EXTREME-ULTRAVIOLET EMISSION FROM THE MILLISECOND PULSAR J0437-4715

We report the first detection of extreme ultraviolet (EUV) emission fr om a millisecond pulsar. The EUV flux is not consistent with standard models used to describe the X-ray flux from this object. The size of a n EUV-emitting hot polar cap disagrees with the size derived from the X-ray data by a factor from similar to 3 to 25. However, a blackbody w ith a temperature of similar to 5.7 x 10(5) K and an area of similar t o 3 km(2) can explain both EUV and X-ray observations below 0.4 keV. A lternatively, if the EUV emission is independent of the X-ray emission and is due entirely to a thermalized neutron star surface, we place a limit on the surface temperature of 1.6-4.0 x 10(5) K. Surface reheat ing would be required to explain this temperature according to standar d neutron star cooling models because of the pulsar's 5 Gyr age. The E UV data rule out reheating by crust-core friction, accretion from the interstellar medium, accretion from the white dwarf companion, and hea ting by a particle-wind generated nebula. We use models of pulsar rehe ating by magnetic monopole catalysis of nucleon decay to establish an upper limit to the flux of monopoles in the Galaxy from one to three o rders of magnitude below existing limits.