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 ${10}^4$ would constrain the axion mass $M_a$<3×${10}^{\mathrm{-}3}$ eV if the core is nonsuperfluid and at temperature $T_c$∼2×${10}^8$ K. This would improve the limits $M_a$≲4×${10}^{\mathrm{-}2}$ eV from neutron-star cooling and $M_a$<1×${10}^{\mathrm{-}2}$ eV from red-giant evolution. If the core is superfluid throughout, a factor of ${10}^5$ 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×${10}^{\mathrm{-}7}$ photons/${\mathrm{cm}}^2$ sec from a very young hot ($T_c$∼7×${10}^8$ K) pulsar within the Galaxy could set a firm bound on the axion mass, since neutron superfluidity is not expected above this temperature.