Accreting binaries containing neutron stars with low magnetic fields ( ) are believed to be the progenitors of millisecond radio pulsars. A G neutron star accreting at - will reach ms spin periods in yr . The luminosity, rapid variability, and magnetospheric QPO observed in Z sources point to the presence of Eddington-rate accretion onto weakly magnetic neutron stars. In atoll and some Z sources, X-ray bursts provide yet stronger evidence for neutron stars with very weak magnetic fields. X-ray binaries, in particular low-mass X-ray binaries (LMXBs), are among the most X-ray luminous objects in the galaxy, and yet millisecond X-ray pulsations remain undetected. Indeed, in the majority of accreting X-ray binary systems containing neutron stars the spin period of the neutron star is unknown, especially among LMXBs.
Detecting millisecond X-ray pulsars is one of XTE's primary goals. Millisecond X-ray pulsars are the most important missing link in our understanding of the evolutionary history of LMXB's and their connection with millisecond radio pulsars. The spin period is a critical ingredient in models of magnetospheric QPO, and thereby an important means for testing the theory of accretion torques. The spin period provides an estimate of magnetic field strength, assuming the neutron star has reached its equilibrium period, as does the dependence of the magnetospheric QPO frequency on luminosity when the spin period is known.
Spin periods make precise orbital determinations possible. Millisecond spin periods would enable the kind of high precision orbital measurements that are now possible only in the radio, and would be ideal for measuring the small orbital changes, so far observed only in a small number of high-mass binaries, that place severe constraints on models of binary evolution and stellar structure. Finally, X-ray pulsars are potentially laboratories for probing the internal structure of neutron stars by measuring their response to external torques on short time scales. Glitches in radio pulsars probe the internal structure of isolated, relatively young, high magnetic-field neutron stars with very different environments and evolutionary histories than the neutron stars in LMXBs.
Previous searches for spin periods in LMXBs resulted in upper limits to the modulation depth of 0.3% 3% for spin periods ranging from milliseconds to a few seconds. From this we know that pulsations, if present, are weak. Low intrinsic modulation depth may prevail in LMXBs because the weak magnetic field is ineffective at channeling material to the polar cap. Atmospheric scattering and gravitational lensing may further suppress the observed pulsed amplitude.
Like scattering, orbital motion of the neutron star around its companion causes dephasing of the pulsar signal. Unlike scattering, we can correct for dephasing due to orbital motion if we know the orbital parameters. Computational limitations have forced most previous observers to approximate the orbital corrections with a single parameter.
We are undertaking a systematic, deep search for pulsations in LMXBs using Fourier techniques aimed at maximizing the likelihood of detection by covering the widest possible range in spin frequency, orbital period and radius, pulse shape, and modulation depth. We achieve a significant improvement over previous efforts through a combination of the large collecting area, high time resolution, pointing flexibility, and wide energy range of XTE, and the availability of parallel supercomputers and networks of high performance workstations that enable us to perform not only longer transforms for improved sensitivity, but to use a variety of different techniques such as two and three parameter orbital searches, harmonic sums, and overresolved transforms to probe with uniform sensitivity areas of pulse-shape and orbital-parameter phase space never before investigated. What we are proposing is an observational program coupled with an intensive computational and human effort combining the computational facilities and expertise of the high-energy astrophysics groups at Caltech, NRL, MIT, and the University of Amsterdam.