Axion mass limits may be improved by pulsar x-ray measurements

Axions thermally emitted by a neutron star would be converted into x rays in the strong magnetic field surrounding the star. The present observational limit of pulsed x rays from the Vela pulsar (PSR 0833-45) is not small enough to bound the axion mass. An increase in x-ray sensitivity by a factor of 10⁴ would constrain the axion mass Mₐ3×{}10^-3 eV if the core is nonsuperfluid and at temperature Tc{~}2×{}10⁸ K. This would improve the limits Mₐ{}4×{}10^-2 eV from neutron-star cooling and Mₐ1×{}10^-2 eV from red-giant evolution. If the core is superfluid throughout, a factor of 10⁵ in sensitivity would be needed. A search for modulated hard x rays from PSR 1509-58 or other young pulsars is suggested. A limit on pulsed hard x rays 5×{}10^-7 photons/cm² sec from a very young hot (Tc{~}7×{}10⁸ K) pulsar within the Galaxy could set a firm bound on the axion mass, since neutron superfluidity is not expected above this temperature.