SN 1987A gamma-ray limits on the conversion of pseudoscalars

Pseudoscalar particles θ usually couple electromagnetically by an interaction of the form 14gθFF, allowing them to convert to photons in the presence of magnetic fields. Notably, new low-mass pseudoscalars emitted from supernova (SN) 1987A would have been converted to γ-rays in the intervening magnetic field of the galaxy. Therefore, measurements by the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS) can limit the inverse coupling constant to g−1 > 1 × 1011 GeV, assuming the pseudoscalar is massless. This is an improvement over other astrophysical limits of a factor of about 2.5.