Neutron Stars, X-Rays, and Gravitational Waves

Abstract

Bright X-ray sources in our galaxy include ^~100 accreting neutron star binaries. Accretion torque models explain spinup observed in binary pulsars (with magnetic fields 10¹²–10¹³ gauss) and predict that the shortest spin period reached by this process decreases as the magnetic field decreases. Systems where no pulses have been seen are candidates for millisecond binary X-ray pulsars. Period searches have set limits on millisecond pulsed flux in the range of 0.1% – 1%. Current upper limits can be interpreted in terms of known effects that minimize pulsed fraction and reduce detectability, but residual pulsation at lower levels is still expected. The way to reach these levels is with a proposed 100 m² instrument, called XLA, that could be mounted on the NASA Space Station. XLA is a general purpose instrument having many applications besides pulsar searching. In some binaries the limit on the spin period of the star may not be set by the magnetic field but by the physics of rotating equilibrium configurations. A gravitational radiation reaction instability can produce a X-ray/gravitational wave pulsar. Discovering the X-ray pulsar with XLA would permit a gravitational wave antenna to be tuned resonantly to the frequency found in X-rays.