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

Abstract

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}$3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ eV if the core is nonsuperfluid and at temperature ${T}_{c}$\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ K. This would improve the limits ${M}_{a}$\ensuremath{\lesssim}4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ eV from neutron-star cooling and ${M}_{a}$1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ photons/${\mathrm{cm}}^{2}$ sec from a very young hot (${T}_{c}$\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}${10}^{8}$ K) pulsar within the Galaxy could set a firm bound on the axion mass, since neutron superfluidity is not expected above this temperature.