Detection of pulsed X-rays from the binary millisecond pulsar J0437 – 4715

Abstract

THE X-ray properties of pulsars (highly magnetized rotating neutron stars) depend mainly on the age of the star¹. For young (≤ 10⁴ 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 magnetosphere^2–5. 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 fields^6,7. No thermal emission is expected for neutron stars older than ∼10⁶ yr (refs 8–10) in the absence of surface heating processes^11–15. Here we report the discovery of pulsed X-ray emission from a ∼2 × 10⁹-yr-old neutron star: the binary milli-second 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 friction^11–13 or by energetic particles streaming onto the polar cap from the pulsar's magnetosphere^14,15.