2000ApJ...529..447G

The Rossi X-ray Timing Explorer spacecraft observed the X-ray pulsar GX 1+4 for a period of 34 hr on 1996 July 19/20. The source faded from an intensity of ∼20 mcrab to a minimum of ≤0.7 mcrab and then partially recovered toward the end of the observation. This extended minimum lasted ∼40,000 s. Phase-folded light curves at a barycentric rotation period of 124.36568±0.00020 s show that near the center of the extended minimum the source stopped pulsing in the traditional sense but retained a weak dip feature at the rotation period. Away from the extended minimum, the dips are progressively narrower at higher energies and may be interpreted as obscurations or eclipses of the hot spot by the accretion column. The pulse profile changed from leading-edge bright before the extended minimum to trailing-edge bright after it. Data from the Burst and Transient Source Experiment show that a torque reversal occurred less than 10 days after our observation. Our data indicate that the observed rotation departs from a constant period with a P{dot}/P value of ~-1.5% per year at a 4.5 σ significance. We infer that we may have serendipitously obtained data, with high sensitivity and temporal resolution about the time of an accretion disk spin reversal. We also observed a rapid flare that had some precursor activity, close to the center of the extended minimum.