE AXION

An axion with a mass greater than 1 eV should be detectable through its decay into two photons. In this paper I discuss the astrophysical and cosmological limits which define a small window of allowed axion mass above 3 eV. A firm upper bound to the axion's mass of m(a) less-than-or-equal-to 8 eV is derived by considering the effect of decaying axions upon the diffuse extragalactic background radiation and the brightness of the night sky due to axions in the halo of our Galaxy. The intergalactic light of clusters of galaxies is shown to be an ideal place to search for an emission line arising from the radiative decay of axions. An unsuccessful search for this emission line in three clusters of galaxies is then detailed. Limits to the presence of any intracluster line emission are derived with the result that axions with masses between 3 and 8 eV are excluded by the data, effectively closing this window of axion mass, unless a severe cancellation of axionic decay amplitudes occurs. The intracluster flux limits are then used to constrain the amplitude of any such model dependence.