EXTREME-ULTRAVIOLET EMISSION FROM THE MILLISECOND PULSAR J0437-4715

We report the first detection of extreme ultraviolet (EUV) emission from 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 an 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 with a temperature of similar to 5.7 x 10(5) K and an area of similar to 3 km(2) can explain both EUV and X-ray observations below 0.4 keV. Alternatively, 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 reheating would be required to explain this temperature according to standard neutron star cooling models because of the pulsar's 5 Gyr age. The EUV data rule out reheating by crust-core friction, accretion from the interstellar medium, accretion from the white dwarf companion, and heating by a particle-wind generated nebula. We use models of pulsar reheating by magnetic monopole catalysis of nucleon decay to establish an upper limit to the flux of monopoles in the Galaxy from one to three orders of magnitude below existing limits.