PULSAR PSR-1744-24A - TIMING, ECLIPSES, AND THE EVOLUTION OF NEUTRON-STAR BINARIES

High-precision timing measurements have yielded improved astrometric, binary, and rotational parameters of the eclipsing binary millisecond pulsar PSR 1744-24A. The timing position of the pulsar shows it to be 40" from the core of the globular cluster Terzan 5, coincident with a steep-spectrum continuum source. The observed negative pulsar period derivative, P = - 1.9 +/- 0.2 x 10(-20), is biased by acceleration of the system in the gravitational fields of the cluster and, possibly, the Galactic center. A strong attenuation of the pulsed signal occurs near the eclipse edges, and excess time delays of the pulsed signal are observed during eclipses. Eclipse characteristics vary as a function of frequency and, even at a single frequency, orbital phases of eclipse edges are somewhat variable. At high frequency the eclipses are not always total. The upper limit on changes in the orbital period, /P(b)/ less-than-or-equal-to 5 x 10(-12), puts a lower limit of 40 Myr on the time scale of orbital evolution. The implications of these observations for the nature of the companion star, the eclipse mechanism, and the evolution and fate of the PSR 1744-24A system are discussed, as is its possible association with the X-ray burster XB 1745 - 25.