DETECTION OF PULSED X-RAYS FROM THE BINARY MILLISECOND PULSAR J0437-4715

THE X-ray properties of pulsars (highly magnetized rotating neutron stars) depend mainly on the age of the star1. For young (less-than-or-equal-to 10(4) yr) pulsars, virtually all of the X-ray emission is pulsed, appearing as sharp bursts with a power-law spectrum; these emissions are thought to arise from the acceleration of electrons and positrons in the pulsar's magnetosphere2-5. Older pulsars, on the other hand, exhibit broad X-ray pulses, a lower pulsed fraction and black-body spectra; this is ascribed to thermal emission from the surface of the cooling neutron star, the modulation of the X-rays reflecting temperature inhomogeneities induced by the strong magnetic fields6,7. No thermal emission is expected for neutron stars older than approximately 10(6) yr (refs 8-10) in the absence of surface heating processes11-15. Here we report the discovery of pulsed X-ray emission from a approximately 2 x 10(9)-yr-old neutron star: the binary millisecond pulsar J0437 - 4715. The spectral properties of the pulsed emission suggest a thermal origin, requiring substantial heating of the neutron-star surface either by internal friction11-13 or by energetic particles streaming onto the polar cap from the pulsar's magnetosphere14,15.